Essential Biochemistry

Transcription

1 Essential Biochemistry Third Edition Charlotte W. Pratt Kathleen Cornely Lecture Notes for Chapter 14 The Citric Acid Cycle

2 The citric acid cycle Citric acid cycle (TCA cycle; Kerbs cycle) is a central pathway of 2- carbon intermediates metabolism Amphibolic 雙向代謝 ( 合成與分解 ) Relates to both catabolism and anabolism Cyclic metabolic pathway Glycolysis and gluconeogenesis are linear The significance of will be shown later

3 14-1 The Pyruvate Dehydrogenase Reaction 丙酮酸 (pyruvate) 的氧化脫羧反應 丙酮酸先滲透過粒腺體外膜, 再以 transporter protein 運送穿過粒腺體內膜中開始脫羧, 故 TCA 循環均在粒腺體內膜中進行 將 Acetyl unit 轉移至 Coenzyme A 將糖解所得小分子導入 TCA 循環的重要步驟

4 The pyruvate dehydrogenase complex contains multiple copies of three different enzymes 3C 裂解為 2C, 還原能轉移至 NAD + 形成 NADH E1, E2, E3 三大部分構成 pyruvate dehydrogenase Model of the pyruvate dehydrogenase complex from B. stearothermophilus

5 E1 of pyruvate dehydrogenase uses thiamine pyrophosphate. Step 1 Pyruvate Hydroxyethyl-TPP + CO 2 E1 Thiamin = Vitamin B 1 Cofactor for several decarboxylation reactions (TPP) 何種輔因子 / 維生素被使用於丙酮酸的脫羧反應而形成 Acetyl -CoA?

6 Step 2: From E1-TPP to E2-Lipoamide E2 uses a lipoamide cofactor ( 輔因子 ). 何種輔因子被使用協助於乙醯基 (Acetyl) 的轉移而形成 Acetyl -CoA?

7 Step 3: Acetyl Transfer to CoA E2 E2

8 Step 4: Restoration of E2 E2 Lipoamide restoration by E3 FAD E3 E3 E2 E2 FAD:Flavin Adenine Dinucleotide 何種維生素可協助 Lipoamide 還原其雙硫鍵?

9 Step 5: Restoration of E3 E3 FAD restored by NAD + 何種維生素可協助 FAD 還原其雙硫鍵並產生 NADH?

10 Reactions of the pyruvate dehydrogenase complex E3 E1 E2 維生素 B3 維生素 B2 3. 還原氧化態的 FAD 維生素 B1 硫辛酰胺 1. Pyruvate 氧化脫羧 哪些維生素涉及 Pyruvate 的脫羧反應? 2. 轉移 Acetyl unit, 合成 Acetyl-CoA

11

12 TCA 生成 NADH 的目的? 生成之 NADH 與 FADH 2, 透過氧化磷酸化的電子傳遞鏈使質子不斷打出粒腺體 利用外部質子濃度差異, 產生能量大量生成 ATP ( 詳細機制於 15 章討論 )

13 14-2 The Eight Reactions of the Citric Acid Cycle 草醯乙酸 檸檬酸 L- 蘋果酸 異檸檬酸 延胡索酸 α- 酮戊二酸 琥珀酸 琥珀醯輔酶 A

14 1. Citrate synthase adds an acetyl group to oxaloacetate Glycolysis Pyruvate Fatty acids ΔG 0 =-31.5kJ mol -1 檸檬酸合成酶

15 Citrate synthase undergoes a conformational change upon binding oxaloacetate, allowing for binding of acetyl-coa. No Substrates Bound Substrates Bound 2014 John Wiley & Sons, Inc. All rights reserved.

