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The hexokinase, phosphofructokinase and pyruvate kinase steps of glycolysis (1,3 and 10, below) are the only ones that are irreversible, and are also the steps where glycolysis is regulated.
Is it necessary for a regulatory step in glycolysis to be irreversible, and if so does this apply to metabolic pathways generally?
As far as glycolyis is concerned, the answer is straightforward. In certain cells and tissues there is a pathway working in the opposite direction - gluconeogenesis - in which the 'irreversible' steps of glycolysis are, in fact (and of necessity), reversed by a different enzymic reaction in which the position of the equilibrium is in the opposite direction.
Obviously if it is metabolically appropriate for glycolysis to occur it is inappropriate for gluconeogenesis to occur. The only way of turning e.g. glycolysis off while at the same time turning gluconeogenesis on is by regulating the activity of the different enzymes at these three steps.
Glycolysis is, therefore a special case in sharing many reactions with another pathway working in the reverse direction. What about pathways in which the interconversions only proceed in one direction. Classic examples are biosynthetic pathways that are regulated by what is known as 'feedback' or 'end-product' inhibition. An example (for which I happen to have my own diagram) is the synthesis of isoleucine from threonine in bacteria:
When the concentration of isoleucine increases to a certain amount (sufficient for the cell's needs) this inhibits the enzyme threonine deaminase, preventing the wasteful conversion of threonine to isoleucine.
The main point is not the position of equilibrium of the threonine deaminase reaction (I haven't checked it yet) but that it is the first unique step in the synthesis pathway. Hence regulating this step prevents the unnecessary removal of threonine in a way that does not allow the wasteful accumulation of intermediates.
There is an old party game in which you pass a pair of scissor to your neighbour saying “I pass you these scissors crossed” or “I pass you these scissors uncrossed”. The initiates then tell you whether you have performed the operation correctly. The uninitiated player assumes that what is important is whether the scissors are crossed. In fact, it is whether or not your legs are crossed. Beware of false associations.
3 Regulatory Enzymes and rate limiting step of Glycolysis
Glycolysis (Glyco=Glucose lysis= splitting) is the oxidation of glucose (C 6) to 2 pyruvate (3 C) with the formation of ATP and NADH.
It is also called as the Embden-Meyerhof Pathway
Glycolysis is a universal pathway present in all organisms:
from yeast to mammals.
It is a universal anaerobic process where oxygen is not required.
In metabolic pathways, enzymes catalyzing essentially irreversible reactions are potential sites of control. In glycolysis, the reactions catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase are virtually irreversible hence, these are the regulatory enzymes in Glycolysis.
This is our 5 minute video Regulatory Enzymes and rate limiting step of Glycolysis
Regulatory Enzyme 1 : Hexokinase
Step 1 : Phosphorylation of glucose to glucose-6 phosphate (Hexokinase)
This reaction requires energy and so it is coupled to the hydrolysis of ATP to ADP and Pi.
• Enzyme: hexokinase. It has a low Km for glucose hexokinase phosphorylates glucose that enters the cell
• Irreversible step. So the phosphorylated glucose gets trapped inside thecell. Glucose transporters transport only free glucose
Hexokinase Activators:AMP/ADP (indicating low energy or ATP therefore activates hexokinase
If G6P accumulates in the cell, there is feedback inhibition of hexokinase till the G6P is consumed
Regulatory Enzyme 2 and Rate limiting step : Phosphofructokinase (PFK)
Step 3 : Phosphorylation of fructose-6- bisphosphate.(PFK)
The rate limiting step is the slowest (irreversible) step in a pathway, which determines how fast the whole pathway can be carried out.
PFK Activators: AMP/ADP, Fructose-2,6-bisphosphate
Citrate inhibits PFK by enhancing the inhibitory effect of ATP.
Fructose 2,6-bisphosphate (PFK-2) activates PFK by increasing its affinity for fructose 6-phosphate and diminishing the inhibitory effect of ATP
This reaction is unique to Glycolysis therefore rate limiting step
Regulatory Enzyme 3: pyruvate kinase.
Step 10 : Enolphosphate is a high energy bond. It is hydrolyzed to form the enolic form of pyruvate with the synthesis of ATP. Irreversible step
Enol pyruvate quickly changes to a more stable keto pyruvate.
