III Bioenergetics and Metabolism 13 Principle of Bioenergetics 14 Glycolysis and the Catabolism 15 The Citric Acid Cycle （三羧酸循环） 16 Oxidation of Fatty Acid 17 Amino Acid Oxidation and the Production of Urea 18 Oxidative Phosphorylation and Photophosphrylation 19 Carbohydrate Biosynthesis 20 Lipid Synthesis 21 Biosynthesis of Amino Acids, Nucleotides, and Related Molecules 22 Integration and Hormonal Regulation of Mammalian Metabolism

Essential Question: How is pyruvate oxidized under aerobic conditions, and what is the chemical logical that dicates how is process occurs?

Key Questions 1.How is pyruvate oxidatively decarboxylated to acetyl-CoA? 2.How is the chemical logic of the TCA cycle? 3.How are two CO2 molecules produces from acetyl-CoA? 4.How are the energetic consequences of the TCA cycle? 5.Can the TCA cycle provide intermediates for biosynthesis? 6.How is the TCA cycle regulates?

*** Production of Acetate （乙酸） *** Reaction of the Citric Acid Cycle *** Regulation of the Citric Acid Cycle *** The Glyoxylate Cycle

Citric Acid Cycle Acytyl-CoA + 3NAD + +GDP +Pi +FAD +2H 2 O 2CO 2 + CoA + 3NADH +GTP +FADH 2

Pyruvate Is Oxidized to Acetyl-CoA and CO2

The Pyruvate Dehydrogenase Complex Requires Five Coenzymes Coenzyme A (CoA ，辅酶 A), Thiamine Pyrophosphate (TPP ，硫胺素焦磷酸 ), Lipoate （硫辛酸）, Flavin Adenine Dinucleotide (FAD ，黄素腺嘌呤二 核苷酸 ), Nicotinamide Adenine Dinucleotide (NAD ，烟酰 胺腺嘌呤二核苷酸 ).

The Pyruvate Dehydrogenase Complex Requires Five Coenzymes

lipoate can serve both as an electron carrier and as an acyl carrier. two thiol groups

The Pyruvate Dehydrogenase Complex Consists of Three Distinct Enzymes The pyruvate dehydrogenase contains three enzymes 1) pyruvate dehydrogenase [ 丙酮酸脱氢酶 ] (E1) 2) dihydrolipoyl transacetylase [ 二氢硫辛酰转乙酰酶 ] (E2) 3) dihydrolipoyl dehydrogenase [ 二氢硫辛酰脱氢酶 ] (E3) E1E2E3

Electron micrograph of the pyruvate dehydrogenase complex isolated from E. coli E1E2E3 Lipoate TPP FAD Acetate + HS-CoA Acetly-CoA NAD + + H + + 2e DNAH

Intermediates Remain Bound to the Enzyme Surface Step 1; pyruvate reacts with the bound thiamine pyrophosphate (TPP) of El. step 2; the transfer of two electrons and the acetyl group from TPP to the oxidized form of the lipoyllysyl group （硫辛酰赖氨酰基团） of the core E2, to form the acetyl thioester （乙酰硫酯） of the reduced lipoyl group.

Step 3; is a transesterification （转酯反应） in which the -SH group of CoA replaces the -SH group of E2 to yield acetyl CoA and the fully reduced (dithiol ，双巯基 ) form of the lipoyl group. step 4 E3 promotes transfer of two hydrogen atoms from the reduced lipoyl groups of E2 to the FAD prosthetic group of E3, restoring the oxidized form of the lipoyllysyl group of E2. Step 5 the reduced FADH2 group on E3 transfers a hydride ion to NAD +, forming NADH. The enzyme complex is now ready for another catalytic cycle.