16 Citrate Synthase Mechanism 2014 John Wiley & Sons, Inc. All rights reserved.

17 Citrate Synthase Mechanism 2014 John Wiley & Sons, Inc. All rights reserved.

18 Citrate Synthase Mechanism 2014 John Wiley & Sons, Inc. All rights reserved.

19 Citrate Synthase Mechanism 2014 John Wiley & Sons, Inc. All rights reserved.

20 順式烏頭酸酶 Reaction 2: Aconitase Isomerizes Citrate to Isocitrate

21 Reaction 3: Isocitrate Dehydrogenase Releases the First CO 2 Mn 2+ :Stabilize the negative charges

22 Reaction 4: α-ketoglutarate Dehydrogenase Releases the Second CO 2 TCA cycle 中, 哪些酶可產生 NADH/ 脫羧 / 產生 CO 2

23 Fates of Carbons in the Citric Acid Cycle 在一次 TCA 循環後, 排出的兩個 CO 2 分子的碳都不是來自 Acetyl unit 的 但是就淨反應而言,2 分子碳進入 TCA 循環, 分解產生 2 分子 NADH, 形成 2 分子完全氧化態的 CO 2 排出體外, 將 6 個碳的 Glucose 能量完全轉換為 ATP Carbon Carbon from Acetyl-CoA 2014 John Wiley & Sons, Inc. All rights reserved.

24 Reaction 5: Succinyl-CoA Synthetase Catalyzes Substrate-Level Phosphorylation ΔG = kj mol -1 ΔG = 30.5 kj mol -1

25 Reaction 6: Succinate Dehydrogenase Generates Ubiquinol

26 Reaction 7: Fumarase Catalyzes a Hydration Reaction

27 Reaction 8: Malate Dehydrogenase Regenerates Oxaloacetate ΔG 0 =+29.7 kj mol -1 Malate 脫氫形成 Oxaloacetate 所需高能量, 主要由何者合成時獲得?

28 The citric acid cycle is an energygenerating catalytic cycle 能量換算 1NADH = 2.5 ATP 1QH = 1.5 ATP Glucose 裂解為 2 Pyruvate 2NADH 2ATP 2 Pyruvate 脫羧為 2 Acetyl-CoA 2NADH 2 Acetyl-CoA 進行 2 次 TCA 循環 2 * 3 NADH (Dehydrogenase) 2 * 1 QH 2 (6Fumarate 7Malate) 2 * 1 GTP (5Succinate 6Fumarate) 一分子 Glucose 共生成 10 NADH,2ATP,2GTP,2QH 2 故共生成 32 ATP x CO 2 一分子 Glucose/Pyruvate/Acetyl-CoA 分別可獲得多少 ATP?

29 The citric acid cycle is regulated at three steps Three metabolically irreversible steps: Step 1 citrate synthase Step 3 isocitrate dehydrogenase Step 4 α-ketoglutarate dehydrogenase

30 The citric acid cycle is regulated at three steps Citrate synthase 受到四者抑制 NADH Citrate Succinyl-CoA ATP ADP 可活化 α-ketoglutarate dehydrogenase 受到二者抑制 NADH Succinyl-CoA ADP 與 Ca 2+ 可活化 Isocitrate dehydrogenase 受到一者抑制 NADH Ca 2+ 可活化 TCA 循環中, 何者的濃度增加能抑制最多 TCA 相關酶之活性?

31 14-3 Anabolic and Catabolic Functions of the Citric Acid Cycle TCA 循環提供了三大營養素的生合成前驅物

32 An Example of Amino Acid Formation from a Citric Acid Cycle Intermediate Glutamine, arginine, proline Purine and Pyrimidine nucleotides 胺基酸與核苷酸, 主要可由 TCA 循環中何者轉換?

33 The citrate transport system 何種機制可連結 TCA 循環 與 脂質的生成或分解利用?

34 回補反應 Anaplerotic reactions replenish citric acid cycle intermediates Acetyl-CoA

35 Anaplerotic reactions replenish citric acid cycle intermediates John Wiley & Sons, Inc. All rights reserved.

36 During exercise, some pyruvate can be converted to α-ketoglutarate to boost the activity of the citric acid cycle John Wiley & Sons, Inc. All rights reserved.

37 lactate 2014 John Wiley & Sons, Inc. All rights reserved. 當肌肉高度運動時,Pyruvate 可轉化為何者以加速 TCA 循環?