Metabolism TCA Cycle Looking back at glycolysis Glucose

Metabolism: TCA Cycle

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

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

THE TCA CYCLE ü 8 steps, most common entry point is Co. A (C 2) ü Key: Oxidation of one acetyl group to two CO 2 ü 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 ü Pyruvate + Co. A + NAD+ -> Acetyl Co. A + CO 2 + NADH ü NET REACTION: Pyruvate is oxidatively decarboxylated to form acetyl-Co. A ü Pyruvate dehydrogenase uses TPP, Co. ASH, lipoic acid, FAD and NAD+

Entry into the TCA cycle üThree basic steps

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

Entry into the TCA cycle ü Step 2: Oxidation of hydroxymethyl group on TPP (pyruvate dehydrogenase E 1)

Entry into the TCA cycle üStep 3: Acetyl transfer to Co. A (dihydrolipoyl transacetylase E 2)

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

Amazing pyruvate dehydrogenase complex üFlexible linkages allow lipoamide to move between different active sites üEight catalytic trimers

Begin the TCA cycle!

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

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

RXN 2: Citrate to Isocitrate üIsomerization for proper oxidative decarboxylation later üAconitase used

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

RXN “ 4”: Ketoglutarate to Succinyl Co. A ü Another oxidative decarboxylation ü Resembles pyruvate decarboxylation! ü Ketoglutarate dehydrogenase

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

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

RXN “ 6”: Succinate to Fumarate üOxidation üSuccinate dehydrogenase

RXN “ 7”: Fumarate to malate üBy Fumarase

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

Net of TCA Cycle ü Acetyl Co. A + 3 NAD+ + FAD + GDP + Pi + 2 H 2 O -> 2 CO 2 + 3 NADH + FADH 2 + GTP + 2 H+ + Co. A ü SUMMARY 1. C 2 enters and joins oxaloacetate (C 4). Two C atoms leave as CO 2 2. Four pairs of H leave in four redox rxns 3. One compound with high phosphorylation transfer potential (GTP) is generated 4. Two molecules of water are consumed

TCA Cycle

TCA is regulated

TCA is a source of precursors