南京农业大学：《生物化学 Biochemistry》课程教学资源（PPT课件）第十二章 氨基酸代谢

第12章氨基酸代谢

第12章 氨基酸代谢

第一节 The nitrogen cycle Nitrogen exists predominantly in an oxidized state in the environment, occurring principally as n2 in the atmosphere or as nitrate ion (NO3) in the soils and oceans. Its acquisition by biological systems is accompanied by its reduction to ammonium ion (NH4) and the incorporation of nh4 into organic lin kage as amino group The reduction of NO3to NH4 occurs in green plants, various fungi, and certain bacteria in a two-step meta bolic pathway known as nitrate assimilation

第一节 The nitrogen cycle Nitrogen exists predominantly in an oxidized state in the environment, occurring principally as N2 in the atmosphere or as nitrate ion (NO3 - ) in the soils and oceans. Its acquisition by biological systems is accompanied by its reduction to ammonium ion (NH4 + ) and the incorporation of NH4 + into organic linkage as amino group. The reduction of NO3 - to NH4 + occurs in green plants, various fungi, and certain bacteria in a two-step metabolic pathway known as nitrate assimilation

第一节 The nitrogen cycle The formation of nH4 from gas is termed nitrogen fixation. fixation is an exclusively prokaryotic process. No animals are capable of either nitrogen fixation or nitrate assimilation Animals release excess nitrogen in a reduced form. either as nhat or as organic nitrogenous compounds such as urea. The release of n occurs both during life and as a consequence of microbial decomposition following death

第一节 The nitrogen cycle The formation of NH4 + from N2 gas is termed nitrogen fixation. N2 fixation is an exclusively prokaryotic process. No animals are capable of either nitrogen fixation or nitrate assimilation. Animals release excess nitrogen in a reduced form, either as NH4 + or as organic nitrogenous compounds such as urea. The release of N occurs both during life and as a consequence of microbial decomposition following death

第一节 The nitrogen cycle Various bacteria return the reduced forms of nitrogen back to the environment by oxidizing them. The oxidation of nh4 to NO3 is performed by nitrifying bacteria. Nitrate nitrogen also returns to the atmosphere as n2 as result of the metabolic activity of denitrifying bacteria

第一节 The nitrogen cycle Various bacteria return the reduced forms of nitrogen back to the environment by oxidizing them. The oxidation of NH4 + to NO3 - is performed by nitrifying bacteria. Nitrate nitrogen also returns to the atmosphere as N2 as result of the metabolic activity of denitrifying bacteria

第一节 The nitrogen cycle The Nitrogen Cycle: Aerobic te N assimil tuIo n Nitrification NO NH rganic Nitrate respiration (dissimilation) NO no nitrification NoO Anaerobic

第一节 The nitrogen cycle

Sources of amino acids for animals Dietary proteins are digested into amino acids in the gastrointestinal〔胃 i)tract via the action of pepsin, trypsin, chymotrypsin, carboxypeptidases and aminopeptidases

Dietary proteins are digested into amino acids in the gastrointestinal(胃 肠) tract via the action of pepsin, trypsin, chymotrypsin, carboxypeptidases and aminopeptidases. Sources of amino acids for animals

Pepsin: the first enzyme Gastric glands in discovered(18th centur) stomach linin Parietal cells (secrete HCl Chief cells (secrete pepsinogen) Stomach Gastric mucosa Pancreas pepsinogen (secretes gastrin) pepsI Pancreatic duet Exocrine cells of Da互 creas Rough ER pH Zymogen Zymogens granule active proteases Collecting duct Villi of small intestine Small o Villus Intestine. and amin Intestinal mucosa Degradation absorption (absorbs amino acids) of dietary proteins Essential amino acids

Proteins (but not pepsin) unfolded Absorbed as tri- & dipeptides, and amino acids Degradation & absorption of dietary proteins Pepsin: the first enzyme discovered (18th century). proteases Essential amino acids

Overall fate of excess amino acids Amino acids can not be stored in animals: excess being completely oxidized to release energy or converted to storable fuels(fatty acids or carbohydrates)

Amino acids can not be stored in animals: excess being completely oxidized to release energy or converted to storable fuels (fatty acids or carbohydrates). Overall fate of excess amino acids

氨的去路 第二节 Amino acid degradation 1.氧化脱氨基 氨基酸在酶的作用下脱去氨基生成相应酮酸的过 程,叫氧化脱氨基作用。 RdH分 RED R<工 A KMS M L

第二节 Amino acid degradation 1. 氧化脱氨基 氨基酸在酶的作用下脱去氨基生成相应酮酸的过 程，叫氧化脱氨基作用。 一. 氨的去路 RCHCOO —— | - NH3 + RCCOO —— - NH3 + | RCCOO+ H —— | - + O | FAD （ ）FMN FADH FMNH2 （ ）2 HO2 NH3 HO22 O2 氨基酸氧化酶

氨的去路 2.脱氢酶作用GDH / IIa-KG+ NH4+ NADH(P)+h+

Glu + NAD(P) + H2O → a-KG + NH4 + + NADH(P) + H+ 2. 脱氢酶作用-GDH 一. 氨的去路 NH3 + | HCCOO —— - | CH2 | CH2 | COO- + NADP（）+ + HO2 NH+ 4 + + NADPH （）+ H + O | CCOO—- | CH2 | CH2 | COO- L- 谷氨酸 -α酮戊二酸 谷氨酸脱氢酶