Module 4.2: Primary Structure - Biology

Module 4.2: Primary Structure - Biology

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Learning Objective

  • Distinguish between primary, secondary, tertiary and quaternary structure.
  • Predict the result of treating a protein with different cleavage reagents.
  • Generate the primary sequence from sequencing data.

Overview of Protein Structure

A protein is composed of amino acids attached in a linear order. This basic level of protein structure is called it's primary structure and derives from the formation of peptide bonds between the individual amino acids. Each amino acid in the linear polymer is referred to as a residue. The order, or sequence of the amino acids is determined by information encoded in the cell's genes. An example of a protein sequence is shown below where the one letter abbreviations are used for each of the 20 amino acids used in cellular protein synthesis.

Amino acid sequence of Human Estrogen Receptor: Amino acids are indicated using the single letter code.

Higher order structure is determined by the Primary Structure

Proteins do not exist as linear threads in the cells but rather as spontaneously folded higher order structures. The higher order structure is determined by the amino acids in the primary structure. Usually the sequence alone is sufficient to generate higher order structures, but some proteins require chaparones to help them fold.

The stages or levels of protein structure are:

  • Primary Structure: The amino acid sequence of the protein, with no regard for the conformation of the amino acids.
  • Secondary Structure: interactions involving only mainchain (also known as backbone) atoms resulting in α-helices and β-sheets. Mainchain atoms are the N-Cα-C=O atoms that form the backbone of the protein polymer.
  • Tertiary Structure: long range interactions resulting in the 3-D Folding of a single polypeptide chain.
  • Quaternary Structure: The interaction of two or more peptide chains to make a functional protein.
    • a homodimer contains two identical chains, represented as (alpha_{2})
    • a homotrimer contains three identical chains, represented as (alpha_{3})
    • a heterodimer contains two different chains, represented as (alphaeta)
    • a heterotrimer can contain two (e.g. (alpha_2eta)) identical chains, or three different chains, as in (alpha,eta,gamma)
    • a heterotetramer often contains two pairs of identical chains, such as in (alpha_2eta_2), but can contain four different chains, e.g. (alpha eta gamma delta)

learn by doing

Example - Structure Hierarchy in Hemoglobin

The oxygen transport protein, hemoglobin, is shown in this Jmol. The heme groups, which are colored purple, are responsible for binding the oxygen. The protein component of hemoglobin is colored gray. Hemoglobin looks complicated, but we can understand its structure using a hierarchical description of the structure.

Tertiary Structure is the complete description of the structure of both the mainchain and sidechain atoms of one poly-peptide chain. Clicking on the button will show you the tertiary structure of one of the sub-units of hemoglobin. Of course, the tertiary structure is built-up from secondary structural elements, which you can highlight with a pink ribbon by clicking here

Quaternary Structure is the complete description of the structure of all of the different poly-peptide chains that comprise the functional molecule. Clicking on the button will show you the complete quaternary structure of hemoglobin. You can click here to color each of the separate chains in hemoglobin. Of course, the quaternary structure is also built-up from secondary structural elements, which you can view by clicking here

Primary Structure is the sequence of amino acids. Hemoglobin has four separate polypeptide chains, the first few amino acids of the first chain (chain A) will appear after clicking the button.

Secondary Structure describes the local structure of just the main chain atoms. Each subunit of hemoglobin contains a number of alpha-helical secondary structural elements. Clicking on the button will show you one of these.

Which of the following levels of structure describes only the local structure of the mainchain atoms?

a. Primary

b. Secondary

c. Tertiary


The two key words in this question are local and structure.


b. (primary structure is just the chemical structure of the protein, i.e. the order of the amino acids; the structure of the sidechain atoms are also specificied in the tertiary structure.)

Determining Primary Structure

We will focus on N-terminal sequencing of the actual protein using Edman degradation. Fragmentation of the peptide may be required in the case of larger proteins. Note that protein sequences can be also be inferred from the DNA sequence and experimentally using mass spectroscopy.

Edman Degradation: The detailed chemical mechanism of Edman degradation will not be discussed here, however an overview of the Edman chemistry is shown here:

The protein is treated with phenyl isothiocyanate (PITC). PITC reacts with the amino terminus, producing a derivatized protein. The modified amino-terminal residue can be cleaved off, producing the intact protein that is one residue shorter and the PTH-derivative of the amino terminal amino acid. The PTH derivative can be analyzed to determine the original amino terminal amino acid. The cycle can be repeated again, identifying the second amino acid in the original peptide. Under optimal conditions it is possible to determine the first 80-100 residues of a protein.

Sequencing long Proteins: It is generally not possible to sequence an entire protein from the amino terminus. To extend the sequence information the protein is fragmented into smaller peptides. After cleavage, the individual peptide fragments are separated from each other and each is independently subject to N-terminal sequencing using the Edman degradation method. Three common fragmentation reactions are:

Cyanogen bromide (CNBr) cleaves the peptide bond after Methionine residues. As an example:

[Ser−Met−Gly−Ala−Phe−Arg−Leu−Ilestackrel{CNBr}{longrightarrow}Ser−Met + Gly−Ala−Phe−Arg−Leu−Ile onumber]

Chymotrypsin hydrolyzes the peptide bonds that follow large hydrophobic residues, e.g. Phenylalanine, Tyrosine, Tryptophan. As an example:

[Ser−Met−Gly−Ala−Phe−Arg−Leu−Ilestackrel{Chymotrypsin}{longrightarrow}Ser−Met−Gly−Ala−Phe + Arg−Leu−Ile onumber]

Trypsin hydrolyzes the peptide bonds that follow positively charged residues, e.g. Lysine and Arginine. As an example:

[Ser−Met−Gly−Ala−Phe−Arg−Leu−Ilestackrel{Trypsin}{longrightarrow}Ser−Met−Gly−Ala−Phe−Arg + Leu−Ile onumber]

If only two fragments are produced by the cleavage reaction, then it is straightforward to reconstruct the sequence using the known sequence of the original protein. However if the original protein is cleaved into three or more fragments, then it is not possible to determine the correct order of fragments using a single cleavage agent. Multiple overlapping fragments have to be used to determine the correct ordering, as illustrated below.


Sequence Determination


In this example I have assumed that 6 cycles of Edman degradation are possible. After that, impurities and side reactions prevent the reliable identification of the amino acid. Note that in practice 30-100 cycles can be accomplished, giving the sequence of the first 30-100 residues of the protein.

