Transcript Lecture 29
Lecture 30 Pyruvate Oxidation and the Citric Acid Cycle Aerobic Fate of Pyruvate: Overview Proteins Glycogen Fats Glucose Pyruvate NADH Acetyl-CoA 3 NADH FADH2 Citric acid (Krebs) cycle CO2 GTP CO2 Mitochondria: Site of Oxidations Pyruvate Oxidation to Acetyl-CoA Lipoic acid, FAD, thiamine pyrophosphate CoASH CO2 NAD+ NADH O H 3C C O C O O Pyruvate dehydrogenase complex H 3C C S CoA Pyruvate Dehydrogenase Complex Complex enzyme – 60 polypeptides of 3 kinds In mitocondrion matrix Regulated – Inhibited by NADH and GTP – Stimulated by insulin Pyruvate Dehydrogenase Complex Three enzymes and 5 coenzymes Enzyme Coenzyme(s) MW Enz Enz2 Enz3 TPP 96 kDa Lipoate, CoA 70 kDa + FAD, NAD 56 kDa Subunits 24 24 12 Total MW: 4.5 x 106! Also present: regulatory protein kinase, phosphatase Lipoic Acid: Acyl- and Redox Carrier S S Lipoic acid H N Lysine NH Oxidized form C O CH3 O SH SH C S HS Reduced form Acylated form On enzyme 2 O Thiamine Pyrophosphate (TPP) H NH 2 C CH2 N N S H3C H3C N R1 H3C O CH2 CH2 O P H+ O C N TPP anion (stable) S R2 O O P O O Pyruvate Dehydrogenase Mechanism O H 3C C O OH C O H + H 3C C O C O C R1 C R1 N N S H 3C H 3C S R2 R2 Pyruvate Dehydrogenase Mechanism OH H 3C C O C O C R1 N H 3C S R2 H 3C CO2 OH C C R1 N H 3C S R2 Pyruvate Dehydrogenase Mechanism H 3C OH OH C H 3C C R1 N H 3C C C S Resonance R2 R1 N H 3C S R2 Pyruvate Dehydrogenase Mechanism OH OH H 3C H3C C H+ C R1 N H 3C S R2 C H C R1 N H3C S R2 Hydroxyethyl-TPP Pyruvate Dehydrogenase Mechanism OH H3C C O H TPP E1 S H3C S FAD E2 E3 C TPP E1 HS S FAD E2 E3 Pyruvate Dehydrogenase Mechanism CoA SH Acetyl-CoA O O H3C C TPP E1 HS H3C C HS S CoA TPP S HS FAD FAD E2 E3 E1 E2 E3 Pyruvate Dehydrogenase Mechanism TPP E1 HS S HS S FAD E2 E3 TPP E1 FADH2 E2 E3 Pyruvate Dehydrogenase Mechanism S TPP E1 S NAD+ NADH + H+ S FADH2 E2 E3 TPP E1 S FAD E2 E3 PDH: The Overall Reaction CoASH NAD+ O H3C C NADH CO2 O C O Pyruvate dehydrogenase complex Acetyl-CoA enter Krebs cycle NADH passes e– to O2 H3C O C S CoA Krebs Cycle Summary C2 NADH C4 C6 NADH CO2 C5 FADH 2 C4 GTP NADH CO2 Importance of the Krebs Cycle 1. Central energy-yielding path 2. Point of convergence of catabolism of fats, CHO, protein 3. Source of precursors for biosynthesis Citrate Synthase: “Condensing Enzyme” Acetyl-CoA O S CoA ∆Gº= –32.2 kJ/mol C O CH3 O C CoASH CH2 COO C O CH OOC C OH CH COO COO Oxaloacetate (OAA) Citric Acid (citrate) Aconitase Citrate CH2COO H2O cis-aconitate CH2COO HO C COO C COO H C COO C COO H H Isocitrate CH2COO H C COO HO C COO H H2O ∆Gº = +13.3 kJ/mol Isocitrate Dehydrogenase H2C COO NAD(P) + NAD(P)H + H+ H2C CH COO HO C H COO COO CH2 CO2 O C COO -Ketoglutarate Isocitrate ∆Gº= –20.9 kJ/mol -Ketoglutarate Dehydrogenase Complex -KG H2C Succinyl-CoA COO CoASH CH2 C CO2 H2C COO O Compare with pyruvate DH CH2 C + NAD NADH ∆Gº= –33.5 kJ/mol COO O S CoA -Ketoglutarate Dehydrogenase Complex Cofactors: FAD, NAD+, lipoate, TPP, CoASH Mechanism PDH complex Succinic Thiokinase Succinyl-CoA H2C COO Succinate GDP + P i CH2 C O S CoA GTP H2C COO H2C COO ∆Gº= –2.9 kJ/mol Succinic Thiokinase Multistep reaction: Pi attack GTP acyl-phosphate Enz-his- P GDP Enz-his succinate Nucleoside Diphosphate Kinase GTP + ADP GDP + ATP ∆Gº= 0 Also uses – CTP – UTP So NTPs are in equilibrium with each other Succinate Dehydrogenase COO H C H H C H FAD FADH2 COO C H COO C COO Succinate Fumarate ∆Gº= 0 H Enzyme is membrane bound Fumarase (Fumarate Hydratase) H2O COO C H H C COO HO C H H C H COO COO Fumarate ∆Gº= –3.8 kJ/mol L-malate Malate Dehydrogenase COO HO C H H C H NAD+ NADH + H+ COO O C H C COO L-malate H COO Oxaloacetate ∆Gº= +29.7 kJ/mol [OAA] normally < 10-6 M Overall Reaction of Krebs Cycle GDP O H3C GTP 2CO2 C SCoA FAD FADH 2 + 3NAD 3NADH Energry conserved from oxidation reactions Fate of NADH, FADH2ATP ADP + P i ATP NADH + O2 Chain of electron carriers NAD H 2O FADH 2 Each NADH3 ATP Each FADH22 ATP FAD Energy Yield from Glucose Oxidation Glucose Glycolysis 2 ATP 2 NADH ATP Equivalents 2 6 2 Pyruvate 2 NADH 6 2 Acetyl-CoA + 2 CO 2 Krebs Cycle 6 NADH 2 FADH 2 2 GTP 4 CO2 18 4 2 38 ATP Citric Acid Cycle: Metabolic Hub ala, ser, cys, gly, thr ala, lys, ile, val, leu Pyruvate phe, tyr, leu, lys, trp, ile asp thr, met Acetyl-CoA OAA Citrate Krebs cycle phe, tyr lipids Fumarate -KG glu, pro, arg glu, arg, his, pro Succinate ile, val, met met, lys porphyrins heme, chlorophyll Regulation of the Citric Acid Cycle Krebs Cycle in Motion QuickT ime™ and a Microsoft Video 1 decompressor are needed to see t his picture. Jon Maber Dept of Biochemistry and Molecular Biology The University of Leeds, UK http://bmbwww.leeds.ac.uk/designs/tcasteps