蛋白质降解及氨基酸代谢 Proteins Degradation& Amino acids Metabolism 宋潇达

1 上节课内容： 蛋白质的酶促降解 氮平衡，蛋白质的酶促降解，酶原， 氨基酸的一般代谢 脱氨基作用 1. 氧化脱氨基作用 L- 谷氨酸脱氢酶 2. 转氨基作用 谷丙转氨酶（ GPT ），谷草转氨酶（ GOT ）。 转氨酶的辅酶只有一种，即磷酸吡哆醛，是 VB6 的磷酸酯。 3. 联合脱氨基作用 氨的去路 1. 氨对动物是有毒的 ①合成尿素 ②合成谷氨酰胺 ③合成非必需氨基酸或其 他含氮物 丙氨酸 - 葡萄糖循环 2. 尿素的合成 鸟氨酸循环 氨甲酰磷酸合成酶Ⅰ（ CPS- Ⅰ）激活氨结合 CO2 形成氨甲酰磷酸。 鸟氨酸转氨甲酰酶催化氨甲酰磷酸转移到鸟氨酸上生成瓜氨酸。 精氨琥珀酸合成酶催化瓜氨酸与天冬氨酸缩合生成精氨琥珀酸。这是尿素中第 2 个氮原子的来源。

2 精氨琥珀酸酶催化精氨琥珀酸裂解为精氨酸和延胡索酸（后者可进入三羧 酸循环，并转变为草酰乙酸，转氨后又形成天冬氨酸）。 精氨酸酶水解精氨酸生成尿素，并重新产生鸟氨酸，进入第二轮循环。 调节：氨甲酰磷酸合成酶Ⅰ是变构酶，乙酰谷氨酸（ AGA ）是该酶的激 活剂，而精氨酸又是 AGA 合成酶的激活剂，因此，精氨酸浓度增高时， 尿素生成加速。精氨琥珀酸合成酶活性最低，是限速酶。

目录 Section 1. 总览 Section 2. 蛋白质代谢 细胞外蛋白质降解 细胞内蛋白质降解 Section 3. 氨基酸分解 脱氨基反应 丙氨酸 - 葡萄糖循环 鸟氨酸循环 碳骨架代谢 个别氨基酸代谢 一碳单位 Section 4. 氨基酸合成 氨基酸的合成 硒代半胱氨酸合成与硒蛋白 吡咯赖氨酸合成与意义 以氨基酸为前体合成的其他化合物（与疾病的关系） 3

骨架碳的去路 生酮氨基酸：生成乙酰辅酶 A ， 乙酰乙酸，可以产生酮体的氨 基酸 如亮氨酸，赖氨酸等 生糖氨基酸：生成丙酮酸，三 羧酸循环中间体（ α 酮戊二酸， 琥珀酸乙酰辅酶 A ，富马酸， 草酰乙酸）转化成葡萄糖 如 同时生酮，生糖氨基酸：色氨 酸，苯丙氨酸，酪氨酸，异亮 氨酸，苏氨酸 4 Section 3. 氨基酸分解 碳骨架代谢

1. 生成丙酮酸（ Pyruvate ）氨基酸家族 6

全部先转化为 Glutamate 2. 生成 α- 酮戊二酸氨基酸家族 7

Histidine 8

Arginine && Proline 9

3. 生成琥珀酰辅酶 A （ SuccinylCoA ）氨基酸家族 全部先转化为丙酰辅酶 A ，后续同奇数脂肪酸的氧化。 Thr 也可通过 α- 酮酸脱羧酶生成丙酰辅酶 A 10

Methionine 11

Ile, Val and Leu These non-polar, branched chain amino acids follow a similar sequence of three reactions: 1. Transamination giving an a-ketoacid. 2. Oxidative decarboxylation giving an acylCoA. 3. AcylCoA dehydrogenase yields a,b unsaturation. The residue from Ile follows b-ox to give acetylCoA and propionylCoA. The residue from Val adds HOH, is then oxidized twice to yield an acid with a b-carbonyl. This decarboxylates to give propionylCoA. 12