A: the first six cycles of edman degradation produced, Ala, Gly, Met, Ser, Thr, and Gly, in that order. therefore the amino terminal sequence is:


B: A new sample of the peptide was treated with CNBr. The two peptides (CNBr-1, CNBr-2) that were produced were isolated and each was subject to Edman Degradation, giving the following sequences (The residues in bold were determined by Edman degradation, the remainder of the peptide is present, but not detectable).

CNBr-1: (Ala-Gly-Met)CNBr-2: (Ser-Thr-Gly-Val-Val-Lys)(-Gly-Ser-Ala-Phe-Leu)

C: A new sample of the peptide was treated with Trypsin. The two peptides (Trp1, Trp2) that were produced were isolated and each was subject to Edman Degradation. The sequence of these two peptides was:

Trp1: (Gly-Ser-Ala-Phe-Leu)Trp2: (Ala-Gly-Met-Ser-Thr-Gly-)(Val-Val-Lys)

Strategy: Find overlaps between fragments obtained with different cleavage reagents and use these overlaps to correctly pair the peptides obtained from one sequencing reaction. The overlaps can be readily identified by finding a cleavage site in a peptide that would be cut by another cleavage reagent (e.g. Trypsin) and then identifying the correct fragment based on the expected amino-terminal sequence. For example, the sequence from the Edman degradation of the intact peptide contains a Met residue, so you would look for overlaps between the intact sequence and the two CNBr fragments:

[Ala−Gly−Met−Ser−Thr−GlyAla−Gly−Met onumber]

[Ala−Gly−Metspacespacespacespace Ser−Thr−Gly−Val−Val−Lys onumber]

[CNBr−1spacespacespacespacespacespacespacespacespacespacespacespace CNBr−2 onumber]

[Combinespace tospace give: onumber]

[Ala−Gly−Met−Ser−Thr−Gly−Val−Val−Lys onumber]

The partial sequence above contains a (Lys) residue. Therefore one of the Trypsin fragments should start with a (Gly) residue. Of the two Trypsin fragments, Trp1 starts with a (Gly) residue. Therefore Trp1 must be the second fragment, allowing completion of the sequence:

[Ala-Gly-Met-Ser-Thr-Gly-Val-Val-Lys space Gly-Ser-Ala-Phe-Leu onumber]

Review Quiz

did i get this

1. The primary structure refers to:

a. the conformation of multiple chains.

b. the conformation of a single protein chain.

c. the conformation of the sidechains.

d. the order of amino acids in a protein.

e. the first structure observed for proteins.


d. (The primary structure is the order of amino acids in the protein.)

2. If the peptide, (Val-Lys-Glu-Met-Ser-Trp-Arg-Ala), was digested with chymotrypsin, which of the following fragments would be produced?

a. (Val-Lys + Glu-Met-Ser + Trp-Arg-Ala).

b. (Val-Lys-Glu-Met-Ser-Trp + Arg-Ala).

c. (Val-Lys-Glu-Met-Ser + Trp-Arg-Ala).

d. (Val-Lys-Glu + Met-Ser-Trp-Arg-Ala).

e. (Val-Lys-Glu-Met + Ser-Trp-Arg-Ala).


b. (Chymotrypsin is specific for cleavage after the large aromatic residues, e.g. Phe, Tyr, and Trp.)

3. Digestion of a protein by trypsin produces three fragments: T1, T2, T3. How many different sequences could be constructed for the original protein from these peptides?

a. 1

b. 3

c. 6


Do you know the order of the trypsin fragments?


c. (A total of six different ordering of the three fragments is possible, e.g. T1-T2-T3, T1-T3-T2, T2-T1-T3, T2-T3-T1, T3-T1-T2, T3-T2-T1)

Module 4.2: Primary Structure - Biology

Communities are complex entities that can be characterized by their structure (the types and numbers of species present) and dynamics (how communities change over time). Understanding community structure and dynamics enables community ecologists to manage ecosystems more effectively.

Foundation Species

Figure 1. Coral is the foundation species of coral reef ecosystems. (credit: Jim E. Maragos, USFWS)

Foundation species are considered the “base” or “bedrock” of a community, having the greatest influence on its overall structure. They are usually the primary producers: organisms that bring most of the energy into the community. Kelp, brown algae, is a foundation species, forming the basis of the kelp forests off the coast of California.

Foundation species may physically modify the environment to produce and maintain habitats that benefit the other organisms that use them. An example is the photosynthetic corals of the coral reef (Figure 1). Corals themselves are not photosynthetic, but harbor symbionts within their body tissues (dinoflagellates called zooxanthellae) that perform photosynthesis this is another example of a mutualism. The exoskeletons of living and dead coral make up most of the reef structure, which protects many other species from waves and ocean currents.

Biodiversity, Species Richness, and Relative Species Abundance

Biodiversity describes a community’s biological complexity: it is measured by the number of different species (species richness) in a particular area and their relative abundance (species evenness). The area in question could be a habitat, a biome, or the entire biosphere. Species richness is the term that is used to describe the number of species living in a habitat or biome. Species richness varies across the globe (Figure 2). One factor in determining species richness is latitude, with the greatest species richness occurring in ecosystems near the equator, which often have warmer temperatures, large amounts of rainfall, and low seasonality. The lowest species richness occurs near the poles, which are much colder, drier, and thus less conducive to life in Geologic time (time since glaciations). The predictability of climate or productivity is also an important factor. Other factors influence species richness as well. For example, the study of island biogeography attempts to explain the relatively high species richness found in certain isolated island chains, including the Galápagos Islands that inspired the young Darwin. Relative species abundance is the number of individuals in a species relative to the total number of individuals in all species within a habitat, ecosystem, or biome. Foundation species often have the highest relative abundance of species.

Figure 2. The greatest species richness for mammals in North and South America is associated with the equatorial latitudes. (credit: modification of work by NASA, CIESIN, Columbia University)

Keystone Species

Figure 3. The Pisaster ochraceus sea star is a keystone species. (credit: Jerry Kirkhart)

A keystone species is one whose presence is key to maintaining biodiversity within an ecosystem and to upholding an ecological community’s structure. The intertidal sea star, Pisaster ochraceus, of the northwestern United States is a keystone species (Figure 3).

Studies have shown that when this organism is removed from communities, populations of their natural prey (mussels) increase, completely altering the species composition and reducing biodiversity. Another keystone species is the banded tetra, a fish in tropical streams, which supplies nearly all of the phosphorus, a necessary inorganic nutrient, to the rest of the community. If these fish were to become extinct, the community would be greatly affected.

