南京农业大学生化课件氨基酸代谢5-12-1.ppt

Dietary proteins are digested into amino acids in the gastrointestinal(胃肠) tract via the action of pepsin, trypsin, chymotrypsin, carboxypeptidases and aminopeptidases. Amino acids can not be stored in animals: excess being completely oxidized to release energy or converted to storable fuels (fatty acids or carbohydrates). Serum aminotransferases have been used as clinical markers of tissue damages Damaged heart or liver cells leak aminotransferases. Blood aspartate aminotransferase and alanine aminotransferase are usually examined for indications of illness. NH4+ in hepatocytes (肝细胞) is convert ed into urea for excretion via the urea cycle in most terrestrial vertebrates Urea is formed from ammonia, CO2 (as bicarbonate) and Asp. The pathway was also discovered by Hans Krebs in 1932 (five years before he discovered the citric acid cycle). Four ATP molecules are consumed to produce each urea. The rate of urea synthesis is controlled at two levels Allosteric （别构）regulation: N-acetylglutamate, by binding to a site which hydrolyzes （水解） Gln in another isozyme, positively regulates carbamoyl phosphate synthetase I activity. Gene regulation: syntheses of the urea cycle enzymes are all increased during starvation (when energy has to be obtained from muscle proteins!) or after high protein uptake. The rates of transcription of the five genes encoding the enzymes are increased. Genetic defects of the urea cycle enzymes lead to hyperammonemia and brain damage High levels of ammonia lead to mental disorder or even coma and death. Ingenious strategies for coping with the deficiencies have been devised based on a thorough understanding of the underlying biochemistry. Strategy I: diet control, provide the essential amino acids in their a-keto acid forms. Strategy II: when argininosuccinate lyase is deficient, ingesting a surplus of Arg will help (ammonia will be carried out of the body in the form of argininosuccinate, instead of urea). Strategy III: when carbamoyl phosphate