1.7 Protein digestion and absorption
The process of digestion is defined as the ‘process by which macromolecules in food are broken down into their component small-molecule subunits’.
In the case of proteins, what are the ‘macromolecules’ and what are the ‘small-molecule subunits'? What type of bond has to be broken to separate the subunits?
The macromolecules are, of course, the proteins or polypeptides themselves, and the subunits are the amino acids. The bonds holding the subunits together are peptide bonds (see Section 6.1).
This breakdown would happen impossibly slowly without the involvement of digestive enzymes, which are themselves proteins. Enzymes are often named by adding the ending ‘-ase’ to the name of the substance on which they work. So, the enzymes that break down peptide bonds are called peptidases (protein-digesting enzymes). Although all amino acids are joined by the same peptide bond, the type of R group on the amino acids on either side of the bond affects the action of the peptidases so much that several different enzymes are usually needed to digest a protein molecule completely.
Protein digestion starts in the stomach, the walls of which secrete hydrochloric acid.
What effect would you expect hydrochloric acid to have on proteins? (Hint: recall the effect of adding vinegar to the water in which eggs are being boiled.)
Acids cause proteins to coagulate, by affecting the bonds holding globular proteins into their normal shape.
An enzyme, called pepsin, produced by cells lining the wall of the stomach, starts to attack some of the peptide bonds and splits the long protein chains into shorter polypeptides. Then more peptidases are released from the pancreas into the small intestine, where they split the polypeptide chains into even smaller lengths and begin to remove individual amino acids from the ends of the chains. Digestion of virtually all the protein in the food into individual amino acids is completed by more peptidases released directly from the cells lining the small intestine. The amino acids are then transported across the wall of the small intestine into the bloodstream. The blood carries them to all the cells of the body, where they can be absorbed and used by each type of cell to make its own particular types of protein by linking them together again, in the order determined by the DNA in the chromosomes.
You should now try to summarise this information about protein digestion and absorption in the form of a diagram or flow chart, or a mix of both. The process of trying out various kinds of diagrams, and discovering which ways are more successful and which less so, is as valuable in terms of clarifying your understanding of the topic as the end-product will be. So, take a sheet of paper, and try to summarise the information on protein digestion in a diagrammatic way. Then compare your version, or versions, with those at the end of this study period. Make a note of how you think you could improve on your version if you were asked to do a similar exercise again.
Figure 10 shows two possible ways in which the information on protein digestion can be summarised, though your version could be just as good, or even better, and be quite different from either of these.
Protein digestion is normally very efficient and virtually all of the protein (98%) that is eaten by an adult is fully digested to amino acids and absorbed. However, the digestive systems of newborn babies are somewhat less efficient, which has important functional implications. Mothers' milk contains antibodies, which are a type of protein important in providing immunity against infections. If the infant digestive system were as efficient as that of an adult, these antibodies would be digested like any other protein. However, a slightly less efficient digestive system allows the antibodies to remain intact and to pass from the gut into the bloodstream, so providing essential immunity for the new baby. However, other proteins, such as those in cows' milk could also pass through to the baby's bloodstream undigested and can be the source of some allergies or food intolerances later in life. Artificial (formula) milks for babies are designed to minimise this risk, but this is an important reason in favour of breastfeeding where possible.