Pyruvte kinase Activators:AMP/ADP , Fructose-1,6-bisphosphate
Inhibitors: ATP, Acetyl CoA, Alanine
If fructose 1,6 bisphosphate is formed, it acts a allosteric feedforward activator and drives the pyruvate kinase reaction forward.
Alanine, an aminoacid derived from pyruvate, is a negative
effector of catabolism.
Glycolysis - Whole process of gylcolysis
Reaction 1: Phosphorylation of glucose to glucose-6 phosphate.
This reaction requires energy and so it is coupled to the hydrolysis of ATP to ADP and Pi.
Enzyme: hexokinase. It has a low Km for glucose thus, once glucose enters the cell, it gets phosphorylated.
This step is irreversible. So the glucose gets trapped inside the cell. (Glucose transporters transport only free glucose, not phosphorylated glucose)
Reaction 2: Isomerization of glucose-6-phosphate to fructose 6- phosphate. The aldose sugar is converted into the keto isoform.
This is a reversible reaction. The fructose-6-phosphate is quickly consumed and the forward reaction is favored.
Reaction 3: is another kinase reaction. Phosphorylation of the hydroxyl group on C1 forming fructose-1,6- bisphosphate.
Enzyme: phosphofructokinase. This allosteric enzyme regulates the pace of glycolysis.
Reaction is coupled to the hydrolysis of an ATP to ADP and Pi.
This is the second irreversible reaction of the glycolytic pathway.
Reaction 4: fructose-1,6-bisphosphate is split into 2 3-carbon molecules, one aldehyde and one ketone: dihyroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP).
Reaction 5: DHAP and GAP are isomers of each other and can readily inter-convert by the action of the enzyme triose-phosphate isomerase.
GAP is a substrate for the next step in glycolysis so all of the DHAP is eventually depleted. So, 2 molecules of GAP are formed from each molecule of glucose
Step-wise reactions of glycolysis (continued)
Glycolysis: Energy balance sheet
- Hexokinase: - 1 ATP
- Phosphofructokinase: -1 ATP
- GAPDH: +2 NADH
- Phsophoglycerate kinase: +2 ATP
- Pyruvate kinase: +2 ATP
Total/ molecule of glucose: +2 ATP, +2 NADH
Fate of Pyruvate
- NADH is formed from NAD+during glycolysis.
- The redox balance of the cell has to be maintained for further cycles of glycolysis to continue.
•NAD+can be regenerated by one of the following reactions /pathways:
Pyruvate is converted to lactate
Pyruvate is converted to ethanol
In the presence of O2, NAD+is regenerated by ETC. Pyruvate is converted to acetyl CoA which enters TCA cycle and gets completely oxidized to CO 2.
F2,6BP REGULATION BY ONCOGENES AND TUMOR SUPPRESSORS AND ITS ROLE IN CANCER CELL METABOLISM
The link between glucose metabolism and cancerous transformation has long been known (Warburg 1956), although the mechanisms underlying this connection are still being explored. Various glycolytic enzymes are commonly elevated in cancer cells, including PFK-1, leading to the elevated glycolytic flux characteristic of these cells (Moreno-Sanchez et al. 2007). There is also a link between the regulation of cellular F2,6BP levels and various processes involved in cancer. The inducible PFK-2 isoform PFKFB3 is often highly expressed in human cancers of brain astrocyles, colon, prostate, breast, ovary, and thyroid when compared with adjacent normal tissues (Atsumi et al. 2002 Bando et al. 2005 Kessler et al. 2008). The tight regulation of PFKFB3, which coordinates its levels with cell cycle progression, suggests a role for F2,6BP in cell proliferation (Almeida et al. 2010 Tudzarova et al. 2011). Moreover, when PFKFB3 is silenced in HeLa cells, this prevents the sharp and short increase in protein levels in late G1, and the cells fail to progress to S phase (Tudzarova et al. 2011). It should be noted that, in these cells, elevation of PFKFB3 coincided with a rise in lactate production, supporting its contribution to glycolytic flux. Another study in HeLa cells demonstrated that cell viability and anchorage-independent cell growth are also compromised by PFKFB3 silencing (Calvo et al. 2006). Heterozygotic genomic deletion of PFKFB3 in ras-transformed mouse fibroblasts reduced the invasive capacity of these cells (Telang et al. 2006). Reciprocally, cells transformed by oncogenes such as v-src/vfps or ras showed elevated F2,6BP as well as increased glycolytic flux (Bosca et al. 1986 Kole et al. 1991), demonstrating the link between oncogenic transformation and metabolic adaptation conferred by F2,6BP. However, there is evidence that in some cells the relationship between PFKFB3 expression and intracellular F2,6BP levels is not so straightforward, with the elevated PFKFB3 expression associated with immortalization being accompanied by a decrease in intracellular F2,6BP (Telang et al. 2006). Interestingly, in this case F2,6BP levels were also not directly correlated with glycolytic flux. The authors speculate that this could be the outcome of elevated glycolysis leading to negative feedback compensation or increased use of F2,6BP as glycolytic substrate following its conversion to F6P.
