Proteins Are Degraded to Amino Acids

II. Transducing and Storing Energy
23. Protein Turnover and Amino Acid Catabolism
23.1. Proteins Are Degraded to Amino Acids
Dietary protein is a vital source of amino acids. Proteins ingested in the diet are digested into amino acids or small
peptides that can be absorbed by the intestine and transported in the blood. Another source of amino acids is the
degradation of defective or unneeded cellular proteins.
23.1.1. The Digestion and Absorption of Dietary Proteins
Protein digestion begins in the stomach, where the acidic environment favors protein denaturation. Denatured proteins
are more accessible as substrates for proteolysis than are native proteins. The primary proteolytic enzyme of the stomach
is pepsin, a nonspecific protease that, remarkably, is maximally active at pH 2. Thus, pepsin can be active in the highly
acidic environment of the stomach, even though other proteins undergo denaturation there.
Protein degradation continues in the lumen of the intestine owing to the activity of proteolytic enzymes secreted by the
pancreas. These proteins, introduced in Chapters 9 and 10 (Sections 9.1 and 10.5), are secreted as inactive zymogens and
then converted into active enzymes. The battery of enzymes displays a wide array of specificity, and so the substrates are
degraded into free amino acids as well as di- and tripeptides. Digestion is further enhanced by proteases, such as
aminopeptidase N, that are located in the plasma membrane of the intestinal cells. Aminopeptidases digest proteins from
the amino-terminal end. Single amino acids, as well as di- and tripeptides, are transported into the intestinal cells from
the lumen and subsequently released into the blood for absorption by other tissues (Figure 23.1).
23.1.2. Cellular Proteins Are Degraded at Different Rates
Protein turnover the degradation and resynthesis of proteins takes place constantly in cells. Although some proteins
are very stable, many proteins are short lived, particularly those that are important in metabolic regulation. Altering the
amounts of these proteins can rapidly change metabolic patterns. In addition, cells have mechanisms for detecting and
removing damaged proteins. A significant proportion of newly synthesized protein molecules are defective because of
errors in translation. Even proteins that are normal when first synthesized may undergo oxidative damage or be altered in
other ways with the passage of time.
The half-lives of proteins range over several orders of magnitude (Table 23.1). Ornithine decarboxylase, at
approximately 11 minutes, has one of the shortest half-lives of any mammalian protein. This enzyme participates in the
synthesis of polyamines, which are cellular cations essential for growth and differentiation. The life of hemoglobin, on
the other hand, is limited only by the life of the red blood cell, and the lens protein, crystallin, by the life of the organism.
II. Transducing and Storing Energy
23. Protein Turnover and Amino Acid Catabolism
23.1. Proteins Are Degraded to Amino Acids
Figure 23.1. Digestion and Absorption of Proteins. Protein digestion is primarily a result of the activity of enzymes
secreted by the pancreas. Aminopeptidases associated with the intestinal epithelium further digest proteins. The amino