《生物化学》课程PPT教学课件（英文版）Chapter 16 The Citric Acid Cycle

Chapter 16 The citric Acid Cycle The common pathway leading to complete oxidation of carbohydrates, fatty acids, and amino acids to CO2. 5 A pathway providing many precursors for biosynthesis

Chapter 16 The Citric Acid Cycle The common pathway leading to complete oxidation of carbohydrates, fatty acids, and amino acids to CO2 . A pathway providing many precursors for biosynthesis

gel. The cellular respiration (complete oxidation of fuels) can be divided into three stages StageI All the fuel molecules are oxidized to generate a common two-carbon unit. acetyl-coA Stage II The acetyl-CoA is completely oxidized into CO2, with electrons collected by NAD and FAD via a cyclic pathway (named as the citric acid cycle Krebs cycle, or tricarboxylic acid cycle) Stage lll Electrons of NADH and FADH, are yo cansferred to o, via a series carr riers, producin g and a H gradient, which will promote ATP formation

1. The cellular respiration (complete oxidation of fuels) can be divided into three stages • Stage I All the fuel molecules are oxidized to generate a common two-carbon unit, acetyl-CoA. • Stage II The acetyl-CoA is completely oxidized into CO2 , with electrons collected by NAD and FAD via a cyclic pathway (named as the citric acid cycle, Krebs cycle, or tricarboxylic acid cycle). • Stage III Electrons of NADH and FADH2 are transferred to O2 via a series carriers, producing H2O and a H+ gradient, which will promote ATP formation

Stage I Intermembrane Amino Fatty Acetyl-CoA spac君 acids acid Glycolysis ansporter Citric acid cycle Pyruvate HSCOA Acetyl COA 3NADH FA ATP,.PP FADH, HSCOA Acetyl-CoA tare Mitochondrial T Acetyl-CoA NADH NAD ADP FAD Oxal 2+H+102→H2O NADH NAD FAD Citric Transporter acid cycle Electron transpon complexes Transport of metabolites Pyruvate dehydrogenase and citric acid cycle into the mitochondrion NADH ATP synthesis by Fo F FADHg Fatty acid metabolism Using proton-motive (reduced Stage 3 Electron transport from ATP export AD Electron transfer and oxidative cygan: generation of phosphorylation oton.motive force 2H+3o Respiratory Mitochondria is the major site for (electron transfer) ADP+P fuel oxidation to generate ATP. ATP

Mitochondria is the major site for fuel oxidation to generate ATP

2. Pyruvate is oxidized to acetyl- CoA by the catalysis of pyruvate R dehydrogenase complex Pyruvate is first transported into mitochondria via a specific transporter on the inner membrane Pyruvate is converted to acetyl-COA and CO2 by oxidative decarboxylation. DI The pyruvate dehydrogenase complex is a huge multimeric assembly of three kinds of enzymes having 60 subunits in bacteria and more in mammals Pyruvate is first decarboxylated after binding to the prosthetic group (TPP)of pyruvate dehydrogenase (E), forming hydroxyethyl-TPP707374329

2. Pyruvate is oxidized to acetyl￾CoA by the catalysis of pyruvate dehydrogenase complex • Pyruvate is first transported into mitochondria via a specific transporter on the inner membrane. • Pyruvate is converted to acetyl-CoA and CO2 by oxidative decarboxylation. • The pyruvate dehydrogenase complex is a huge multimeric assembly of three kinds of enzymes, having 60 subunits in bacteria and more in mammals. • Pyruvate is first decarboxylated after binding to the prosthetic group (TPP) of pyruvate dehydrogenase (E1 ), forming hydroxyethyl-TPP