Studies in different cancer cell lines have demonstrated that the increased levels of PFK-2 activity are achieved through various mechanisms, including elevated transcription, activation of the enzyme through posttranslational modification, and reduced proteosomal degradation (Okar et al. 2001 Rider et al. 2004 Bando et al. 2005 Almeida et al. 2010). For example, hypoxia-inducible factor 1 (HIF1), which is commonly stabilized in cancer cells, induces increased transcription of the PFKFB genes but to a different extent depending on the cell type (Minchenko et al. 2003). PFKFB3 phosphorylation, which enhances enzymatic activity, is increased in human tumor cells and is correlated with increased proliferation of COS7 cells in culture (Bando et al. 2005). Phosphorylation of the different isoforms of PFKFB may be achieved by several kinases, including AKT and AMP-activated protein kinase, protein kinase A, and protein kinase C, among others (Marsin et al. 2000, 2002 Rider et al. 2004 Mukhtar et al. 2008 Moon et al. 2010), allowing the regulation of glycolysis in response to various signaling pathways. Whereas several isoforms of PFK-2 are expressed in cancer cells, it is assumed that the inducible FKBFB3 has the dominant effect on cellular F2,6BP levels and glycolytic flux rate because of its high kinase activity (Telang et al. 2006 Yalcin et al. 2009b). Indeed, a small-molecule inhibitor designed to specifically target PFKFB3 reduced F2,6BP levels, glucose uptake, and tumor growth when administered to tumor-bearing mice (Clem et al. 2008). As an efficient glycolytic activator, which is tightly regulated in normal cells and highly expressed in transformed ones, PFKFB3 thus provides an attractive target to cancer therapy. Development of specific inhibitors that would directly inhibit the metabolically modified cancer cells and spare the normal cells could provide an effective method to utilize the central role of F2,6BP levels for cancer treatment.
Interestingly, another link between PFKFB3 and cell cycle regulation in transformed cells has been suggested because one of its splice variants harbors a nuclear localization signal (Yalcin et al. 2009a). This variant is the dominantly expressed splice variant in several tumor cell lines, and it is the only one that localizes to the nucleus. Both kinase activity and nuclear localization were found to be important for the induction of cell proliferation and resulted in elevated expression of cell cycle proteins. This study suggests that the effects of F2,6BP on cancer cells can be mediated at many levels, requiring correct localization as well as timing.
7.7 Regulation of Cellular Respiration
Cellular respiration must be regulated in order to provide balanced amounts of energy in the form of ATP. The cell also must generate a number of intermediate compounds that are used in the anabolism and catabolism of macromolecules. Without controls, metabolic reactions would quickly come to a stand still as the forward and backward reactions reached a state of equilibrium. Resources would be used inappropriately. A cell does not need the maximum amount of ATP that it can make all the time: At times, the cell needs to shunt some of the intermediates to pathways for amino acid, protein, glycogen, lipid, and nucleic acid production. In short, the cell needs to control its metabolism.
A variety of mechanisms is used to control cellular respiration. Some type of control exists at each stage of glucose metabolism. Access of glucose to the cell can be regulated using the GLUT proteins that transport glucose (Figure 7.18). Different forms of the GLUT protein control passage of glucose into the cells of specific tissues.
