Protein digestion
Protein metabolism denotes the various biochemical processes responsible for the synthesis of proteins and amino acids, and the breakdown of proteins (and other large molecules, too) by catabolism.Dietary proteins are first broken down to individual amino acids by various enzymes and hydrochloric acid present in the gastro-intestinal tract.[1] These amino acids are further broken down to α-keto acids which can be recycled in the body for generation of energy, and production of glucose or fat or other amino acids. This break-down of amino acids to α-keto acids occurs in the liver by a process known as transamination
Protein digestion occurs in the stomach and duodenum in which 3 main enzymes, pepsin secreted by the stomach and trypsin and chymotrypsin secreted by the pancreas, break down food proteins into polypeptides that are then broken down by various exopeptidases and dipeptidases into amino acids. The digestive enzymes however are mostly secreted as their inactive precursors, the zymogens. For example, trypsin is secreted by pancreas in the form of trypsinogen, which is activated in the duodenum by enterokinase to form trypsin. Trypsin then cleaves proteins to smaller polypeptides.
Protein metabolism is dependent on a vast number of endogenous mediators. These mediators define the balance between anabolic and catabolic processes. Insulin is the major anabolic hormone and also has an important role in amino acid and protein homeostasis. During injury and stress two major alterations in insulin are noted: a catecholamine-mediated suppression of insulin release and an insulin resistance, leading to a release of skeletal muscle amino acid for gluconeogenesis and, at the same time, a decreased utilisation of glucose by insulin-dependent tissues. This mechanism provides glucose to the insulin-dependent tissues that are important for survival and