文档介绍：Chapter 18 Amino Acid Oxidation and the Production of Urea
1. The surplus amino acids in animals can pletely oxidized or converted to other storable fuels
Amino acids in excess (from diet, protein turnover) can neither be stored, nor excreted, but oxidized to release energy or converted to fatty acids or glucose.
Animals also utilize amino acid for energy generation during starvation or in diabetes mellitus.
anisms can also use amino acids as an energy source when the supply is in excess.
Plants almost never use amino acids as an energy source (neither fatty acids).
2. Dietary proteins are digested into amino acids in the gastrointestinal tract
Pepsin cleaves polypeptides into smaller peptides in stomach (N-terminal side of Y, F, W residues).
Trypsin (C-terminal side of K, R) and chymotrypsin (C-terminal side of F, W, and Y) further cleave the peptides in small intestine.
Carboxypeptidase and aminopeptidase cleave the small peptides into amino acids, which are then absorbed and eventually delivered to liver.
Pepsin
Chymotrypsin,trypsin,
and exopeptidases
Amino acids
3. The amino groups and carbon skeletons of amino acids take separate but interconnected pathways
The amino group is reused or excreted, as ammonia, urea (via the urea cycle) or uric acid.
The carbon skeletons (a-keto acids) generally find their way to the citric acid cycle for further oxidation or conversion.
The degradation of the carbon skeletons may be plicated but similar to that of fatty acids in some cases.
4. Liver is the major site of amino acid degradation in vertebrates
a-ketoglutarate (from amino acids), glutamate (from free ammonia), pyruvate (from muscle amino acids) collects amino groups (in forms of Glu, Gln and Ala) to liver mitochondria for further processing.
The excess NH4+ is excreted directly in bony fishes, as urea in most terrestrial vertebrates, or as uric acid in birds and terrestrial reptiles.
The carbons in both urea and uric acid are highly oxidized (with most of