Some reactions are controlled by having two different enzymes—one each for the two directions of a reversible reaction. Reactions that are catalyzed by only one enzyme can go to equilibrium, stalling the reaction. In contrast, if two different enzymes (each specific for a given direction) are necessary for a reversible reaction, the opportunity to control the rate of the reaction increases, and equilibrium is not reached.
A number of enzymes involved in each of the pathways—in particular, the enzyme catalyzing the first committed reaction of the pathway—are controlled by attachment of a molecule to an allosteric site on the protein. The molecules most commonly used in this capacity are the nucleotides ATP, ADP, AMP, NAD + , and NADH. These regulators, allosteric effectors, may increase or decrease enzyme activity, depending on the prevailing conditions. The allosteric effector alters the steric structure of the enzyme, usually affecting the configuration of the active site. This alteration of the protein’s (the enzyme’s) structure either increases or decreases its affinity for its substrate, with the effect of increasing or decreasing the rate of the reaction. The attachment signals to the enzyme. This binding can increase or decrease the enzyme’s activity, providing feedback. This feedback type of control is effective as long as the chemical affecting it is attached to the enzyme. Once the overall concentration of the chemical decreases, it will diffuse away from the protein, and the control is relaxed.
Control of Catabolic Pathways
Enzymes, proteins, electron carriers, and pumps that play roles in glycolysis, the citric acid cycle, and the electron transport chain tend to catalyze non-reversible reactions. In other words, if the initial reaction takes place, the pathway is committed to proceeding with the remaining reactions. Whether a particular enzyme activity is released depends upon the energy needs of the cell (as reflected by the levels of ATP, ADP, and AMP).
The control of glycolysis begins with the first enzyme in the pathway, hexokinase (Figure 7.19). This enzyme catalyzes the phosphorylation of glucose, which helps to prepare the compound for cleavage in a later step. The presence of the negatively charged phosphate in the molecule also prevents the sugar from leaving the cell. When hexokinase is inhibited, glucose diffuses out of the cell and does not become a substrate for the respiration pathways in that tissue. The product of the hexokinase reaction is glucose-6-phosphate, which accumulates when a later enzyme, phosphofructokinase, is inhibited.
Phosphofructokinase is the main enzyme controlled in glycolysis. High levels of ATP, citrate, or a lower, more acidic pH decrease the enzyme’s activity. An increase in citrate concentration can occur because of a blockage in the citric acid cycle. Fermentation, with its production of organic acids like lactic acid, frequently accounts for the increased acidity in a cell however, the products of fermentation do not typically accumulate in cells.
The last step in glycolysis is catalyzed by pyruvate kinase. The pyruvate produced can proceed to be catabolized or converted into the amino acid alanine. If no more energy is needed and alanine is in adequate supply, the enzyme is inhibited. The enzyme’s activity is increased when fructose-1,6-bisphosphate levels increase. (Recall that fructose-1,6-bisphosphate is an intermediate in the first half of glycolysis.) The regulation of pyruvate kinase involves phosphorylation by a kinase (pyruvate kinase kinase), resulting in a less-active enzyme. Dephosphorylation by a phosphatase reactivates it. Pyruvate kinase is also regulated by ATP (a negative allosteric effect).
If more energy is needed, more pyruvate will be converted into acetyl CoA through the action of pyruvate dehydrogenase. If either acetyl groups or NADH accumulate, there is less need for the reaction and the rate decreases. Pyruvate dehydrogenase is also regulated by phosphorylation: A kinase phosphorylates it to form an inactive enzyme, and a phosphatase reactivates it. The kinase and the phosphatase are also regulated.
Citric Acid Cycle
The citric acid cycle is controlled through the enzymes that catalyze the reactions that make the first two molecules of NADH (Figure 7.9). These enzymes are isocitrate dehydrogenase and α-ketoglutarate dehydrogenase. When adequate ATP and NADH levels are available, the rates of these reactions decrease. When more ATP is needed, as reflected in rising ADP levels, the rate increases. α-Ketoglutarate dehydrogenase will also be affected by the levels of succinyl CoA—a subsequent intermediate in the cycle—causing a decrease in activity. A decrease in the rate of operation of the pathway at this point is not necessarily negative, as the increased levels of the α-ketoglutarate not used by the citric acid cycle can be used by the cell for amino acid (glutamate) synthesis.
