Does pepsin digest plant protein?

Does pepsin digest plant protein?

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This may sound trivial, but…

Protein is sourced from plants and animals. Pepsin and HCl digest meat (animal protein). Does pepsin also digest plant-based proteins?

I took a look at few articles online and this. None seems to explain what I am after.

Yes, pepsin does digest plant proteins also. Pepsin is a protein and its job is to cut other proteins (also known as polypeptides) into smaller pieces. Proteins are made of amino acids and there are 20 amino acids found in proteins. Pepsin cuts the bond between two amino acids to break the protein into smaller pieces. When pepsin encounters a tryptophan (Trp), tyrosine (Tyr), and/or phenylalanine (Phe), it cuts the bond between that amino acid and the one next one, as shown below.

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Proteins from plants contain these amino acids so pepsin can cleave plant proteins.


Pepsin is one example of a group of enzymes termed "acid proteases." In the case of pepsin, this name is doubly appropriate. Pepsin works its best in strong hydrochloric acid. But the similarity with the other enzymes pictured here refers to a second type of acid. The active site of the acid proteases rely on two acidic aspartate amino acids, which activate a water molecule and use it to cleave protein chains. These aspartates are pictured on the next page.

The acid proteases have evolved to fill several functional roles in different organisms. Pepsin, shown at upper left (PDB entry 5pep ), is optimized for digestion of food in the acidic environment of the stomach. It is very promiscuous, cleaving proteins in many different places. Chymosin, shown at upper right (PDB entry 4cms ), is made by young calves to break down milk proteins. A purified form of chymosin, taken from calf stomach, has been used for centuries to curdle milk in the production of cheese. Cathepsin D, shown at lower left (PDB entry 1lyb ), digests proteins inside lysozomes, the tiny stomachs inside cells. Other cellular acid proteases, such as renin (not shown, PDB entry 1hrn ), are designed to make very specific cuts in one particular protein, aiding in the maturation of a hormone or structural protein. Endothiapepsin, shown at lower right (PDB entry 4ape ), is made by a fungus and excreted into the surrounding environment, breaking up the surrounding proteins and allowing the fungus to feed on the pieces.

Exploring the Structure

Pepsin and Pepstatin

Pepsin (PDB entry 1pso) uses a pair of aspartate residues to perform the protein cleavage reaction, shown here with red oxygen atoms. In an example of parallel evolution (where two organisms independently develop the same method for solving a problem), the mechanism is similar to that used by HIV protease. This structure also includes a non-cleavable inhibitor in the active site (shown in orange). It contains a hydroxyl group, shown here in purple, that mimics the place that water attaches during the cleavage reaction. Three crosslinks that strengthen the folded protein are also shown, formed between sulfur atoms (yellow) in cysteine. To explore this structure in more detail, click on the image for an interactive JSmol.

Related PDB-101 Resources


  1. 1pso: Fujinaga, M., Chernaia, M.M., Tarasova, N.I., Mosimann, S.C., James, M.N. (1995) Crystal structure of human pepsin and its complex with pepstatin. Protein Sci. 4: 960-972
  2. 1hrn: Tong, L., Pav, S., Lamarre, D., Pilote, L., LaPlante, S., Anderson, P.C., Jung, G. (1995) High resolution crystal structures of recombinant human renin in complex with polyhydroxymonoamide inhibitors. J.Mol.Biol. 250: 211-222
  3. 1lyb: Baldwin, E.T., Bhat, T.N., Gulnik, S., Hosur, M.V., Sowder 2nd., R.C., Cachau, R.E., Collins, J., Silva, A.M., Erickson, J.W. (1993) Crystal structures of native and inhibited forms of human cathepsin D: implications for lysosomal targeting and drug design. Proc.Natl.Acad.Sci.USA 90: 6796-6800
  4. 3psg: Hartsuck, J.A., Koelsch, G., Remington, S.J. (1992) The high-resolution crystal structure of porcine pepsinogen. Proteins 13: 1-25
  5. 4cms: Newman, M., Safro, M., Frazao, C., Khan, G., Zdanov, A., Tickle, I.J., Blundell, T.L., Andreeva, N. (1991) X-ray analyses of aspartic proteinases. IV. Structure and refinement at 2.2 A resolution of bovine chymosin. J.Mol.Biol. 221: 1295-1309
  6. 5pep: Cooper, J.B., Khan, G., Taylor, G., Tickle, I.J., Blundell, T.L. (1990) X-ray analyses of aspartic proteinases. II. Three-dimensional structure of the hexagonal crystal form of porcine pepsin at 2.3 A resolution. J.Mol.Biol. 214: 199-222
  7. 4ape: Pearl, L., Blundell, T. (1984) The active site of aspartic proteinases. FEBS Lett. 174: 96-101

December 2000, David Goodsell

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Pepsin, the powerful enzyme in gastric juice that digests proteins such as those in meat, eggs, seeds, or dairy products. Pepsin is the mature active form of the zymogen (inactive protein) pepsinogen.

