The Citric Acid Cycle

Transcription

1 Mary K. Campbell Shawn O. Farrell Chapter 19 The Citric Acid Cycle Paul D. Adams University of Arkansas

2 Major pathways of glucose utilization

3 C 6 H 12 O 6 +6O 2 +6H 2 O 6CO 2 +12H 2 O+energy

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6 Mary K. Campbell Shawn O. Farrell Chapter 19 The Citric Acid Cycle Paul D. Adams University of Arkansas

7 The Central Role of the Citric Acid Cycle Cellular aerobic metabolism The citric acid cycle (CA cycle) Electron transport (Chapter 20) Oxidative phosphorylation (Chapter 20) Metabolism consists of Catabolism( 分解代謝 ): the oxidative breakdown of nutrients Anabolism( ( 合成 ): the reductive synthesis of biomolecules The citric acid cycle is amphibolic( ( 兩義 ):catabolism and anabolism. It is the central metabolic pathway

8 The Central Relationship of the Citric Acid Cycle to Catabolism cytosol Mitochondria matrix Mitochondria inner membrane

9 Where does the Citric Acid Cycle Take Place? In eukaryotes, cycle takes place in the mitochondrial matrix

10 Features of Cycle Pyruvate transporter Mitochondria matrix Irreversible

11 Features of Cycle

12 Pyruvate is Converted to Acetyl-CoA Pyruvate dehydrogenase (PDH) complex is responsible for the conversion of pyruvate to CO 2 and the acetyl portion of acetyl-coa Five enzymes in PDH complex: pyruvate dehydrogenase, dihydrolipoyl transacetylase, dihydrolipoyl dehydrogenase, pyruvate dehydrogenase kinase, pyruvate dehydrogenase phosphatase Why?

13 The Mechanism of the Pyruvate Dehydrogenase (PDH) Complex Reduced lipoylysine Co-factor: Thiamin pyrophosphate (TPP), lipoic acid, Flavin adenine dinucleotide (FAD)

14 Rate-limiting step

15 Summary The two-carbon unit needed at the start of the citric acid cycle is obtained by converting pyruvate to acetyl-coa This conversion requires the three primary enzymes of the pyruvate dehydogenase complex, as well as, the cofactors TPP, FAD, NAD+, and lipoic i acid The overall reaction of the pyruvate dehydogenase complex is the conversion of pyruvate, NAD +, and CoA-SH to acetyl-coa, NADH + H +, and CO 2

16 Diseases related to the PDH complex (TPP): 1. Thiamine deficiency: unable to oxidize pyruvate, alcholism 2. Beriberi( 腳氣病 ), thiamine deficiency disease: loss of neural function

17 Individual Reactions of the Citric Acid Cycle In step 1, there is a condensation of acetyl-coa with oxaloacetate to form citrate ΔG = kj mol -1, therefore, the reaction is exergonic step 1 Reaction is catalyzed by citrate synthase, an allosteric enzyme that is inhibited by NADH, ATP, and succinyl-coa

18 Individual Reactions of the Citric Acid Cycle (Cont d) step 2 isomerization

19 Individual Reactions of the Citric Acid Cycle (Cont d) In step 3, there is an oxidation of isocitrate followed by decarboxylation to form α-ketoglutarate and CO 2 The reaction is catalyzed by isocitrate dehydrogenase, an allosteric enzyme, which is inhibited by ATP and NADH, and activated by ADP and NAD + step 3 isocitrate ATP NADH ADP NAD + decarboxylation

20 Individual Reactions of the Citric Acid Cycle (Cont d) In step 4, there is an oxidative decarboxylation of α- ketoglutarate to succinyl-coa step 4 α-ketoglutarate dehydrogenase complex This reaction is catalyzed by the α ketoglutarate This reaction is catalyzed by the α-ketoglutarate dehydrogenase complex, which is, like pyruvate dehydrogenase, a multienzyme complex and requires coenzyme A, thiamine pyrophosphate, lipoic acid, FAD, and NAD +