Electron Transport Chain
Specific enzymes of the electron transport chain are unaffected by feedback inhibition, but the rate of electron transport through the pathway is affected by the levels of ADP and ATP. Greater ATP consumption by a cell is indicated by a buildup of ADP. As ATP usage decreases, the concentration of ADP decreases, and now, ATP begins to build up in the cell. This change is the relative concentration of ADP to ATP triggers the cell to slow down the electron transport chain.
Link to Learning
Visit this site to see an animation of the electron transport chain and ATP synthesis.
For a summary of feedback controls in cellular respiration, see Table 7.1.
Glycolysis and ATP production
In the glycolytic pathway the glucose molecule is degraded to two molecules of pyruvate.
In the first phase, the preparatory phase, two ATP are consumed per molecule of glucose in the reactions catalyzed by hexokinase and PFK-1. In the second phase, the payoff phase, 4 ATP are produced through substrate-level phosphorylation in the reactions catalyzed by phosphoglycerate kinase and pyruvate kinase. So there is a net gain of two ATP per molecule of glucose used. In addition, in the reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase, two molecules of NADH are produced for each glucose molecule.
Energy Changes of Glycolytic Reactions
The overall ΔG°’ of glycolysis is -85 kJ/mol (-20.3 kcal/mol), value resulting from the difference between the ΔG°’ of the conversion of glucose into two pyruvate molecules, -146 kJ/mol (-34,9 kcal/mol), and the ΔG°’ of the formation of ATP from ADP and Pi, 2 x 30.5 kJ/mol = 61 kJ / mol (2 x 7.3 kcal/mol = 14.6 kcal/mol). Here are the two reactions.
Glucose + 2 NAD + → 2 Pyruvate + 2 NADH + 2 H +
2 ADP + 2 Pi → 2 ATP + 2 H2O
The sum of the two reactions gives the overall equation of glycolysis.
Glucose + 2 NAD + + 2 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 2 H + + 2 ATP + 2 H20
Thus, under standard conditions, the amount of released energy stored within ATP is (61/146) x 100 = 41.8%.
Notice that the overall equation of glycolysis can also be derived by considering all the reagents, ATP, NAD + , ADP, and Pi and all the products.
Glucose + 2 ATP + 2 NAD + + 4 ADP + 2 Pi → 2 Pyruvate + 2 ADP + 2 NADH + 2 H + + 4 ATP + 2 H20
Cancelling the common terms on both sides of the equation, we obtain the overall equation shown above.
Glycolysis and ATP production under anaerobic conditions
Under anaerobic conditions, regardless of what is the metabolic fate of pyruvate, conversion to lactate, ethanol or other molecules, there is no additional production of ATP downstream of glycolysis.
Therefore under these conditions, glycolysis extracts only a small fraction of the chemical energy of the glucose molecule, energy equal to 2840 kJ/mol (679 kcal/mol) released as a result of its conversion to CO2 and H2O. Indeed, only 146 kJ/mol are released in the conversion of a glucose molecule to two pyruvate molecules, equal to 5%, [(146/2,840) x 100], of the available chemical energy. Therefore, pyruvate still contains most of the chemical energy of the hexose.
Similarly, the 4 electrons carried by NADH produced in step 6 of glycolysis cannot be used for ATP production.
In lactic acid fermentation, the ΔG°’ associated with the conversion of a glucose molecule to two molecules of lactate is -183.6 kJ/mol (-43.9 kcal/mol), and 33.2% of such free energy, [(61/183.6) x 100] is stored within ATP, whereas it is 41.8% in the conversion of a glucose molecule to two molecules of pyruvate.
It should be noted that under actual conditions the amount of free energy required for the synthesis of ATP from ADP and Pi is much higher than that required under standard conditions, namely, approximately 50% of the energy released is stored within ATP.
Glycolysis and ATP production under aerobic conditions
Under aerobic conditions, in cells with mitochondria, the amount of chemical energy that can be extracted from glucose and stored within ATP is much greater than under anaerobic conditions.
If we consider the two NADH produced during glycolysis, the flow of their 4 reducing equivalents along the mitochondrial electron transport chain allows the production of 2-3 ATP per electron pair through oxidative phosphorylation. Therefore, 6 to 8 ATP are produced when one molecule of glucose is converted into two molecules of pyruvate, 2 from glycolysis and 4-6 from oxidative phosphorylation.