Pepsin was first recognized in 1836 by the German physiologist Theodor Schwann. In 1929 its crystallization and protein nature were reported by American biochemist John Howard Northrop of the Rockefeller Institute for Medical Research. (Northrop later received a share of the 1946 Nobel Prize for Chemistry for his work in successfully purifying and crystallizing enzymes.)

Glands in the mucous-membrane lining of the stomach make and store pepsinogen. Impulses from the vagus nerve and the hormonal secretions of gastrin and secretin stimulate the release of pepsinogen into the stomach, where it is mixed with hydrochloric acid and rapidly converted to the active enzyme pepsin. The digestive power of pepsin is greatest at the acidity of normal gastric juice (pH 1.5–2.5). In the intestine the gastric acids are neutralized (pH 7), and pepsin is no longer effective.

In the digestive tract pepsin effects only partial degradation of proteins into smaller units called peptides, which then either are absorbed from the intestine into the bloodstream or are broken down further by pancreatic enzymes.

Small amounts of pepsin pass from the stomach into the bloodstream, where it breaks down some of the larger, or still partially undigested, fragments of protein that may have been absorbed by the small intestine.

Chronic backflow of pepsin, acid, and other substances from the stomach into the esophagus forms the basis for reflux conditions, particularly gastroesophageal reflux disease and laryngopharyngeal reflux (or extraesophageal reflux). In the latter, pepsin and acid travel all the way up to the larynx, where they can cause damage to the laryngeal mucosa and produce symptoms ranging from hoarseness and chronic cough to laryngospasm (involuntary contraction of the vocal cords) and laryngeal cancer.

Pepsin is prepared commercially from swine stomachs. Crude pepsin is used in the leather industry to remove hair and residual tissue from animal hides prior to their being tanned. It is also used in the recovery of silver from discarded photographic films by digesting the gelatin layer that holds the silver compound.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Kara Rogers, Senior Editor.

Breakdown the Fat

The same portion of grilled tenderloin also contains 8.3 grams of fat. Fat digestion doesn't begin until the food reaches the small intestine. Here, bile salts emulsify the fat molecules so that pancreatic enzymes called lipases can more easily break the fats down into free fatty acids and monoglycerides.

These then pass into the epithelial cells that line the intestines, where they're converted into triglycerides. Bundled with cholesterol and covered with protein, the triglycerides are ready for absorption into the bloodstream for transportation to where the body needs them.

Related Testing

As mentioned earlier, the stomach protects itself from the digestive properties of pepsin by creating an adherent layer of bicarbonate-rich mucus lining. As such, pepsin should always remain in the stomach and should never regurgitate back to the upper tracts. As long as the lower esophageal sphincter functions accurately, pepsin resides withing the stomach and the duodenum, and the esophagus lining remains intact. However, a weak esophageal sphincter allows pepsin to reach not only the esophagus but also the upper airways. Gastroesophageal reflux disease (GERD) and laryngopharyngeal reflux (LPR) are two disease processes characterized by weak esophageal sphincters.[2]

The salivary pepsin test is a non-invasive, low-cost test that can detect the presence of pepsin in saliva, as the name implies. It has shown some promise as a useful਍iagnostic tool for LPR. However, further research should assess the sensitivity, specificity, and clinical utility of the test. On the contrary, the results are not that encouraging with GERD, and the test is no longer as helpful as previously thought.[4][5]

Dietary vitamin B-12 is bound to a protein. In order for the vitamin to be absorbed, the protein must be stripped away. Gastric juices and pepsin are part of the cleavage process. Pepsin also cleaves nonheme iron -- which is found in plant foods such as cereals, fruits and vegetables -- from a protein to facilitate absorption.

Carolyn Robbins began writing in 2006. Her work appears on various websites and covers various topics including neuroscience, physiology, nutrition and fitness. Robbins graduated with a bachelor of science degree in biology and theology from Saint Vincent College.

Where is pepsin produced?

The Chief cells secrete pepsin in the form of an inactive enzyme precursor called pepsinogen. When pepsinogen contacts hydrochloric acid from the parietal cells, it breaks down rapidly, forming pepsin 15) . Pepsin begins the digestion of nearly all types of dietary protein into polypeptides. This enzyme is most active in an acidic environment, which is provided by the hydrochloric acid in gastric juice.

Figure 2. Stomach cells (gastric glands)

Footnote: Lining of the stomach. Gastric glands include mucous cells, parietal cells, and chief cells. The mucosa of the stomach is studded with gastric pits that are the openings of the gastric glands.