21 Individual Reactions of the Citric Acid Cycle (Cont d) Next, the thioester bond of succinyl-coa if hydrolyzed in the formation of succinate The two CH 2 -COO - groups of succinate are equivalent This is the first energy-yielding y g step (GTP) of the cycle The overall reaction is slightly exergonic step 5

22 Individual Reactions of the Citric Acid Cycle (Cont d) Next, there is an oxidation of succinate to fumarate step 6 Then, the hydration of fumarate to L-malate occurs step 7

23 Individual Reactions of the Citric Acid Cycle (Cont d) Then, malate is oxidized to Oxaloacetate step 8

24 Oxidation of Pyruvate Forms CO 2 and ATP

25 Summary In the citric acid cycle and the pyruvate dehydrogenase reaction, one molecule of pyruvate is oxidized to three molecules of CO 2 as a result of oxidative decarboxylation The oxidations are accompanied by reductions involving NAD + to NADH, FAD to FADH 2 GDP is phosphorylated to GTP (substrate GDP is phosphorylated to GTP (substrate phosphorylation)

26 Control of the Citric Acid Cycle There are 3 points of control within the cycle: Citrate synthase: inhibited by ATP, NADH, and succinyl CoA; also product inhibition by citrate Isocitrate t dehydrogenase: d activated t by ADP and NAD +, inhibited by ATP and NADH α-ketoglutarate t l t t dehydrogenase d complex: inhibited by ATP, NADH, and succinyl CoA; activated by ADP and NAD + There is one control point outside the cycle Pyruvate dehydrogenase: inhibited by ATP and NADH; also product inhibition by acetyl-coa

27 Control of the Citric Acid Cycle (Cont d)

28 Energetics of the Citric Acid Cycle ATP

29 Control of the Citric Acid Cycle (Cont d)

30 The Citric Acid Cycle in Catabolism The catabolism of proteins, carbohydrates, and fatty acids all feed into the citric acid cycle at one or more points Protein Protein

31 Summary All metabolic pathways are related, and all of them operate simultaneously In catabolic pathways, nutrients, many of which are macromolecules, are broken down to smaller molecules, such as sugars, fatty acids, and amino acids Small molecules are processed further, and the end products of catabolism frequently enter the citric acid cycle, which plays a key role in metabolism

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33 The Citric Acid Cycle in Anabolism The citric acid cycle is the source of starting materials for the biosynthesis of other compounds If a component of the citric acid cycle is taken out for biosynthesis, it must be replaced oxaloacetate, for example, is replaced by the carboxylation of pyruvate A reaction that replenishes a citric acid cycle intermediate is called an anaplerotic reaction ( 補給反應 )

34 The Citric Acid Cycle in Anabolism (Cont d)

35 The Citric Acid Cycle in Anabolism (Cont d)

36 Lipid Anabolism Lipid anabolism begins with acetyl-coa and takes place in the cytosol acetyl-coa is produced mainly in mitochondria from catabolism of fatty acids and carbohydrates an indirect transfer mechanism exists involving citrate Citrate + CoA-SH + ATP -----> Acetyl-CoA + Oxaloacetate + ADP + P i the oxaloacetate thus formed provides a means for the production of the NADPH needed d for biosynthesis i

37 Lipid Anabolism (Cont d) Oxaloacetate + NADH + H > Malate + NAD + Malate + NADP > Pyruvate + CO NADPH + H The the net effect of these two reactions is replacement of NADH by NADPH While there is some NADPH produced d by this means, its principal source is the pentose phosphate pathway The anabolic reactions that produce amino acids and many other biomolecules begin with CA cycle molecules that are transported into the cytosol

38 Summary of Anabolism in the Citric Acid Cycle

39 Summary The citric acid cycle plays a central role in anabolic pathways as well as in catabolism Pathways that give rise to sugars, fatty acids, and amino acids all originate with components of the citric i acid cycle

40 The Link To Oxygen The citric acid cycle is considered part of the aerobic metabolic process because of its link to the electron transport chain and oxidative phosphorylation NADH and FADH 2, two important cofactors generated by the citric acid cycle, ultimately pass their electrons to oxygen