Note: The amount of ATP produced from the reducing equivalents of NADH depends upon the mechanism by which they are shuttled into mitochondria.
On the other hand, if we analyze the coordinated and consecutive action of glycolysis, the pyruvate dehydrogenase complex, citric acid cycle, mitochondrial electron transport chain and oxidative phosphorylation, much more energy can be extracted from glucose and stored within ATP. In this case, according to what reported by Lehninger, 30 to 32 ATP are produced for each glucose molecule, although recent estimates suggest a net production equal to 29.85 ATP/glucose, or 29.38 ATP/glucose if also ATP formed from GTP, in turn produced by the citric acid cycle, is exported. Considering both estimates, the production of ATP is about 15 times greater than under anaerobic condition.
There are present different substrates for Gluconeogenesis such as Glycerol, lactate, and Glucogenic amino acids.
Gluconeogenesis – Glycerol |Image Source: https://biologyreader.com/gluconeogenesis.html
Glycerol is produced from the triglyceride hydrolysis in the adipose tissue. After that, it moves to the liver via the bloodstream.
Glycerol enters into the Gluconeogenesis pathway in two sequential steps:
- In the first step, Glycerol is phosphorylated to form Glycerol 3 phosphate with the help of the enzyme Glycerol kinase. In this reaction, one ATP molecule is used.
- After that, the Glycerol 3 phosphate is oxidized to form dihydroxyacetone phosphate. In this reaction, one molecule of NAD is reduced to NADH. The reaction is catalyzed by the enzyme Glycerol 3 phosphodehydrogenase.
- It also clears the metabolites which are accumulated in blood such as lactate and glycerol etc.
Gluconeogenesis – lactate |Image Source: https://biologyreader.com/gluconeogenesis.html
Lactate is produced via anaerobic glycolysis in RBC and muscles. After that, Lactate moves to the liver via the bloodstream.
In liver lactate converted into Pyruvate by the enzyme lactate dehydrogenase. Now Pyruvate enters into the Gluconeogenesis pathway and produces glucose.
Glucogenic amino acids
Glucogenic amino acids are produced by the hydrolysis of tissue proteins. Some examples of Glucogenic amino acids are Succinyl Co-A, α-ketoglutarate, fumarate, oxaloacetate, and fumarate.
Glucogenic amino acids enter into the Gluconeogenesis pathway via two entry points such as pyruvate and oxaloacetate.
Hormonal regulation action
In glycolysis and gluconeogenesis, hormones (glucagon and insulin) regulate pathways at points where different enzymes are used. As shown in figure 5, glucagon is secreted by alpha cells of the pancreas into the bloodstream when there is a decrease in blood glucose level (James, 2010), stimulating gluconeogenesis by increasing the enzyme PEPCK and inhibiting pyruvate kinase in glycolysis (enhancing release of glucose from glycogen) This hormonal regulation prevents hypoglycemia (Eric & Tony, 2009). Insulin is secreted from the beta cells of pancreas, when blood glucose levels are high (James, 2010). Therefore, gluconeogenesis is inhibited by reducing the enzyme PEP carboxykinase and glycolysis pathway is stimulated by activating pyruvate kinase (converting glucose into glycogen). This hormonal regulation prevents hyperglycemia (Eric & Tony, 2009).
Malfunction of blood glucose regulation can cause many diseases. Examples for high glucose levels: diabetes mellitus (type I insulin-dependent and type II non-insulin dependent), liver disease and hyperthyroidism. Examples of low glucose levels: hypothyroidism and hyperinsulinism. Diabetes is a common failure of metabolism regulation. However, it can lead to serious complications if not controlled (Izak, 2001).
In conclusion, it is very important for metabolic pathways to be coordinated to make sure that organism maintains life by performing efficiently and effectively. There are several different ways in which metabolic processes are regulated: different types of enzymes play a huge role in regulation of pathways, allosteric regulators either increase or decrease the rate of reactions. Feedback inhibition prevents excess product being made and committed steps in pathways, ensures that the rest of the pathway takes place. However, there are many consequences, health problems, if the organism’s regulation malfunctions. Without metabolism, organisms would not be alive.