Protein Digestion

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Investigate Digestion of Protein by Pepsin
This video depicts a lab investigation that shows how proteins can be broken apart into smaller molecules using enzymes.
1. Label three test tubes A, B and C.
2. Cut some egg white into cubes.
3. Put three egg cubes into each teat tube.
4. Add 10 ml pepsin (an enzyme) to test tube A.
5. Add 10ml hydrochloric acid to test tube B (Caution: use safety goggles and gloves).
6. Ad 5ml of acid and 5 ml of pepsin to test tube C.
7. Cover each test tube and leave for a week.
8. Compare the egg cubes in the the three test tubes.

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Ask an Expert: effects of pepsin on milk

Hi Science Buddies!
I'm doing a science project on milk and pepsin this year, and I don't know how I'm supposed to replicate digestion! I have to do this in order to demonstrate how pepsin breaks down the protein in milk, and which kinds of milk have their protein broken down the best for digestion. I want to replicate it up until the point after protein is broken down by pepsin. I don't know how to replicate everything! I've done some research, but I still don't know how I'm supposed to do it. I think I could demonstrate the mixing of the milk and saliva in the mouth by mixing milk and water ("saliva") in a blender, but I don't know what to do after that. HELP! I DON'T KNOW WHAT TO DO! I'M DESPERATE! I'M SORRY I SOUND SO FRANTIC, BUT I'M PARANOID! PLEASE HELP! THANK YOU!
yours in paranoia,
Kriti Gaur

P.S. I also wanted to know which forms I could test this in, because I'm using Vitamin D milk, 2% reduced fat milk, whole milk, chocolate milk,and soy milk in my experiment. I wanted to do my experiment so that the milk (IV) would be in its liquid state. Is that possible? Because if there are suggestions with dry milk, that's OK too, I guess.

Re: effects of pepsin on milk

Post by Walker » Sun Oct 03, 2010 10:36 pm

Since you want to learn about how pepsin effects milk proteins in normal digestion, you'll want to develop an experimental protocol that replicates the normal "working environment" of pepsin pretty well. Pepsin is produced in the stomach, so you should think about what it's like in there. By the time food in general gets to the stomach, it is well mixed (so your blender idea sounds like a good one), acidic (the stomach has special cells that produce natural acids to keep the pH very low) and warm (near body temperature of about 37 degrees Celsius), so it's reasonable to predict that pepsin will work best when it's added to a solution that's well-mixed, acidic, and warm (these are predictions that you could easily test if you want!)

If you add pepsin to a warm, acidic, well-mixed solution of milk proteins (nonfat dry milk, casein, or whey protein could be good choices for milk protein sources), you can expect the pepsin to start chopping up the proteins into smaller pieces. An important decision you'll need to make is how you are going to measure this activity of the pepsin. A fairly simple way, if you have access to electrophoresis equipment, would be to separate the milk proteins by size by using electricity to push them through a polyacrylamide gel. You could stain the proteins in the gel with a dye like Coomassie Blue, so then you'd actually be able to see the protein bands in the gel shift down to smaller sizes as the pepsin works. There are also biochemical tests that you could use to measure the activity of the pepsin, for example by tracking the change in spectrophotometric absorbance of a protein solution over time, or performing a biochemical test that produces a color change in the test tube based on the presence of more small peptides in the digested samples. If you post back with some information about the equipment you have to work with, the experts here can help you more with suggestions about what a good assay may be for you to use.

Once you have a good pepsin activity assay working, you'll need to decide what question you're most interested in asking about the effect of pepsin on the milk proteins. For example, you could find out how well pepsin works on different types of milk proteins, such as casein vs. whey proteins (they're both available in purified forms at nutritional supplement stores), or you could study the effectiveness of pepsin in different environments, such as at different pH values or temperatures, or you could find out whether the pepsin digestibility of milk is different when the milk is organic, or lactose-reduced, or ultra-pasteurized. your curiosity is the limit!


In decision, the survey carried out was equal for the information required and indicated that temperature decidedly affects the rate at which an enzyme reacts. Equally long as the method is executed good this is a great experiment to look at temperature and its consequence on enzyme activity nevertheless as with any scientific survey human input is a important factor and could impact the quality of consequences. Another experiment may necessitate to be carried out to find what the optimal temperature is on a more specific graduated table, something closer to organic structure temperature would assist to detect a more precise optimal temperature, 35 & A deg c – 40 & A deg degree Celsius for illustration. Equally good as happening out an exact optimal temperature a farther survey to happen out the optimal pH of pepsin could be done to further heighten the enzymes rate of reaction, focused around the acidic pH in the human tummy.

Watch the video: pepsin digestion of protein experiment Trimed Video (May 2022).


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