Dietary proteins furnish the amino acids for synthesis of tissue proteins and for other special metabolic functions. Synthesis includes the rebuilding of tissue proteins that are being broken down continuously as well as the formation of the new proteins during growth. The process of synthesis is called anabolism; the process of breakdown, catabolism. For anabolism, the amino acids may be pictured as being activated and arranged in their proper places on the surface of an appropriate catalyst, there to be joined together through peptide linkages into a specific long polypeptide chain. This chain may then wind off the catalytic surface into a structure characteristic of the tissue protein formed by that catalyst.
Some tissue proteins are continuously being broken down and resynthesized, thereby creating a dynamic state in which nitrogen may flow from one tissue to another. This concept was revealed by the work of Whipple1 and
Allison JB. NITROGEN BALANCE AND THE NUTRITIVE VALUE OF PROTEINS. JAMA. 1957;164(3):283–289. doi:10.1001/jama.1957.62980030004015
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