Metabolism of Proteins

Metabolism of Proteins
Since proteins are composed of amino acids, of which 20 kinds commonly occur, the central topic of our consideration is amino acid metabolism. Amino acid metabolism is complex. For one thing, each of the 20 amino acids requires a separate pathway for biosynthesis and degradation. For another, amino acids are precursors to tissue proteins, enzymes, nucleic acids, and other nitrogenous constituents that form the fabric of cells. The central purpose of carbohydrate and fat oxidation is to provide energy needed to construct and maintain these vital macromolecules.

Let us begin with the amino acid pool in blood and extracellular fluid from which the tissues draw their requirements. When animals eat proteins, most are digested in the gut, releasing the constituent amino acids, which are then absorbed (Figure 4-18). Tissue proteins also are hydrolyzed during normal growth, repair, and tissue restructuring; their amino acids join those derived from protein foodstuffs to enter the amino acid pool. A portion of the amino acid pool is used to rebuild tissue proteins, but most animals ingest a surplus of protein. Since amino acids are not excreted as such in any significant amounts, they must be disposed of in some other way. In fact, amino acids can be and are metabolized through oxidative pathways to yield high-energy phosphate. In short, excess proteins serve as fuel as do carbohydrates and fats. Their importance as fuel obviously depends on the nature of the diet. In carnivores that ingest a diet of almost pure protein and fat, nearly half of their high-energy phosphate is derived from amino acid oxidation.

Before an amino acid molecule may enter the fuel depot, nitrogen must be removed by deamination (the amino group splits to form ammonia and a keto acid) or by transamination (the amino group is transferred to a keto acid to yield a new amino acid). Thus amino acid degradation yields two main products, carbon skeletons and ammonia, which are handled in different ways. Once nitrogen atoms are removed, the carbon skeletons of amino acids can be completely oxidized, usually by way of pyruvic acid or acetic acid. These residues then enter regular routes used by carbohydrate and fat metabolism.

The other product of amino acid degradation is ammonia. Ammonia is highly toxic because it reacts with - ketoglutaric acid to form glutamic acid (an amino acid). Any accumulation of ammonia effectively removes - ketoglutarate from the Krebs cycle (see Figure 4-13) and inhibits respiration. Disposal of ammonia offers little problem to aquatic animals because it is soluble and readily diffuses into the surrounding medium through respiratory surfaces. Terrestrial forms cannot get rid of ammonia so conveniently and must detoxify it by converting it to a relatively nontoxic compound. The two principal compounds formed are urea and uric acid, although a variety of other detoxified forms of ammonia are excreted by different invertebrate and vertebrate groups. Among vertebrates, amphibians and especially mammals produce urea. Reptiles and birds, as well as many terrestrial invertebrates, produce uric acid.

The key feature that seems to determine choice of nitrogenous waste is availability of water in the environment. When water is abundant, the chief nitrogenous waste is ammonia. When water is restricted, it is urea. And for animals living in truly arid habitats, it is uric acid. Uric acid is highly insoluble and easily precipitates from solution, allowing its removal in solid form. The embryos of birds and reptiles benefit greatly from excretion of nitrogenous waste as uric acid, because the waste cannot be eliminated through their shells. During embryonic development, harmless, solid uric acid is retained in one of the extraembryonic membranes. When a hatchling emerges into its new world, accumulated uric acid, along with the shell and membranes that supported development, is discarded.