Invasive Species

Figure 4. Aquatic invasive species in the United States: (a) purple loosestrife and (b) zebra mussel. (credit a: modification of work by Liz West credit b: modification of work by M. McCormick, NOAA)

Invasive species are non-native organisms that, when introduced to an area out of their native range, threaten the ecosystem balance of that habitat. Many such species exist in the United States, as shown in Figures 4–6. Whether enjoying a forest hike, taking a summer boat trip, or simply walking down an urban street, you have likely encountered an invasive species.

Invasive species like purple loosestrife (Lythrum salicaria) and the zebra mussel (Dreissena polymorpha) threaten certain aquatic ecosystems (Figure 4).

Some forests are threatened by the spread of common buckthorn (Rhamnus cathartica), garlic mustard (Alliaria petiolata), and the emerald ash borer (Agrilus planipennis) (Figure 5).

The European starling (Sturnus vulgaris) may compete with native bird species for nest holes.

Figure 5. Invasive species in US forests: (a) common buckthorn, (b) garlic mustard, and (c) the emerald ash borer. (credit a: modification of work by E. Dronkert credit b: modification of work by Dan Davison credit c: modification of work by USDA)

4.6 Structure Classification Schemes

The previous chapters gave a broad overview of protein structures. There are two notable endeavors to classify all proteins. SCOP and CATH. Intuitively one might ask the question whether there is a limited amount of principal folds existing. Interestingly no new folds were identified after 2008 respectively 2012, depending on the algorithm used.

4.6.1 SCOP: Structural Classification of Proteins

Nearly all proteins have structural similarities with other proteins and, in some of these cases, share a common evolutionary origin. A knowledge of these relationships is crucial to our understanding of the evolution of proteins and of development.

The scop database aims to provide a detailed and comprehensive description of the structural and evolutionary relationships between all proteins whose structure is known, including all entries in Protein Data Bank (PDB). It is available as a set of tightly linked hypertext documents which make the large database comprehensible and accessible. In addition, the hypertext pages offer a panoply of representations of proteins, including links to PDB entries, sequences, references, images and interactive display systems. The data can be directly accessed on the SCOP webpage.

Structural annotation in SCOP is done both manually and automatically.

Proteins are classified to reflect both structural and evolutionary relatedness. Many levels exist in the hierarchy, but the principal levels are family, superfamily and fold, described below. The exact position of boundaries between these levels are to some degree subjective. The evolutionary classification is generally conservative: where any doubt about relatedness exists, new divisions at the family and superfamily levels were made. Thus, some researchers may prefer to focus on the higher levels of the classification tree, where proteins with structural similarity are clustered.

The different major levels in the hierarchy are (from top to bottom):

  • Superfamilies: Bridging together protein families with common functional and structural features inferring probable common ancestors
  • Family: Proteins with related sequence but typically with distinct function.
  • Proteins: Sequences of essentially with essentially the same function (Different species, different isoforms)
  • Classes: Folds with similar structure
  • Folds: Similar structural elements

Proteins are defined as having a common fold if they have same major secondary structures in same arrangement and with the same topological connections. Different proteins with the same fold often have peripheral elements of secondary structure and turn regions that differ in size and conformation. In some cases, these differing peripheral regions may comprise half the structure. Proteins placed together in the same fold category may not have a common evolutionary origin: the structural similarities could arise just from the physics and chemistry of proteins favoring certain packing arrangements and chain topologies.

Andreeva A,Howorth D,Chandonia JM,Brenner SE,Hubbard TJP, Chothia C and Murzin AG (2007) Data growth and its impact on the SCOP database: new developments Nucleic Acids Research, 2008, Vol. 36

4.6.2 CATH: Classification of protein structures

CATH is a hierarchical classification of protein domain structures, which clusters proteins at four major levels, class(C), architecture(A), topology(T) and homologous superfamily (H). Annotation of domains is both manual and automatic

  • Class (similar to class from SCOP): Is defined by the secondary structure content (All alpha, all beta, alpha/beta etc.).
  • Architecture: Clustering of structurally similar arrangement of secondary elements, independent of their connectivity however
  • Topology or fold family: Structural grouping depending on both overall 3D shape and connectivity
  • Homologous superfamilies: Grouping of protein domains with (predicted to have) a common ancestor.

Sillitoe I, Lewis, TE, Cuff AL, Das S, Ashford P, Dawson NL, Furnham N, Laskowski RA, Lee D, Lees J, Lehtinen S, Studer R, Thornton JM, Orengo CA. CATH: comprehensive structural and functional annotations for genome sequences. Nucleic Acids Res. 2015 Jan doi: 10.1093/nar/gku947

Part A

During this module you will learn about the structure and function of each endocrine gland. You will be able to identify the major cells of each gland, list the signalling molecules they produce and briefly outline the role they play in the body.


Endocrine cells are organized into glands and the DNES.

They release signaling molecules by a number of mechanisms: neurocrine, paracrine, autocrine, juxtacrine and traditional endocrine.

Find out more about these cells their mechanisms of action and get an overview of the different endocrine glands in this video (8:11)


The pituitary gland is an endocrine gland that produces several hormones responsible for the regulation of growth, reproduction and metabolism.

It has two parts: adenohypophysis and neurohypophysis

The secretions from each are either produced in the hypothalamus or are regulated by secretions from the hypothalamus. Find out more about this master gland and the part of the brain that controls it during this video (33:25).


Ever wonder why we sleep at night and are awake during the day or even get sad and depressed during the winter months when days are short and nights are long?

Look no further than this pine cone shaped gland in the brain that seretes melatonin.

Find out more in this video (8:00)


The thyroid gland makes thyroid hormone that regulates our basal metabolic rate.

Find out about the thirsty cells that perform this huge task on a daily basis in this video (8:39)


There are usually four spherical parathyroid glands that are located on the posterior surface of the thyroid gland.

Find out about the important role they play in calcium regulation during this video (3.47)


The adrenal glands are pyramidal shaped glands that sit at the superior pole of each kidney.

In section the glands have two structurally and functionally distinct regions: the outer adrenal cortex (composed of three different zones that secretes a number of steroid hormones) and the inner adrenal medulla famous for secreting epinephrine and norepinephrine.

Find out more about the histology of the adrenal glands in this video (7:29)


You have seen the pancreas already during Unit 3 when you looked at its exocrine components and the enzymes it secreted involved in digestion.

Here we will focus very briefly on its endocrine component, the islets of Langerhans, and discover the cells involved in regulating blood sugar levels.

