MCAT Biochemistry Review

Chapter 11: Lipid and Amino Acid Metabolism

11.7 Protein Catabolism

Protein is very rarely used as an energy source because it is so important for other functions; routinely breaking down protein would result in serious illness. However, under conditions of extreme energy deprivation, proteins can be used for energy. In order to provide a reservoir of amino acids for protein building by the cell, proteins must be digested and absorbed.


Metabolism is directed toward conserving tissues to the greatest extent possible, especially the brain and heart. Digestion of protein compromises muscle—potentially that of the heart—so it is unlikely to occur under normal conditions.

Digestion of protein begins in the stomach with pepsin and continues with the pancreatic proteases trypsinchymotrypsin, and carboxypeptidases A and B, all of which are secreted as zymogens. Protein digestion is completed by the small intestinal brush-border enzymes dipeptidase andaminopeptidase. The main end products of protein digestion are amino acids, dipeptides, and tripeptides. Absorption of amino acids and small peptides through the luminal membrane is accomplished by secondary active transport linked to sodium. At the basal membrane, simple and facilitated diffusion transports amino acids into the bloodstream. Figure 11.13 illustrates the major transport mechanisms involved in moving amino acids across the luminal and basal membranes of intestinal cells.

Figure 11.13. Absorption of Amino Acids and Peptides in the Intestine

Protein obtained from the diet or from the body (during prolonged fasting or starvation) may be used as an energy source. Body protein is catabolized primarily in muscle and liver. Amino acids released from proteins usually lose their amino group through transamination or deamination. The remaining carbon skeleton can be used for energy. Amino acids are classified by their ability to turn into specific metabolic intermediates: glucogenic amino acids (all but leucine and lysine) can be converted into glucose through gluconeogenesis; ketogenic amino acids (leucine and lysine, as well as isoleucine, phenylalanine, threonine, tryptophan, and tyrosine, which are also glucogenic as well) can be converted into acetyl-CoA and ketone bodies.

The amino groups removed by transamination or deamination constitute a potential toxin to the body in the form of ammonia, and must be excreted safely. The urea cycle, shown in Figure 11.14, occurs in the liver and is the body's primary way of removing excess nitrogen from the body. The MCAT is highly unlikely to test on the steps and intermediates of the urea cycle directly, but it is provided here as a point of reference.

Figure 11.14. The Urea Cycle

The fate of the side chain from each amino acid depends on its chemistry. Basic amino acid side chains feed into the urea cycle, while the other side chains act like the carbon skeleton and produce energy through gluconeogenesis or ketone production.

MCAT Concept Check 11.7:

Before you move on, assess your understanding of the material with these questions.

1.    True or False: Bodily proteins will commonly be broken down to provide acetyl-CoA for lipid synthesis.

2.    Where does the bulk of protein digestion occur?

3.    During protein processing, what is the eventual fate of each of the following components: carbon skeleton, amino group, and side chains?

·        Carbon skeleton:

·        Amino group:

·        Side chains: