Citric Acid Cycle

Electron Transport and Oxidative phosphorylation Gylcolysis Electron Transport and Oxidative phosphorylation TCA Cycle

(aka Citric Acid Cycle, Krebs Cycle) The TCA Cycle (aka Citric Acid Cycle, Krebs Cycle) 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

Entry into the TCA Cycle Pyruvate is translocated from the cytosol to the mitochondria Pyruvate is oxidatively decarboxylated to form acetyl-CoA Pyruvate dehydrogenase uses TPP, CoASH, lipoic acid, FAD and NAD+ Acetyl-CoA then enters TCA cycle thru citrate synthase

Pyruvate Dehydrogenase Complex Composed of three enzymes: pyruvate dehydrogenase (E1) (cofactor = TPP) Dihydrolipoamide acetyltransferase (E2) (cofactor = Lipoamide, CoA) Dihydrolipoamide dehydrogenase (E3) (cofactor = FAD, NAD+)

Pyruvate Dehydrogenase

Citrate Synthase Only step in TCA cycle that involves the formation of a C-C bond

Aconitase Isomerization of Citrate to Isocitrate Citrate is a poor substrate for oxidation So aconitase isomerizes citrate to yield isocitrate which has a secondary -OH, which can be oxidized Aconitase uses an iron-sulfur cluster to position citrate (binds –OH and carboxyl of central carbon)

Isocitrate Dehydrogenase Oxidative decarboxylation of isocitrate to yield  -ketoglutarate Classic NAD+ chemistry (hydride removal) followed by a decarboxylation Isocitrate dehydrogenase is a link to the electron transport pathway because it makes NADH Rxn is metabolically irreversible

 -Ketoglutarate Dehydrogenase A second oxidative decarboxylation This enzyme is nearly identical to pyruvate dehydrogenase - structurally and mechanistically Five coenzymes used - TPP, CoASH, Lipoic acid, NAD+, FAD

Succinyl-CoA Synthetase A substrate-level phosphorylation A nucleoside triphosphate is made (ATP in plants/bacteria and GTP in animals) Its synthesis is driven by hydrolysis of a CoA ester

Succinate Dehydrogenase An oxidation involving FAD Mechanism involves hydride removal by FAD and a deprotonation This enzyme is actually part of the electron transport pathway 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 Enzyme inhibited by malonate

Fumarase Hydration across the double bond trans-addition of the elements of water across the double bond Stereospecific reaction

Malate Dehydrogenase An NAD+-dependent oxidation The carbon that gets oxidized is the one that received the -OH in the previous reaction This reaction is energetically expensive Go' = +30 kJ/mol

Reduced Coenzymes Fuel ATP Production Acetyl-CoA + 3 NAD+ + Q + GDP + Pi +2 H20  HS-CoA + 3NADH + QH2 + GTP + 2 CO2 + 2 H+ Isocitrate Dehydrogenase 1 NADH=2.5 ATP a-ketoglutarate dehydrogenase 1 NADH=2.5 ATP Succinyl-CoA synthetase 1 GTP=1 ATP Sunccinate dehydrogenase 1 QH2=1.5 ATP Malate Dehydrogenase 1 NADH=2.5 ATP Total of 10 ATPs gained from oxidation of 1 Acetyl-CoA

Regulation of TCA Cycle

TCA Cycle provides intermediates for many biosynthetic processes

The Anaplerotic Reactions The "filling up" reactions PEP carboxylase - converts PEP to oxaloacetate Pyruvate carboxylase - converts pyruvate to oxaloacetate Malic enzyme converts pyruvate into malate