Find out more about the histology of the adrenal glands in this video (2:48)


Endocrine cells are found throughout the body either organized into glands or as diffuse cells scattered throughout other organs (DNES)

Endocrine cells release their signaling molecules via a neurocrine, paracrine, autocrine, juxtacrine or traditional endocrine mechanism

The pituitary gland has an epithelial derived adenohypophysis (anterior) part that has a pars tuberalis, intermedia and distalis and a neural derived neurohypophysis (posterior) which has an infundibulum and pars nervosa.

The adenohypophysis contains three cell populations: basophils and ,acidophils (responsible for the secretion of many signaling molecules) and chromophobes (a presumed stem cell population)

The neurohypophysis contains axons that have their cell bosies in nuclei in the hypothalamus and secrete ADH and oxytcocin, these axons travel into the pituitary via the hypothalamo-hyophyseal tract

The pituitary has a unique blood supply that consists of a portal system of vessels

The hypopthalamus is home to the supraoptic and paraventricular nuclei that send axons into the neurohypohysis and a collection of nuclei called the hypothalamo-hypophyseotropic area responsible for secreting signaling molecules that regulate the activity of cells in the adenohypophysis.

The pineal gland is composed of pinealocytes that secrete melatonin based on light/dark and is responsible for the sleep/wake cycle

The thyroid gland is organized into distinctive colloid filled follicles surrounded by follicular cells that produce the colloid and under the influence of TSH pinocytose it and convert it to T3/T4 to regulate mbasal metabolic rate

The thyroid gland also contains parafollicular cells that secrete calcitonin a signaling molecule that lowers elevated blood calcium levels

The parathyroid glands contain chief cells that secrete parathyroid hormone that elevate low blood calcium levels

The adrenal gland is organized into a cortex (that has a zona glomerulosa, fasciculata and reticularis, responsible for delivering an array of steroid hormones under the influence of ACTH) and a medulla that contains chromaffin cells which release adrenaline and noradrenalin

The endocrine pancreas is composed of islets of Langerhans (embedded among the pancreatic acinar cells) that contain alpha cells (glucagon - to increase blood sugar) and beta cells (insulin - to decrease blood sugar)

This is Part A, Protein Expression, under the module topic Protein Techniques. This topic part has two sections to review: Content Tutorial & Animations.

Click on the following link to visit the University of Utah’s Genetic Science Learning Center:

Once you are at the DNA to Protein web page, view the right side of the screen where it says “Proteins”. Click on the Tour the basics, What is a Protein? to review some general information about the role and production of proteins in the human body. When you have finished the Tour the basics, click on the Interactive Explore section below it to view the following animation, “Test Neurofibromin Activity in a Cell”. The animated activity allows you to see how normal cell division can be affected by a protein mutation. In the activity you will learn about the NF1 gene that codes for the protein neurofibromin and how it interacts with and regulates the function of the Ras protein that promotes cell division.

Overview of Protein Expression Systems: Workflow (Invitrogen)

The following flow chart has been designed by the Invitrogen company and provides a summary of the workflow involved in using a protein expression system to study and validate protein function and expression. Many of the techniques displayed in the flowchart for Sample Preparation, Protein Separation, Protein Characterization and Functional Validation will be explored in the protein module subsets b and c.

Reference: Invitrogen (, HTML Page) “Protein expression enables analysis of protein structure and function. Use of recombinant proteins varies widely—from functional studies in vivo to large-scale production for structural studies and therapeutics. Using the best expression system for your protein and application is the key to your success. Solubility, functionality, speed, and yield are often the most important factors to consider when choosing an expression system. Invitrogen offers a wide variety of expression systems so you’ll be sure to find one that meets your needs. The following table highlights the characteristics of the most popular expression hosts. “ (Invitrogen) The second flowchart provided below was also designed by Invitrogen and organizes the various types of protein expression systems and vectors that can be used in vitro and in vivo to study protein function and expression. As you can see, the following hosts span a diverse array of organisms that include both prokaryotic and eukaryotic systems.

  • Rapid expression screening
  • Toxic proteins
  • Incorporation of unnatural labels or amino acids
  • Functional assays
  • Protein interactions
  • Rapid expression directly from plasmid
  • Open system – easily add components to enhance solubility or functionality
  • Simple format
  • Scalable
  • Compatible with Gateway® Cloning
  • Expression yields over 3mg
  • Structural analysis
  • Antibody generation
  • Functional assays
  • Protein interactions
  • Scalable
  • Low cost
  • Simple culture conditions
  • Compatible with Gayteway® cloning
  • Protein solubility
  • Minimal posttranslational modifications
  • May be difficult to express functional mammalian proteins
  • Structural analysis
  • Antibody generation
  • Functional assays
  • Protein interactions
  • Eukaryotic protein processing
  • Scalable up to fermentation (grams per liter)
  • Simple media requirements
  • Fermentation required for very high yields
  • Growth conditions may require optimization
  • Functional assays
  • Structural analysis
  • Antibody generation
  • Posstranslational modifications similar to mammalian systems
  • Greater yield than mammalian systems
  • Compatible with Gateway® cloning
  • More demanding culture conditions
  • Functional assays
  • Protein interactions
  • Antibody generation
  • Highest level of correct post-translational modifications
  • Highest probability of obtaining fully functional human proteins
  • Compatible with Gateway® Cloning
  • Multimilligram per liter yields only possible in suspension cultures
  • More demanding culture conditions

Reference: Invitrogen (, HTML Page)

Protein Structure [back to top]

Polypeptides are just a string of amino acids, but they fold up to form the complex and well-defined three-dimensional structure of working proteins. To help to understand protein structure, it is broken down into four levels:

This is just the sequence of amino acids in the polypeptide chain, so is not really a structure at all. However, the primary structure does determine the rest of the protein structure. Finding the primary structure of a protein is called protein sequencing, and the first protein to be sequenced was the protein hormone insulin, by the Cambridge biochemist Fredrick Sanger, for which work he got the Nobel prize in 1958.

2. Secondary Structure

This is the most basic level of protein folding, and consists of a few basic motifs that are found in all proteins. The secondary structure is held together by hydrogen bonds between the carboxyl groups and the amino groups in the polypeptide backbone. The two most common secondary structure motifs are the a -helix and the b -sheet .

The a -helix. The polypeptide chain is wound round to form a helix. It is held together by hydrogen bonds running parallel with the long helical axis. There are so many hydrogen bonds that this is a very stable and strong structure. Do not confuse the a-helix of proteins with the famous double helix of DNA. Helices are common structures throughout biology.

The b -sheet. The polypeptide chain zig-zags back and forward forming a sheet of antiparallel strands. Once again it is held together by hydrogen bonds.

