生物化學(xué)原理課件（英文）：Chapter23 TCA cycle

1、楊榮武楊榮武 生物化生物化 學(xué)原理學(xué)原理 第二版第二版 Chapter23 TCA Cycle Overview of TCA cycle All TCA cycle reactions Summary of the TCA cycle Physiological functions of the TCA cycle The Glyoxylate Cycle Anaplerotic reactions of the TCA cycle Regulation of the TCA cycle Outline Overview of TCA cycle aka Citric Acid Cycle,

2、 Krebs Cycle A common metabolic pathway for glucose, aa and fatty acid Pyruvate (actually acetate) from glycolysis is degraded to CO2 Some ATP is produced More NADH is made NADH goes on to make more ATP in electron transport and oxidative phosphorylation All roads lead to TCA cylce Entry into the TC

3、A Cycle Pyruvate Dehydrogenase(PDH) Complex Pyruvate is oxidatively decarboxylated to form acetyl-CoA Pyruvate dehydrogenase uses TPP, CoASH, lipoic acid, FAD and NAD+ Pyruvate dehydrogenase (E1) Dihydrolipoamide transacetylase (E2) Dihydrolipoamide dehydrogenase (E3) lipoic acid and its linkage wit

4、h E2 Pyruvate Dehydrogenase Complex of E. coli The five reactions of the PDH multienzyme complex Arsenic Compounds Are Poisonous in part because they Sequester Lipoamide All TCA cycle reactions Formation of citrate Catalyzed by Citrate Synthase Another example for the induced fit model OAA, the firs

5、t substrate to bind to the enzyme, induce a large conformational change, creating a binding site for the second substrate, acetyl-CoA. When citroyl-CoA forms on the enzyme surface, another conformational change brings the side of a crucial Asp residue into position to cleavage the thioester. This me

6、chanism decreases the likelihood of premature and unproductive cleavage of the thioester bond of acetyl-CoA Formation of Citrate and Fluorocitrate Conformational changes in citrate synthase The Open ConformationThe closed, substrate-binding conformation Isomerization of Citrate to Isocitrate Catalyz

7、ed by Aconitase Citrate is a poor substrate for oxidation So aconitase isomerizes citrate to yield isocitrate which has a secondary -OH, which can be oxidized Note the stereochemistry of the Rxn: aconitase removes the pro-R H of the pro-R arm of citrate! Aconitase uses an iron-sulfur cluster Isomeri

8、zation of Citrate to Isocitrate A mechanism for aconitase reaction Stereochemistry of the Aconitase Reaction Oxidative Decarboxylation of Isocitrate Catalyzed by Isocitrate Dehydrogenase (IDH) Classic NAD+ chemistry (hydride removal) followed by a decarboxylation Isocitrate dehydrogenase is a link t

9、o the electron transport pathway because it makes NADH Know the mechanism! The reaction mechanism of IDH A second oxidative decarboxylation Catalyzed by -Ketoglutarate Dehydrogenase Complex This enzyme is nearly identical to pyruvate dehydrogenase - structurally and mechanistically Five coenzymes us

10、ed - TPP, CoASH, Lipoic acid, NAD+, FAD You know the mechanism if you remember pyruvate dehydrogenase Another target for arsenic compounds Oxidative Decarboxylation of -Ketoglutarate A substrate-level phosphorylation Catalyzed by Succinyl-CoA Synthetase A nucleoside triphosphate is made Its synthesi

11、s is driven by hydrolysis of a CoA ester The mechanism involves a phosphohistidine The substrate-level phosphorylation The reaction mechanism of succinyl-CoA synthetase An oxidation involving FAD Catalyzed by Succinate Dehydrogenase This enzyme is actually part of the electron transport pathway (Com

12、plex II) in the inner mitochondrial membrane The electrons transferred from succinate to FAD (to form FADH2) are passed directly to ubiquinone (UQ) in the electron transport pathway Dehydrogenation of Succinate Hydration across the double bond Catalyzed by Fumarase trans-addition of the elements of

13、water across the double bond The actual mechanism is not known for certain Hydration across the double bond An NAD+-dependent oxidation Catalyzed by Malate Dehydrogenase The carbon that gets oxidized is the one that received the -OH in the previous reaction This reaction is energetically expensive,

14、Go = +30 kJ/mol This and the previous two reactions form a reaction triad that we will see over and over! Regeneration of OAA The Fate of Carbon in TCA Carboxyl C of acetate turns to CO2 only in the second turn of the cycle (following entry of acetate) Methyl C of acetate survives two cycles complet

15、ely, but half of whats left exits the cycle on each turn after that. TCA Cycle Summary Total rxn: Acetyl-CoA+3NAD +FAD+GDP+Pi+2H2O2CO2+3NADH+F ADH2+GTP+2H+CoA One Acetyl-CoA through the cycle produces two CO2, one ATP, four reduced coenzymes Two H2Os are used as substrates Absolutely depends on O2 F

16、unction of the TCA Cycle $Produces More ATPs $As a source of biosynthetic precursors $ A common final metabolic pathway for glucose, aas and fatty acids $Some Immediates act as effectors to regulate other metabolic pathways $Produces CO2 Stoichiometry of Coenzyme Reduction and ATP Formation During t

17、he Complete Oxidation of Glucose Amphibolic functions of the citric acid cycle Anaplerotic Reactions The Glyoxylate Cycle A variant of TCA for plants and bacteria Acetate-based growth - net synthesis of carbohydrates and other intermediates from acetate - is not possible with TCA Glyoxylate cycle of

18、fers a solution for plants and some bacteria and algae The CO2-evolving steps are bypassed and an extra acetate is utilized Isocitrate lyase and malate synthase are the short- circuiting enzymes Glyoxylate Cycle II Isocitrate lyase produces glyoxylate and succinate Malate synthase does a Claisen condensation of acetyl-CoA and the aldehyde group of glyoxylate - classic CoA chemistry! The glyoxylate cycle helps plants grow in the dark! Glyoxysomes borrow three reactions from mitochond