Leucine Transaminase  -Keto acid decarboxylase (Like PDH) 13

Leucine, cont. AcylCoA dehydrogenase Carboxylase 14

Leucine, cont. Hydratase Lyase 15

Conversion of Phe to Tyr Requires tetrahydrobiopterin 16

Tetrahydrobiopterin Formation and regeneration 17

Phe and Tyr Degradation 18

Tryptophan Degradation 19

总结： Amino Acid Pool Molecule Asp, AsnOxaloacetate Ala, Cys, Gly, Ser, Thr, TrpPyruvate Arg, His, Pro, Gln, Glu  -ketoglutarate Met, Ile, Val, ThrSuccinylCoA Asp, Phe, Tyr, Fumarate Phe, Tyr, Trp, Ile, Leu, LysAcetylCoA and/or AcetoacetylCoA 20

21 Section 3. 氨基酸分解 个别氨基酸代谢 α 氨基酸 少部分 骨架胺 二氧化碳 骨架碳 氨 氨 ⌘ 氨基酸脱羧酶 Amino acid decarboxylase Amino acid  Amine + CO 2 Histamine GABA 5-HT

22 Histidine ⌘ 与 H 1 和 H 2 组胺受体发生作用, 能提高胃酸分泌，使各种平滑肌发生痉挛， ⌘ 并使毛细血管扩张，通透性增加 ⌘ 与荨麻疹，哮喘有关 ⌘ 与海鲜过敏有关

23 Glutamate ⌘ 脑内抑制性递质, ⌘ 如果脑内的浓度变低，会焦虑 ⌘ GHB 曾被用作镇静剂 中华人民共和国精神药品 第一类

24 Ornithine ⌘ 促进核酸和蛋白质的合成，与肿瘤有关

25 Cysteine ⌘ 婴幼儿生长发育的必需 ⌘ 增强免疫力和抗疲劳

26 Tryptophan ⌘ 血清素，大脑皮层质及神经突触内含量很高，是一种抑制性神经递质。

27 Section 3. 氨基酸分解 --- 一碳单位 ⌘ 定义：氨基酸在代谢过程中分解生成的含有一个碳原子的基团，称为 一碳基团。一碳基团传递必须要载体。 ⌘ 有两类载体，对应不同的氨基酸 甲基 次甲基 羟甲基 亚甲基 甲酰基 亚氨甲基 （二氧化碳除外）

四氢叶酸 Tetrahydrofolate (FH4) ⌘ FH4 是一碳单位的载体. ⌘ FH4 有三个组成部分 蝶啶 Pteridine 对氨基苯甲酸 p-aminobenzoate 谷氨酸 glutamate 28

FH4 是一碳单位的载体，甲基，亚甲基，甲酰基，甲酰胺基，次甲基 29

四氢叶酸加载一碳单位的互 变 四氢叶酸一碳单位来源： 甘氨酸 丝氨酸 组氨酸 30

A biological methylating agent. SAM synthetase 31 S- 腺苷甲硫氨酸 S-Adenosylmethionine

An SAM Methylation e.g. ethanolamine to choline Methyl transferase Hydrolyase 32

Methionine Regeneration Methyl comes from N 5 -methylFH 4 Methionine synthase 33

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DNA Methylation Using SAM and DNA methylase 35

Section 4. Synthesis of Amino Acids Most bacteria and plants (not mammals) synthesize all 20 common amino acids. Nonessential amino acids for mammals are usually derived from intermediates of glycolysis or the citric acid cycle. Amino acids with the largest energy requirements are usually essential amino acids. In anabolism, the carbons skeletons are normally synthesized first and then -NH 2 added. 36

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Incorporation of NH 3 substrate 1. Glutamate dehydrogenase:  -ketoglutarate 2. Glutamine synthetase:glutamate 3. Transaminase:  -ketoacids & Glu 4. Glutamate synthase (bacteria):  -ketoglutarate glutamine and NAD(P)H  -KG + Gln > 2 Glu 5. Carbamoyl-P Synthetase I feeds Arg synthesis. 38 NAD(P)H

Glutamate Dehydrogenase Ammonia forms a Schiff base with the  -keto group and then reduction occurs giving Glu. Called “reductive amination” in organic. 39

Glutamine Synthetase ATP forms a mixed anhydride using the  -carboxyl of Glu and the  -phosphate of ATP. Ammonia then displaces Pi giving Gln. Gln is a non-toxic carrier of ammonia. GS is the major regulatory enzyme in nitrogen metabolism. 40

Source of Carbon Skeletons Carbon Source Amino acids OxaloacetateAsp, Asn, Met, Thr, Lys, Ile PEP and erythrose-4-PPhe, Tyr, Trp PyruvateAla, Val, Leu Ribose-5-PHis  -ketoglutarateGlu, Gln, Pro, Arg 3-P glycerateSer, Cys, Gly 41