The a -helix and the b -sheet were discovered by Linus Pauling, for which work he got the Nobel prize in 1954. There are a number of other secondary structure motifs such as the b -bend, the triple helix (only found in collagen), and the random coil.

This is the compact globular structure formed by the folding up of a whole polypeptide chain. Every protein has a unique tertiary structure, which is responsible for its properties and function. For example the shape of the active site in an enzyme is due to its tertiary structure. The tertiary structure is held together by bonds between the R groups of the amino acids in the protein, and so depends on what the sequence of amino acids is. There are three kinds of bonds involved:

So the secondary structure is due to backbone interactions and is thus largely independent of primary sequence, while tertiary structure is due to side chain interactions and thus depends on the amino acid sequence.

4. Quaternary Structure

This structure is found in proteins containing more than one polypeptide chain, and simply means how the different polypeptide chains are arranged together. The individual polypeptide chains are usually globular, but can arrange themselves into a variety of quaternary shapes. e.g.:

Haemoglobin , the oxygen-carrying protein in red blood cells, consists of four globular subunits arranged in a tetrahedral (pyramid) structure. Each subunit contains one iron atom and can bind one molecule of oxygen.

Tubulin is a globular protein that polymerises to form hollow tubes called microtubules. These form part of the cytoskeleton, and make cilia and flagella move.

These four structures are not real stages in the formation of a protein, but are simply a convenient classification that scientists invented to help them to understand proteins. In fact proteins fold into all these structures at the same time, as they are synthesised.

The final three-dimensional shape of a protein can be classified as globular or fibrous.

fibrous (or filamentous) structure

The vast majority of proteins are globular, including enzymes, membrane proteins, receptors, storage proteins, etc. Fibrous proteins look like ropes and tend to have structural roles such as collagen (bone), keratin (hair), tubulin (cytoskeleton) and actin (muscle). They are usually composed of many polypeptide chains. A few proteins have both structures: the muscle protein myosin has a long fibrous tail and a globular head, which acts as an enzyme.

This diagram shows a molecule of the enzyme dihydrofolate reductase, which comprises a single polypeptide chain. It has been drawn to highlight the different secondary structures.

This diagram shows part of a molecule of collagen, which is found in bone and cartilage. It has a unique, very strong triple-helix structure.

Author information

Present address: Department of Biochemistry and Biophysics, School of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA

Present address: Lineberger Comprehensive Cancer Center, University of North Carolina, Chapel Hill, NC, USA

Present address: Foghorn Therapeutics, Cambridge, MA, USA

These authors contributed equally: Richard W. Baker, Janice M. Reimer.


Department of Cellular and Molecular Medicine, School of Medicine, University of California San Diego, La Jolla, CA, USA

Richard W. Baker, Janice M. Reimer & Andres E. Leschziner

Department of Physiology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, USA

Peter J. Carman, Bengi Turegun & Roberto Dominguez

Graduate Group in Biochemistry and Molecular Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, USA

Alliance Protein Laboratories, a Division of KBI BioPharma, San Diego, CA, USA

Section of Molecular Biology, Division of Biological Sciences, University of California San Diego, La Jolla, CA, USA

You can also search for this author in PubMed Google Scholar

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R.W.B., J.M.R., P.J.C. and B.T. performed all of the protein production and purification. R.W.B. performed cryo-EM data collection, analysis and model building. J.M.R. performed nucleosome remodeling assays. T.A. performed CD experiments and analyzed CD data. A.E.L. and R.D. supervised the structural and biochemical work. All authors participated in writing and editing the manuscript.

Corresponding author

3.4 Proteins

By the end of this section, you will be able to do the following:

  • Describe the functions proteins perform in the cell and in tissues
  • Discuss the relationship between amino acids and proteins
  • Explain the four levels of protein organization
  • Describe the ways in which protein shape and function are linked

Proteins are one of the most abundant organic molecules in living systems and have the most diverse range of functions of all macromolecules. Proteins may be structural, regulatory, contractile, or protective. They may serve in transport, storage, or membranes or they may be toxins or enzymes. Each cell in a living system may contain thousands of proteins, each with a unique function. Their structures, like their functions, vary greatly. They are all, however, amino acid polymers arranged in a linear sequence.

Types and Functions of Proteins

Enzymes , which living cells produce, are catalysts in biochemical reactions (like digestion) and are usually complex or conjugated proteins. Each enzyme is specific for the substrate (a reactant that binds to an enzyme) upon which it acts. The enzyme may help in breakdown, rearrangement, or synthesis reactions. We call enzymes that break down their substrates catabolic enzymes. Those that build more complex molecules from their substrates are anabolic enzymes, and enzymes that affect the rate of reaction are catalytic enzymes. Note that all enzymes increase the reaction rate and, therefore, are organic catalysts. An example of an enzyme is salivary amylase, which hydrolyzes its substrate amylose, a component of starch.

Hormones are chemical-signaling molecules, usually small proteins or steroids, secreted by endocrine cells that act to control or regulate specific physiological processes, including growth, development, metabolism, and reproduction. For example, insulin is a protein hormone that helps regulate the blood glucose level. Table 3.1 lists the primary types and functions of proteins.

Digestive EnzymesAmylase, lipase, pepsin, trypsinHelp in food by catabolizing nutrients into monomeric units
TransportHemoglobin, albuminCarry substances in the blood or lymph throughout the body
StructuralActin, tubulin, keratinConstruct different structures, like the cytoskeleton
HormonesInsulin, thyroxineCoordinate different body systems' activity
DefenseImmunoglobulinsProtect the body from foreign pathogens
ContractileActin, myosinEffect muscle contraction
StorageLegume storage proteins, egg white (albumin)Provide nourishment in early embryo development and the seedling

Proteins have different shapes and molecular weights. Some proteins are globular in shape whereas, others are fibrous in nature. For example, hemoglobin is a globular protein, but collagen, located in our skin, is a fibrous protein. Protein shape is critical to its function, and many different types of chemical bonds maintain this shape. Changes in temperature, pH, and exposure to chemicals may lead to permanent changes in the protein's shape, leading to loss of function, or denaturation . Different arrangements of the same 20 types of amino acids comprise all proteins. Two rare new amino acids were discovered recently (selenocystein and pirrolysine), and additional new discoveries may be added to the list.

Amino Acids

Amino acids are the monomers that comprise proteins. Each amino acid has the same fundamental structure, which consists of a central carbon atom, or the alpha (α) carbon, bonded to an amino group (NH2), a carboxyl group (COOH), and to a hydrogen atom. Every amino acid also has another atom or group of atoms bonded to the central atom known as the R group (Figure 3.22).

