DIGESTION AND ABSORPTION OF PROTEINS

DENATURATION disruption of the three-dimensional shape of a protein, usually by heat, light, acid, or chemical reaction

The original shape of the protein is not necessarily permanent. Heat, light, change in pH, alcohol, or motion—such as beating egg whites—will change the three-dimensional structure, a process known as denaturation. Very high fevers or extreme changes in the pH of blood may be sufficient to denature proteins within our body and cause them not to function properly. For example, heat or a change in pH could destroy the function of enzymes or damage the protein hemoglobin, which carries oxygen in the blood.

It’s easy to visualize protein denaturation in the kitchen—adding lemon juice to milk renders it more acidic (changes the pH), making the milk curdle. Similarly, cooking (heat) causes meat to become firm and eggs to harden.

In our bodies, denaturation by stomach acid (hydrochloric acid) unfolds a food protein in the stomach. Unfolding allows the enzyme pepsin, produced by cells lining the stomach, to access the peptide bonds and increase the digestibility of the protein. (INFOGRAPHIC 8.5)

INFOGRAPHIC 8.5 Protein Denaturation Denaturation by stomach acid allows the enzyme pepsin to cut protein into shorter strands.
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Question 8.5

image Atrophic gastritis is a condition that results in insufficient hydrochloric acid production by the stomach. Describe why this condition would decrease the efficiency of protein digestion.

Hydrochloric acid denatures proteins, changing the shape so that the enzyme pepsin can access the peptide bonds and break the protein into fragments. Without hydrochloric acid, pepsin would not be able to begin the dismantling of protein.

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Sunnyside up. You can watch denaturation in action by observing an egg change from a gelatinous liquid to a rubbery solid when heated.
Valentina Proskurina/Shutterstock

Once the protein fragments enter the small intestine, enzymes released from the pancreas, known as proteases, break the strings of amino acids into peptides, short chains of amino acids. These peptides are digested further by enzymes on the surface of the intestinal mucosa, and dipeptides, tripeptides, and individual amino acids are then absorbed by mucosal cells of the small intestine with the assistance of various transport proteins. From the mucosal cells the amino acids are transported into the blood by additional transport proteins. (INFOGRAPHIC 8.6)

INFOGRAPHIC 8.6 Protein Digestion and Absorption The digestion of proteins begins in the stomach and is completed within the mucosal cells lining the small intestine.
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Question 8.6

image What is the largest protein fragment that can be absorbed into the mucosal cells lining the small intestine?

The largest protein fragments to be absorbed into the mucosal cells are tripeptides.