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ATP Cost 48

Transamination 49

Proton transfer Enzyme- 50

Asparagine Synthetase Asp is adenylated not phosphorylated like Glu in making the amide form. Asparagine synthetase 51

Proline Synthesis Mixed anhydride reduction to give a semialdehyde. γ-glutamyl kinase Glutamate semialdehyde dehydrogenase 52

Proline Cont. transaminase spontaneous Pyrroline carboxylate reductase 53

Serine Synthesis 3-phosphoglycerate dehydrogenase 3-phosphoserine aminotransferase 3-phosphoserine phosphatase 54

Cysteine Synthesis Cystathionine synthase Cystathionine lyase 55

Ecoli: Phe, Tyr & Trp Pathway to Shikimate 56

Shikimate to Chorismate 57

Chorismate to Phe, Tyr 58

Chorismate to Trp 59

Last step to Trp Attaching indole to Ser in a PLP mediated reaction. 60

Tryptophan Synthase An example of substrate channeling. The α-subunit cleaves glyceraldehyde-3-P to make indole which passes through the channel to the active site in the β-subunit and reacts with serine. 61

Tryptophan Synthase Tunnel or channel allows substrate to move from the  -subunit to the  -subunit. 62

Regulation A representation of feedback inhibition, often acting at the first enzyme in a pathway. e.g. 3-phosphoglycerateDH in Ser synthesis is inhibited by Ser. 63

Serine Synthesis 3-phosphoglycerate dehydrogenase 3-phosphoserine aminotransferase 3-phosphoserine phosphatase 64

Regulation 3-phosphoglycerate dehydrogenase, the first enzyme in the pathway to Ser, is inhibited by Ser. 65

Regulation of Branched Pathways Thr deaminase converts Thr to  -ketobutyrate is inhibited by Ile the end product. It is activated by Val, the product of a parallel path that leads to Val. 66

Regulation Immunologically different enzymes may catalyze the same reaction. E.g. aspartokinase catalyzes: Asp -- >  -aspartyl phosphate --- > Thr, Lys, Met 67

E. Coli Aspartokinases Asp -- >  -aspartyl phosphate then to Thr, Lys, Met 68

69 Section 4. 硒代半胱氨酸合成与硒蛋白 硒代半胱氨酸的合成与翻译偶联

70 硒代 SAM ，及硒代 Met

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73 Section 4. 吡咯赖氨酸合成与意义 1. 发现 生物合成的发现 2011

74 Expanding the Genetic Code

Section 4. Amino acids are Bioprecursors &disease 75

Amino Acids as Precursors Gly, Asp and Gln --- > Purines Asp --- > Pyrimidines Ser --- > Sphingosine His --- > Histamine Tyr --- > Thyroxine, L-Dopamine, Norepinephrine and Epinephrine Trp --- > Serotonin Trp --- > Nicotinamide 76

Amino Acids as Precursors Glu --- >  -aminobutyric acid Gly --- > Serine Gly --- > Heme Gly --- > Glutathione Gly + Arg --- > Ornithine + Creatine Arg --- > Nitric oxide 77

Glutathione Glutathione is easily oxidized and protects cells. 2 G-SH + ox -- > G-S-S-G + red Then: G-S-S-G + NADPH + H + -- > 2 G-SH + NADP + 78

Glutathione Peroxidase An enzyme containing selenocysteine 79

Glutathione Peroxidase Cycle 80

Nitric oxide from Arg Nitric oxide synthase 81

Heme Synthesis 82

Heme Degradation 83

84 苯丙酮尿症（ PKU ）是一种常见的氨基酸代谢病，是由于苯丙氨酸（ PA ） 代谢途径中的酶缺陷，使得苯丙氨酸不能转变成为酪氨酸 常染色体隐性遗传。临床表现不均一，主要临床特征为智力低下、 精神神经症状、湿疹、皮肤抓痕征及色素脱失、脑电图异常

85 白化病（ PKU ）是一种常见的氨基酸代谢病，酪氨酸酶缺乏或功能减退引 起 常染色体隐性遗传，白化病全身皮肤呈乳白或粉红色，毛发为淡白或淡 黄色。由于缺乏黑色素的保护，患者皮肤对光线高度敏感，日晒后易发 生晒斑和各种光感性皮炎。

谢谢大家 ! 86