Chapter 17 - Protein and Amino Acid Metabolism

Abstract

Proteins constantly undergo breakdown and synthesis. During growth, even though there is net deposition of protein, the rates of synthesis and breakdown are increased. Total protein turnover in a well-fed, adult human is estimated at about 300 g/day, of which approximately 100 g is myofibrillar protein, 30 g is digestive enzymes, 20 g is small intestinal cell protein, and 15 g is hemoglobin. The remainder is accounted for by turnover of cellular proteins of various other cells such as hepatocytes, leukocytes, platelets, and oxidation of amino acids, and a small amount is lost as free amino acids in urine. Dietary protein provides organic nitrogen and the essential amino acids. The quantitative estimation of protein requirement must take into account the quality of protein, as determined by its essential amino acid composition. If the concentration of one amino acid is significantly greater or less than that of the others, utilization of the others may be depressed and will be reflected in growth failure. Proteins of animal origin, such as meats, eggs, milk, cheese, poultry, and fish are of good quality since they provide all of the essential amino acids. Plant proteins are generally of poor quality as they lack one or more essential amino acids. The best quality plant proteins are found in legumes and nuts. Intracellular metabolism of amino acids requires their transport across the cell membrane. Transport of L-amino acids occurs against a concentration gradient and is an active process usually coupled to Na + -dependent carrier systems as for transport of glucose across the intestinal mucosa.