*** Production of Acetate *** Reaction of the Citric Acid Cycle *** Regulation of the Citric Acid Cycle *** The Glyoxylate Cycle

Citric Acid Cycle [ 三羧酸循环，柠檬酸循环， Krebs 循环 ] (Tricarboxylic Acid Cycle, Krebs Cycles) A nearly universal metabolic pathway in which the acetyl group of acetyl coenzyme A is effectively oxidized to two CO 2 and four pairs of electrons are transferred to coenzymes. The acetyl group combines with oxaloacetate （草酰乙酸） to form citrate （柠檬酸）, which undergoes successive transformation to isocitrate （异柠檬酸）, α-ketoglutarate （ α- 酮戊二酸）, succinyl- CoA （琥珀酰辅酶 A ）, succinate （琥珀酸）, fumarate （延胡索 酸）, malate （苹果酸）, and oxaloacetate again.

The Citric Acid and Urea Cycles Are Linked

Citric Acid Cycle Acytyl-CoA + 3NAD + +GDP +Pi +FAD +2H 2 O 2CO 2 + CoA + 3NADH +GTP +FADH 2

The Citric Acid Cycle Has Eight Steps 1.Formation of Citrate （柠檬酸） 2.Formation of Isocitrate （异柠檬酸） via cis-Aconitate （顺乌头酸） 3.Oxidation of Isocitrate to α-Ketoglutarate （ α- 酮戊二酸） and CO 2 4.Oxidation of α-Ketoglutarate to Succinyl-CoA （琥珀酰辅酶 A ） and CO 2 5.Conversion of Succinyl-CoA to Succinate （琥珀酸） 6.Oxidation of Succinate to Fumarate （延胡索酸） 7.Hydration of Fumarate to Produce Malate （苹果酸） 8.Oxidation of Malate to Oxaloacetate （草酰乙酸）

The Citric Acid Cycle is catalyzed by Eight Enzymes 1.Citrate synthase （柠檬酸合酶） 2.Aconitase （顺乌头酸酶） 3.Isocitrate dehydrogenase （异柠檬酸脱氢酶） 4.α-ketoglutarate dehydrogenase complex （ α- 酮戊二酸 脱氢酶复合体） 5.Succiny-CoA synthetase （琥珀酰辅酶 A 合成酶） 6.Succinate dehydrogenase （琥珀酸脱氢酶） 7.Fumarase （延胡索酸酶） 8.Malate dehydrogenase （苹果酸脱氢酶）

1. Formation of Citrate Condensation of acetyl-CoA with oxaloacetate （草酰乙酸） to form citrate is catalyzed by citrate synthase （柠檬酸合酶）

Oxaloacetate (yellow), the first substrate to bind to the enzyme, induce a large conformation change, creating a binding site for the second substrate, acetyl-CoA (red).

(2) Formation of Isocitrate via cis-Aconitate The enzyme aconitase ( 顺乌头酸酶， aconitate hydratase) catalyzes the reversible transformation of citrate to isocitrate （异柠檬酸）.

(3) Oxidation of Isocitrate to α-Ketoglutarate and CO 2 Isocitrate dehydrogenase （异柠檬酸脱氢酶） catalyzes oxidative decarboxylation of isocitrate to form α- ketoglutarate （ α- 酮戊二酸）.

(4) Oxidation of α-Ketoglutarate to Succinyl-CoA α-ketoglutarate is converted to succinyl-CoA （琥珀酰辅 酶 A ） and CO 2 by the action of the α-ketoglutarate dehydrogenase complex （ α- 酮戊二酸脱氢酶复合体）.

(5) Conversion of Succinyl-CoA to Succinate Succinate （琥珀酸） is formed by succinyl-CoA synthetase ( 琥珀 酰辅酶 A 合成酶， succinic thiokinase) catalysis.

