Metabolism: TCA Cycle

Looking back at glycolysis Glucose + 2Pi + 2 ADP + 2 NAD+ -> 2 pyruvate + 2 ATP + 2 NADH + 2H+ + 2H2O

The Tricarboxylic Acid Cycle Rotondas/Traffic circles facilitate traffic flow for many converging paths Central metabolic hub of cell TCA/Krebs Cycle is the final common pathway for the oxidation of fuel molecules (proteins, fatty acids, carbs) Important source of precursors

THE TCA CYCLE 8 steps, most common entry point is CoA (C2) Key: Oxidation of one acetyl group to two CO2 Function: harvesting high-energy e- (to be used later in oxidative phosphorylation or the e- transport chain

TCA in the Mitochondrion

Entry into the TCA cycle Use of Pyruvate dehydrogenase complex Pyruvate + CoA + NAD+ -> Acetyl CoA + CO2 + NADH NET REACTION: Pyruvate is oxidatively decarboxylated to form acetyl-CoA Pyruvate dehydrogenase uses TPP, CoASH, lipoic acid, FAD and NAD+

Entry into the TCA cycle Three basic steps

Entry into the TCA cycle Step 1: Decarboxylation (pyruvate dehydrogenase E1)

Entry into the TCA cycle Step 2: Oxidation of hydroxymethyl group on TPP (pyruvate dehydrogenase E1)

Entry into the TCA cycle Step 3: Acetyl transfer to CoA (dihydrolipoyl transacetylase E2)

(Step 4: Regenerate lipoamide from dihydrolipoamide) (dihydrolipoyl dehydrogenase)

Amazing pyruvate dehydrogenase complex Flexible linkages allow lipoamide to move between different active sites Eight catalytic trimers

Begin the TCA cycle!

“RXN 1”: Oxaloacetate to Citrate No true “first step” since it is a cycle. But assume here acetyl CoA is the entry point Aldol condensation followed by hydrolysis Citrate synthase

Acetyl CoA must not be wasted/hydrolyzed! Exploring the citrate synthase Oxaloacetate binds first Structural rearragement creating acetyl CoA binding site Efficiency Acetyl CoA binds only after oxaloacetate Catalytic residues are not positioned until citryl CoA is formed

RXN 2: Citrate to Isocitrate Isomerization for proper oxidative decarboxylation later Aconitase used

RNX “3”: Isocitrate to Ketoglutarate Oxidation AND Decarboxylation Rate of ketoglutarate formation important in over-all rate of cycle By isocitrate dehydrogenase

RXN “4”: Ketoglutarate to Succinyl CoA Another oxidative decarboxylation Resembles pyruvate decarboxylation! Ketoglutarate dehydrogenase

RXN “5”: Succinyl CoA to Succinate Succinyl-CoA is a high-energy compound. Energy is transformed to phosphoryl transfer potential Succinyl CoA synthetase

FINAL 3 STEPS Key: reactions of 4C species Regeneration of oxaloacetate from succinate

RXN “6”: Succinate to Fumarate Oxidation Succinate dehydrogenase

RXN “7”: Fumarate to malate By Fumarase

RXN “8”: Malate to Oxaloacetate Oxidation By malate dehydrogenase

Net of TCA Cycle SUMMARY Acetyl CoA + 3 NAD+ + FAD + GDP + Pi + 2H2O -> 2CO2 + 3 NADH + FADH2 + GTP + 2H+ + CoA SUMMARY C2 enters and joins oxaloacetate (C4). Two C atoms leave as CO2 Four pairs of H leave in four redox rxns One compound with high phosphorylation transfer potential (GTP) is generated Two molecules of water are consumed

TCA Cycle

TCA is regulated

TCA is a source of precursors