南京农业大学生物化学课件29

第一节 The nitrogen cycle
Sources of amino acids for animals
Dietary proteins are digested into amino acids in the gastrointestinal(胃
肠) tract via the action of pepsin, trypsin, chymotrypsin, carboxypeptidases and aminopeptidases.
H2O
O
R—C||—COO-
+
+H
NH3
H2O2
O2
2. 脱氢酶作用-GDH
一. 氨的去路
+
NH3
|
H—C—COO|
+
NAD（P）+ +H2O
C| H2
C| H2 COO
谷氨酸脱氢酶
O ||
+
NH4 +
C—COO|
+ NAD（P）H
+
+
H
C| H2
C| H2 COO
L- 谷氨酸
α-酮戊二酸
Glu + NAD(P) + H2O a-KG + NH4+ + NADH(P) + H+
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 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.
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 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.
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
3. 转氨基作用
一. 氨的去路
转氨基作用是α-氨基酸和α-酮酸之间氨基的转移作用。 一种α-氨基酸的α-氨基借助转氨酶（transaminase）的催化 作用转移到α-酮酸的羰基上，结果生成新的酮酸，而原来 的α-酮酸则形成相应的氨基酸。
R | 1
R | 2
转氨酶
R | 2
R|1
H—C |—NH2
| C H— N H 2
+
C=O |
பைடு நூலகம்
H—C |—NH2
+
C=O |
COOH COOH
COOH COOH
3. 转氨基作用
一. 氨的去路
谷丙转氨酶催化的转氨基作用机理
CH3 | H— C— N H 2 | COOH
丙氨酸
HO— H 3C—
H
C |
=
O —
C
H
2
—
O
O || —P |
—
-
O
-
O
磷酸吡哆醛
COOH
| （ C H 2） 2
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.
acids or carbohydrates).
一. 氨的去路 1. 氧化脱氨基
第二节 Amino acid degradation
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。
+
+
R—NC| H H3—COO- 氨基酸氧化酶R—NC|— H3COO-
（FFM ADN）（FFM ADNHH22）
Pepsin: the first enzyme discovered (18th century).
Proteins (but not pepsin) unfolded
proteases
Absorbed as tri- & dipeptides,
and amino acids
Degradation & absorption of dietary proteins