The succinyl-CoA synthetase reaction

6) Oxidation of Succinate to Fumarate The succinate formed from succinyl-CoA is oxidized to fumarate （延胡索酸） by the flavoprotein succinate dehydrogenase （琥珀酸脱氢酶）. Competitive Inhibitor

(7) Hydration of Fumarate to Produce Malate The reversible hydration of fumarate to L-malate （ L- 苹果酸） is catalyzed by fumarase [ 延胡索酸酶 ] (fumarate hydratase). No Active

(8) Oxidation of Malate to Oxaloacetate NAD-linked L-malate dehydrogenase （ L- 苹果酸脱氢酶） catalyzes the oxidation of L-malate to oxaloacetate （草酰乙酸）.

The Energy of Oxidations in the Cycle Is Efficiently Conserved Although the citric acid cycle itself directly generates only one molecule of GTP per turn, the four oxidation steps in the cycle provide a large flow of electrons into the respiratory chain and thus eventually lead to formation of a large number of ATP molecules during oxidative phosphorylation.

The Energy of Oxidations in the Cycle Is Efficiently Conserved

Shuttle Systems （穿梭系统） Are Required for Mitochontrial Oxidation of Cytosolic NADH

Shuttle Systems Are Required for Mitochontrial Oxidation of Cytosolic NADH

Why Is the Oxidation of Acetate So Complicated? 1.The pathway is hub of intermediary metabolism, four and five-carbon end products of many catabolic processes feed into the cycle to serve as fuels. 2.The cycle is the product of evolution. Early anaerobes very probably used some of the reactions of the cycle in linear biosynthetic processes as a source, not of energy, but of biosynthetic precursors

Citric Acid Cycle Components Are Important Biosynthetic Intermediates

Biosynthetic precursors produced by an incomplete citric acid cycle in anaerobic bacteria. These anaerobes lack ketoglutarate dehydrogenase

Anaplerotic Reactions Replenish Citric Acid Cycle Intermediates When intermediates of the citric acid cycle are removed to serve as biosynthetic precursors, the intermediates can be replenished by anaplerotic reactions （添补反应）.

*** Production of Acetate *** Reaction of the Citric Acid Cycle *** Regulation of the Citric Acid Cycle *** The Glyoxylate Cycle

Regulation of key enzymes in metabolic pathways are regulated by allosteric effectors and by covalent modification, to assure production of intermediates and products at the rates required to keep the cell in a stable steady state and to avoid wasteful overproduction of intermediates.

Regulation at: 1 the pyruvate dehydrogenase complex reaction (the conversion of pyruvate into acetyl-CoA). 2 the citrate synthase reaction (the entry of acetyl-CoA into the cycle). 3 the isocitrate dehydrogenase reaction. 4 the α-ketoglutarate dehydrogenase reaction.

*** Production of Acetate *** Reaction of the Citric Acid Cycle *** Regulation of the Citric Acid Cycle *** The Glyoxylate Cycle （乙醛酸循环）

The Glyoxylate Cycle (glyoxylate pathway) A modification of the citric acid cycle occurring in some plants (glyoxysomes [ 乙醛酸循环体 ], membrane- bounded organelles) and microorgannisms, in which isocitrate, instead of being dehydrogenated, is cleaved as described under isocitrate lyase （异柠檬酸裂解酶） to glyoxylate （乙醛酸）. The glyoxylate can then react with another molecule of acetyl-CoA to form malate catalyzed by malate synthase make the citric acid cycle shorter.

Lipid Acetyl-CoA 2Acetyl-CoA + NAD + + 2H 2 O Succinate + 2CoA + NADH + H + Succinate Glucose-6-phosphate Polycarbonhydrates

The Citric Acid and Glyoxylate Cycles Are Coordinately Regulated Isocitrate is a crucial intermediate, standing at the branch point between the glyoxylate and citric acid cycles. Isocitrate dehydrogenase is regulated by covalent modification: a specific protein kinase phosphorylates and thereby inactivates the dehydrogenase.

1.ATP, NADH and FADH 2.Glycolysis: two phases and ten steps of glycolysis 3.Fates of pyruvate under aerobic and anaerobic conditions 4.Three major stages of cellular respiration 5.The citric acid cycle 6.The glyoxylate cycle