Scientists use the name "amino acid" because these acids contain both amino group and carboxyl-acid-group in their basic structure. As we mentioned, there are 20 common amino acids present in proteins. Nine of these are essential amino acids in humans because the human body cannot produce them and we obtain them from our diet. For each amino acid, the R group (or side chain) is different (Figure 3.23).

Visual Connection

Which categories of amino acid would you expect to find on a soluble protein's surface and which would you expect to find in the interior? What distribution of amino acids would you expect to find in a protein embedded in a lipid bilayer?

The chemical nature of the side chain determines the amino acid's nature (that is, whether it is acidic, basic, polar, or nonpolar). For example, the amino acid glycine has a hydrogen atom as the R group. Amino acids such as valine, methionine, and alanine are nonpolar or hydrophobic in nature, while amino acids such as serine, threonine, and cysteine are polar and have hydrophilic side chains. The side chains of lysine and arginine are positively charged, and therefore these amino acids are also basic amino acids. Proline has an R group that is linked to the amino group, forming a ring-like structure. Proline is an exception to the amino acid's standard structure since its amino group is not separate from the side chain (Figure 3.23).

A single upper case letter or a three-letter abbreviation represents amino acids. For example, the letter V or the three-letter symbol val represent valine. Just as some fatty acids are essential to a diet, some amino acids also are necessary. These essential amino acids in humans include isoleucine, leucine, and cysteine. Essential amino acids refer to those necessary to build proteins in the body, but not those that the body produces. Which amino acids are essential varies from organism to organism.

The sequence and the number of amino acids ultimately determine the protein's shape, size, and function. A covalent bond, or peptide bond , attaches to each amino acid, which a dehydration reaction forms. One amino acid's carboxyl group and the incoming amino acid's amino group combine, releasing a water molecule. The resulting bond is the peptide bond (Figure 3.24).

The products that such linkages form are peptides. As more amino acids join to this growing chain, the resulting chain is a polypeptide. Each polypeptide has a free amino group at one end. This end the N terminal, or the amino terminal, and the other end has a free carboxyl group, also the C or carboxyl terminal. While the terms polypeptide and protein are sometimes used interchangeably, a polypeptide is technically a polymer of amino acids, whereas the term protein is used for a polypeptide or polypeptides that have combined together, often have bound non-peptide prosthetic groups, have a distinct shape, and have a unique function. After protein synthesis (translation), most proteins are modified. These are known as post-translational modifications. They may undergo cleavage, phosphorylation, or may require adding other chemical groups. Only after these modifications is the protein completely functional.

Link to Learning

Click through the steps of protein synthesis in this interactive tutorial.

Evolution Connection

The Evolutionary Significance of Cytochrome c

Cytochrome c is an important component of the electron transport chain, a part of cellular respiration, and it is normally located in the cellular organelle, the mitochondrion. This protein has a heme prosthetic group, and the heme's central ion alternately reduces and oxidizes during electron transfer. Because this essential protein’s role in producing cellular energy is crucial, it has changed very little over millions of years. Protein sequencing has shown that there is a considerable amount of cytochrome c amino acid sequence homology among different species. In other words, we can assess evolutionary kinship by measuring the similarities or differences among various species’ DNA or protein sequences.

Scientists have determined that human cytochrome c contains 104 amino acids. For each cytochrome c molecule from different organisms that scientists have sequenced to date, 37 of these amino acids appear in the same position in all cytochrome c samples. This indicates that there may have been a common ancestor. On comparing the human and chimpanzee protein sequences, scientists did not find a sequence difference. When researchers compared human and rhesus monkey sequences, the single difference was in one amino acid. In another comparison, human to yeast sequencing shows a difference in the 44th position.

Protein Structure

As we discussed earlier, a protein's shape is critical to its function. For example, an enzyme can bind to a specific substrate at an active site. If this active site is altered because of local changes or changes in overall protein structure, the enzyme may be unable to bind to the substrate. To understand how the protein gets its final shape or conformation, we need to understand the four levels of protein structure: primary, secondary, tertiary, and quaternary.

Primary Structure

Amino acids' unique sequence in a polypeptide chain is its primary structure . For example, the pancreatic hormone insulin has two polypeptide chains, A and B, and they are linked together by disulfide bonds. The N terminal amino acid of the A chain is glycine whereas, the C terminal amino acid is asparagine (Figure 3.25). The amino acid sequences in the A and B chains are unique to insulin.

The gene encoding the protein ultimately determines the unique sequence for every protein. A change in nucleotide sequence of the gene’s coding region may lead to adding a different amino acid to the growing polypeptide chain, causing a change in protein structure and function. In sickle cell anemia, the hemoglobin β chain (a small portion of which we show in Figure 3.26) has a single amino acid substitution, causing a change in protein structure and function. Specifically, valine in the β chain substitutes the amino acid glutamic. What is most remarkable to consider is that a hemoglobin molecule is comprised of two alpha and two beta chains that each consist of about 150 amino acids. The molecule, therefore, has about 600 amino acids. The structural difference between a normal hemoglobin molecule and a sickle cell molecule—which dramatically decreases life expectancy—is a single amino acid of the 600. What is even more remarkable is that three nucleotides each encode those 600 amino acids, and a single base change (point mutation), 1 in 1800 bases causes the mutation.

Because of this change of one amino acid in the chain, hemoglobin molecules form long fibers that distort the biconcave, or disc-shaped, red blood cells and causes them to assume a crescent or “sickle” shape, which clogs blood vessels (Figure 3.27). This can lead to myriad serious health problems such as breathlessness, dizziness, headaches, and abdominal pain for those affected by this disease.

Secondary Structure

The local folding of the polypeptide in some regions gives rise to the secondary structure of the protein. The most common are the α-helix and β-pleated sheet structures (Figure 3.28). Both structures are held in shape by hydrogen bonds. The hydrogen bonds form between the oxygen atom in the carbonyl group in one amino acid and another amino acid that is four amino acids farther along the chain.

Every helical turn in an alpha helix has 3.6 amino acid residues. The polypeptide's R groups (the variant groups) protrude out from the α-helix chain. In the β-pleated sheet, hydrogen bonding between atoms on the polypeptide chain's backbone form the "pleats". The R groups are attached to the carbons and extend above and below the pleat's folds. The pleated segments align parallel or antiparallel to each other, and hydrogen bonds form between the partially positive hydrogen atom in the amino group and the partially negative oxygen atom in the peptide backbone's carbonyl group. The α-helix and β-pleated sheet structures are in most globular and fibrous proteins and they play an important structural role.