Metabolism is many coordinated chemical reactions occurring within a cell of an organism to sustain life (Berg et al. 2006). Obtaining nutrients, generating wastes, growing, reproducing, adapting to different environments are all chemical processes that occur in a human body to maintain a living state (Deborah, 2009). Many specific enzymes catalyse different chemical reactions in a metabolic pathway. Metabolic pathways are irreversible however the reaction can be reversed by another pathway or an enzyme (Berg et al. 2006). For example, the glycolytic pathway can be reversed by gluconeogenesis.
Metabolic pathways can be separated into catabolic, anabolic and amphibolic reactions. Catabolism breaks down complex molecules like proteins and lipids into smaller and simpler molecules like amino acids and fatty acids this reaction releases chemical energy, adenosine tri-phosphate (ATP) and reduced electron carriers NADH, NADPH and FADH2 (David & Michael, 2005). These cofactors are important in metabolism as they are recycled by oxidative phosphorylation and reused by the glycolysis and the TCA cycle. An example of catabolic reaction can include oxidation of glucose during aerobic respiration glycolysis, breakdown of glucose to pyruvate acid (Joyce, 2007). Anabolic reactions require energy which is obtained from the chemical energy produced in the catabolic process. The energy is used for maintenance and growth of the cell. Small, precursor molecules like monosaccharides, amino acids and nucleotides synthesise into macromolecules like polysaccharides, proteins and nucleic acids. Example of anabolic pathway can include, gluconeogenesis, a builds up a molecule of glucose from pyruvate (David & Michael, 2005). Both catabolic and anabolic pathways together are referred to as amphibolic reaction e.g. the TCA cycle, which involves in both the breakdown and synthesis of molecules (Berg et al. 2006).
Regulation in metabolic pathways is essential to maintain a steady balance within the cell, i.e. homeostasis, controlling the flow intermediates through pathways, conserving energy, preventing excess products being made and exhaustion of substrates and/ or substrate cycles (William & Daphne, 2005). There are many ways in which metabolic regulation is carried out. A number of these processes are incorporated in metabolism.
Enzymes play a huge role in regulation of metabolic pathways. Controlling the amount of enzymes and amending the rate of synthesis coordinates the activity in the cell, increasing or decreasing the catalytic activity is stimulated by certain signals (L. Roux, 2010). Allosteric regulation is when a molecule attaches itself at a site on the enzyme other than the active site, changing enzymes activity as shown in figure 1. An allosteric regulator either increases the enzymes activity, known as allosteric activators, or either decreases the enzymes activity, called allosteric inhibitors (David & Michael, 2005). Zymogens also help regulate enzymes activity they are produced in an inactive form and when this enzyme is required it is transformed into an active form, using proteolysis for this conversion. Zymogens are found inactive in the digestive tract until they are required for digesting this prevents damage to the stomach (Berg et al. 2006). Deficiency of a proteolytic enzyme, can lead to many problems such as alkaline excess in blood which can cause anxiety (Enzyme Essentials, 2006).
Regulation of pathways by Feedback inhibition. The inhibitor is the product made from the reaction further on, in the pathway. When the product builds up, it feeds back into the process, inhibiting the enzymes activity which is involved in its synthesis. Once the product level decreases, the pathway begins again. Feedback inhibition prevents excess product being made (William & Daphne, 2005).
Committed steps are also very unique in regulating the pathways. They occur early on in the pathway, which ensures that the rest of the pathway takes place (Bryant Miles, 2003).
Regulation of metabolism can take place in different compartments in the cell, e.g. in a eukaryotic cell. Compartmentalisation helps to organise diverse or opposing metabolic pathways to take place e.g. mitochondria, in the matrix the TCA cycle takes place and in the inner membrane of the mitochondria the electron transport chain pathway occurs (Bryant Miles, 2003).
The enzyme glyceraldehyde 3-phosphate dehydrogenase (GAPDH) serves two functions in this reaction. First, it dehydrogenates GAP by transferring one of its hydrogen (H⁺) molecules to the oxidizing agent nicotinamide adenine dinucleotide (NAD⁺) to form NADH + H⁺.
Next, GAPDH adds a phosphate from the cytosol to the oxidized GAP to form 1,3-bisphosphoglycerate (BPG). Both molecules of GAP produced in the previous step undergo this process of dehydrogenation and phosphorylation.