Proteins consist of amino acids, 9 of which (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) are essential amino acids, meaning that we cannot synthesize them ourself.
Denaturation of proteins starts in the mouth. In the stomach, HCl and pepsine further break and hydrolize the proteins into big polypeptides. Proteases in the small intestine (e.g. aminopeptidase of the intestinal endothele cells) break the polypeptides further down, into amino acids and dipeptides. Proteases (dipeptidases of the intestinal endothele cells) transport amino acids and some dipeptides into the blood stream, where they’re transported to the liver through the hepatic portal vein.
Huge turn-over of proteins in the body is facilitated by proteasomes, which recognize and break down ubiquitinated proteins into peptides, which are further broken down (proteolysis) into amino acids. Some amino acids are left intact for biosynthesis. Others are skavenged: the amino group enters the urea cycle and the carbon skeletons are used for glucose/glucogen or fatty acid synthesis, if they are not exhaled as CO₂.
Amino acids, the urea cycle and gluconeogenesis
Amino acid degradation takes place mostly in the liver: alpha-amino acid → glutamate → ammonium (NH₄+) → Urea (with NH₂ groep). (Urea cycle take two NH₄+ and Fumerate [Gluconeogenesis lecture]. Fumerate is another of the amino acid breakdown products (not in the urea cycle).)
Muscles can also break down amino acids to some extend, although muscles cannot create urea from ammonium (NH₄+). To this end, NH₄+ is built into alanine, which is tranferred to the liver through the blood stream. In the liver, alanine is converted into pyruvate (which can then be further converted into glucose) and glutamate, from which the NH₄+ group is extracted for the synthesis of urea.
(g protein/kg bm)
(g protein/kg bm)
|½–1 yrs old||1.5||2.0|
|7–10 yrs old||1.0||1.8|
Excess amino acids are used as an energy source, with hightened urea excretion as a result. During pregnancy and some disease, there can be a hightened or deviant amino acid need.