Download 5. TCA Cycle

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

Acetyl CoA + 3 NAD+ + FAD + GDP + Pi + 2H2O -> 2CO2 + 3 NADH +
FADH2 + GTP + 2H+ + CoA
 SUMMARY
1. C2 enters and joins oxaloacetate (C4). Two C atoms
leave as CO2
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