Tertiary Structure

The polypeptide's unique three-dimensional structure is its tertiary structure (Figure 3.29). This structure is in part due to chemical interactions at work on the polypeptide chain. Primarily, the interactions among R groups create the protein's complex three-dimensional tertiary structure. The nature of the R groups in the amino acids involved can counteract forming the hydrogen bonds we described for standard secondary structures. For example, R groups with like charges repel each other and those with unlike charges are attracted to each other (ionic bonds). When protein folding takes place, the nonpolar amino acids' hydrophobic R groups lie in the protein's interior whereas, the hydrophilic R groups lie on the outside. Scientists also call the former interaction types hydrophobic interactions. Interaction between cysteine side chains forms disulfide linkages in the presence of oxygen, the only covalent bond that forms during protein folding.

All of these interactions, weak and strong, determine the protein's final three-dimensional shape. When a protein loses its three-dimensional shape, it may no longer be functional.

Quaternary Structure

In nature, some proteins form from several polypeptides, or subunits, and the interaction of these subunits forms the quaternary structure . Weak interactions between the subunits help to stabilize the overall structure. For example, insulin (a globular protein) has a combination of hydrogen and disulfide bonds that cause it to mostly clump into a ball shape. Insulin starts out as a single polypeptide and loses some internal sequences in the presence of post-translational modification after forming the disulfide linkages that hold the remaining chains together. Silk (a fibrous protein), however, has a β-pleated sheet structure that is the result of hydrogen bonding between different chains.

Figure 3.30 illustrates the four levels of protein structure (primary, secondary, tertiary, and quaternary).

Denaturation and Protein Folding

Each protein has its own unique sequence and shape that chemical interactions hold together. If the protein is subject to changes in temperature, pH, or exposure to chemicals, the protein structure may change, losing its shape without losing its primary sequence in what scientists call denaturation. Denaturation is often reversible because the polypeptide's primary structure is conserved in the process if the denaturing agent is removed, allowing the protein to resume its function. Sometimes denaturation is irreversible, leading to loss of function. One example of irreversible protein denaturation is frying an egg. The albumin protein in the liquid egg white denatures when placed in a hot pan. Not all proteins denature at high temperatures. For instance, bacteria that survive in hot springs have proteins that function at temperatures close to boiling. The stomach is also very acidic, has a low pH, and denatures proteins as part of the digestion process however, the stomach's digestive enzymes retain their activity under these conditions.

Protein folding is critical to its function. Scientists originally thought that the proteins themselves were responsible for the folding process. Only recently researchers discovered that often they receive assistance in the folding process from protein helpers, or chaperones (or chaperonins) that associate with the target protein during the folding process. They act by preventing polypeptide aggregation that comprise the complete protein structure, and they disassociate from the protein once the target protein is folded.

Link to Learning

For an additional perspective on proteins, view this animation called “Biomolecules: The Proteins.”

Primary vs Secondary Structure

I just started in architecture school and I am trying to understand the difference between Primary vs Secondary structure in a structural system.

As far as I can tell, primary probably means all loadbearing elements but what is secondary? Also, is there tertiary structure?

I've never heard of 1st vs 2nd.

You should be able to find what you're looking for here:

Primary - everything without which the building won't stand up. Typically this means columns, braces and beams in steel construction. In concrete add shear walls and slabs. In some super tall or non standards geometry buildings floor slabs are also "activated" meaning they provide diaphragm action, whereas in much other construction they merely rest on primary elements.

Secondary - everything that holds something up (provides "structure") but isn't crucial to the buildings structural integrity. Example of this is facade steel like mullions and transoms or other arrangements to hold cladding, or various structural elements that hold up secondary elements like canopies, ceilings and decorative screens etc.

Tertiary systems come in to play sometimes, especially in large projects, for instance where the module of primary structure is vastly different in pure size from the module of for instance a cladding panel that attaches to it. Think a long span roof with parallel trusses every 30' and a 3x10' roof cladding element: then the secondary grid might create spans of say 10x10' bays and then there's the tertiary following the cladding module.

Kisumu Polytechnic Admission Requirements

Kisumu Polytechnic Admission Requirements vary depending on the course you want to apply.
Take a look at Kisumu Polytechnic Admission Requirements and find one that meets your needs.