The hydroxyethyl group attached to TPP is oxidized and transferred: First two electrons. then the acety group formed are all transferred to the lipoyllysyl (硫辛酰赖氨酰) group of dihydrolipoyl transacetylase (E2). The li e lipo oyllysyl gro oup serves as both electron and acetyl carriers The acetyl group is then transferred (still catalyzed by E2) from acetyllipoamide to CoA-SH, forming acetyl-CoA The oxidized lipoamide group is then regenerated by the action of dihydrolipoyl dehydrogenase(E3), with electrons collected by FAd and then by nad

• The hydroxyethyl group attached to TPP is oxidized and transferred: First two electrons, then the acetyl group formed are all transferred to the lipoyllysyl (硫辛酰赖氨酰）group of dihydrolipoyl transacetylase (E2 ). • The lipoyllysyl group serves as both electron and acetyl carriers. • The acetyl group is then transferred (still catalyzed by E2 ) from acetyllipoamide to CoA-SH,forming acetyl-CoA. • The oxidized lipoamide group is then regenerated by the action of dihydrolipoyl dehydrogenase (E3 ), with electrons collected by FAD and then by NAD+

Substrates of the five reactions catalyzed by pyruvate dehydrogenase complex are efficientl channeled The lipoamide group attached to e swings between E (accepting the electrons and acetyl group) and E, (giving away the electrons), passing the acetyl group to Coenzyme a on E The multienzyme complexes catalyzing the oxidative decarboxylation of a few different kinds of a-keto acids, pyruvate dehydrogenase complex, a ketoglutarate dehydrogenase complex and branched chain a-keto acid dehydrogenase complex show remarkable structure and function relatedness(all have identical E3, Similar El and E

• Substrates of the five reactions catalyzed by pyruvate dehydrogenase complex are efficiently channeled : The lipoamide group attached to E2 swings between E1 (accepting the electrons and acetyl group) and E3 (giving away the electrons), passing the acetyl group to Coenzyme A on E2 • The multienzyme complexes catalyzing the oxidative decarboxylation of a few different kinds of a-keto acids, pyruvate dehydrogenase complex, a￾ketoglutarate dehydrogenase complex and branched chain a-keto acid dehydrogenase complex show remarkable structure and function relatedness (all have identical E3 , similar E1 and E2 )

CO 2 COA-SH NAD NADH TPP, lipoate O S-COA FAD C=0 C pyruvate dehydrogenase CH3 complex(E1+ E2+ E3) CH 3 Pyruvate Acetyl-CoA △G°′=-33.4kJ/mol The of pyruvate in mitochondria: producing acetyl-CoA and Co

The oxidative decarboxylation of pyruvate in mitochondria: producing acetyl-CoA and CO2

005pm Electron micrograph of teg from E coli

Electron micrograph of pyruvate dehydrogenase complexes from E. coli

pyruvate CO, acetvI-CoA E2(dihydrolipoyl transacetylase consisting the core, 24 subunits hydro TPP pyruvate dehydrogenase) bound to the E core, 24 subunits (dihydrolipoyl dehydrogenase) E 3 bound to the F, core, 12 subunit (a protein kinase and phosphoprotein phosphatase, not shown here, are also part of the comple apl e (b) A model of the E. coli complex showing the three kinds of enzymes and the flexible lipoamide arms covalently attached to F

A model of the E. coli pyruvate dehydrognase complex showing the three kinds of enzymes and the flexible lipoamide arms covalently attached to E2 E2 (dihydrolipoyl transacetylase): consisting the core, 24 subunits; E1 (pyruvate dehydrogenase): bound to the E2 core, 24 subunits; E3 (dihydrolipoyl dehydrogenase): bound to the E2 core, 12 subunits. (a protein kinase and phosphoprotein phosphatase, not shown here, are also part of the complex) pyruvate E2 E3 hydroxyethyl-TPP CO2 acetyl-CoA

The E core (a total of 24 subunits f orms a hollow cube X-ray structure of the E, transacetylase core. Onl four out of eight trimers are shown here

X-ray structure of the E2 transacetylase core: Only four out of eight trimers are shown here. The E2 core (a total of 24 subunits) forms a hollow cube