1.0 Electrical and Electronics Engineering
1.1 Diploma Courses (9 terms with Attachment)
Code Course title Minimum requirement Admission time Exam body
1.1.1 Electrical & Electronic Technology (Power Option) module I,II & III (TIVET) KCSE mean grade C- (Minus). C- (Minus) in Maths/Physics. KNEC Craft Certificate for Module II January (regular) September- Part Time KNEC
1.2 Craft Courses (7 terms with Attachment)
1.2.1 Electrical & Electronic Technology Module I & II KCSE mean grade D+ (Plus). D (Plain) in Maths, Physics & English January (regular) September- Part Time KNEC
1.2.2 Telecommunication Engineering Module I & II (TIVET) KCSE mean grade D+ (Plus). D (Plain) in Maths, Physics & English January (regular) September- Part Time KNEC
2.0 Mechanical Engineering
2.1 Diploma Courses (9 terms with Attachment)
Code Course title Minimum requirement Admission time Exam body
2.1.1 Mechanical Engineering (Plant Option) TEP KCSE mean grade C- (Minus). C- (Minus) in Maths/Physics. January & May (regular) KNEC
2.1.2 Mechanical Engineering (Production Option) module I,II & III (TIVET) KCSE mean grade C- (Minus). C- (Minus) in Maths/Physics. KNEC Craft Certificate for Module II January only (regular) KNEC
2.1.3 Automotive Engineering Module I,II & III (TIVET) KCSE mean grade C- (Minus). C- (Minus) in Maths/Physics. KNEC Craft Certificate for Module II January only (regular KNEC
2.2 Craft Courses (7 terms with Attachment)
2.2.1 Mechanical Engineering (Production Option) Module I & II (TIVET) KCSE mean grade D+ (Plus). D (Plain) in Maths, Physics & English September Intake only KNEC
2.2.2 Marine Engineering Module I, II (TIVET) KCSE mean grade D+ (Plus). D (Plain) in Maths, Physics & English January & September KNEC
2.2.3 Motor Vehicle Mechanics Module I & II (TIVET) KCSE mean grade D+ (Plus). D (Plain) in Maths, Physics & English January/September KNEC
3.0 Building and Civil Engineering
3.1 Diploma Courses (9 terms with Attachment)
Code Course title Minimum requirement Admission time Exam body
3.1.1 Building Technology Module I,II & III (TIVET) KCSE mean grade C- (Minus). C- (Minus) in Maths/Physics. KNEC Craft Certificate for Module II January (regular) September- Part Time KNEC
131.2 Civil Engineering (TEP) KCSE mean grade C- (Minus). C- (Minus) in Maths/Physics. KNEC January (regular) September- Part Time KNEC
3.2 Craft Courses (7 terms with Attachment)
3.2.1 Masonry KCSE mean grade D+ (Plus). D (Plain) in Maths, Physics & English. May Intake Only KNEC
3.2.2 Plumbing (TEP) KCSE mean grade D+ (Plus). D (Plain) in Maths, Physics & English. May Intake Only KNEC
3.2.3 Road Construction KCSE mean grade D+ (Plus). D (Plain) in Maths, Physics & English. May Intake Only KNEC
4.0 Applied Sciences
4.1 Diploma Courses (9 terms with Attachment)
Code Course title Minimum requirement Admission time Exam body
4.1.1 Diploma in Medical Laboratory Sciences KCSE mean grade C (Plain). C (Plain) in Biology/Chemistry, English/Kiswahili. C- (Minus) in Maths/Physics 3 Years January/May KNEC & KMLTTB
4.1.2 Analytical Chemistry (TEP) KCSE mean grade C- (Minus). C- (Minus) in Chemistry Maths/Physics. January & May (regular) KNEC
4.1.3 Applied Biology (TEP) KCSE mean grade C- (Minus). C- (Minus) in Biology, Maths/Physics/Chemistry January & May (regular) KNEC
4.2 Craft Courses (7 terms with Attachment)
4.2.1 Science Laboratory Technology (TEP) KCSE mean grade D+ (Plus). D (Plain) in English/Kiswahili, Maths, Physics, Chemistry/Biology May Intake Only KNEC
4.2.2 Certificate in Medical Laboratory Sciences KCSE mean grade C- (Minus). C- (Minus) in Biology/Chemistry, English/Kiswahili. D+ (Plus) in Maths/Physics 2 years May intake only KNEC & KMLTTB
5.0 Institutional Management
5.1 Diploma Courses (9 terms with Attachment)
Code Course title Minimum requirement Admission time Exam body
5.1.1 Food and Beverages Management module I only (TIVET) KCSE Mean Grade of C- (Minus) January only (regular) KNEC
5.2 Craft Courses (7 terms with Attachment)
5.2.1 Food and Beverage Production, Sales & Service (TIVET) Module I only. KCSE mean grade D+ (Plus) September Intake only KNEC
6.0 Mathematics and Computer Stuies
6.1 Diploma Courses (9 terms with Attachment)
Code Course title Minimum requirement Admission time Exam body
6.1.1 Computer Science Module I,II & III (TIVET) KCSE mean grade C- (Minus). C- (Minus) in Maths, Physics & English. KNEC Craft Certificate for Module II January Intake Only KNEC
6.1.2 Diploma Information Communication Technology Module I,II & III (TIVET) KCSE mean grade C- (Minus). KNEC Craft Certificate for Module II January (regular) September- Part Time KNEC
6.2 Craft Courses (7 terms with Attachment)
6.2.1 Certificate in Information Communication Technology Module I, II (TIVET) KCSE mean grade D+ (Plus) Jan & Sept. (Regular) May (Part time) KNEC
7.0 Business Studies
7.1 Diploma Courses (9 terms with Attachment)
Code Course title Minimum requirement Admission time Exam body
7.1.1 Accountancy (B-TEP) KCSE Mean Grade C- (Minus). OR a relevant KNEC Craft Certificate. January & May (regular) KNEC
7.1.2 Business Management Module I,II & III (TIVET) KCSE Mean Grade C- (Minus). OR a relevant KNEC Craft Certificate. Jan & Sept. (Regular) May (Part time) KNEC
7.1.3 Sales and Marketing Module I, II & III (TIVET) KCSE Mean Grade C- (Minus). OR a relevant KNEC Craft Certificate. Jan & Sept. (Regular) May (Part time) KNEC
7.1.4 Supply Chain Management Module I, II & III (TIVET) KCSE Mean Grade C- (Minus). OR a relevant KNEC Craft Certificate. Jan & Sept. (Regular) May (Part time) KNEC
7.1.5 Secretarial Studies Module I,II & II (TIVET) KCSE Mean Grade C- (Minus). OR a relevant KNEC Craft Certificate. January (regular) September- Part Time KNEC
7.1.6 Human Resource Management Module I, II & III (TIVET) KCSE Mean Grade C- (Minus). OR a relevant KNEC Craft Certificate. Jan & Sept. (Regular) May (Part time) KNEC
7.2 Craft Courses (7 terms with Attachment)
7.2.1 Business Management Module I & II (TIVET) KCSE mean grade D (Plain). D (Plain) in Maths & English. January/September (Regular) May (Part time) KNEC
7.2.2 Sales and Marketing Module I & II (TIVET) KCSE mean grade D (Plain). D (Plain) in Maths & English. January/September (Regular) May (Part time) KNEC
7.2.3 Human Resource Management Module I & II (TIVET) KCSE mean grade D (Plain) January/September (Regular) May (Part time) KNEC
7.2.4 Transport Management Module I & II (TIVET) KCSE mean grade D (Plain). D (Plain) in Maths & English. January/September (Regular) May (Part time) KNEC
7.2.5 Secretarial Studies Module I & II (TIVET) KCSE mean grade D (Plain). D (Plain) in Maths & English. January/September (Regular) May (Part time) KNEC
7.2.6 Supply Chain Management Module I & II (TIVET) KCSE mean grade D (Plain). D (Plain) in Maths & English. January/September (Regular) May (Part time) KNEC
7.2.7 Business Education Single and Group Stage I, II & III (TIVET) KCSE mean grade D (Plain). D (Plain) in Maths & English. Jan/May/Sept. KNEC
8.0 Liberal Studies
8.1 Diploma Courses (9 terms with Attachment)
Code Course title Minimum requirement Admission time Exam body
8.1.1 Social Work and Community Development Module I,II & III KCSE mean grade C- (Minus). KNEC Craft Certificate for Module II. KNEC Diploma II for Diploma III January/September (regular) May intake (Part Time) (Evening & Weekends) KNEC
8.2 Craft Courses (7 terms with Attachment)
3.7 Craft Social Community Development KCSE mean grade D+ (Plus) May intake only KNEC

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