1. T R I C A R B O X Y L I C A C I D | C I T R I C A C I D C Y C L E
KREB’S CYCLE
2. OVERVIEW
• Involves the oxidation of acetyl coenzyme A (acetyl
CoA) along with the reduction of coenzymes, which
are subsequently reoxidized to produce ATP.
• Occurs in the mitochondria of the cell. All enzymes
are present in the matrix of the mitochondria
except for succinate dehydrogenase.
5. STEP 1
Cycle begins with the combination of acetyl CoA and oxaloacetate
to produce citrate.
Enzyme: citrate synthetase.
Reaction Type: hydrolysis & nonhydrolytic cleavage.
7. STEP 3
Isocitrate from rearragement of citrate is oxidized. The oxidation of the
hydroxyl group of isocitrate results in an unstable intermediate that
decarboxylates to alpha ketoglutarate
Enzyme: isocitrate dehydrogenase
8. STEP 4
Alpha ketoglutarate is converted to succinyl CoA. Co2 is released, and
succinyl CoA, NADH and hydrogen are produced.
Enzyme: Alpha ketodehydrogenase
9. STEP 5
Succinyl CoA is cleaved to succinate
Cleavage of the high-energy thioester bond of succinyl CoA provides energy
for the substrate level phosphorylation of GDP to GTP.
Enzyme: succinyl CoA synthetase
10. STEP 6
Succinate is oxidized to fumarate.
Succinate transfers two protons together with their electrons to FAD, which forms FADH2.
Enzyme: succinate dehydrogenase
Takes place in the inner mitochondrial membrane
11. STEP 7
Fumarate is converted to malate by the addition of water across the double bond.
Enzyme: Fumarase
12. STEP 8
Oxaloacetate is formed in the last step by oxidation of the hydroxyl
group of malate
Enzyme: malate dehydrogenase
14. NET RESULTS
Acetyl CoA + 3 NAD+ + FAD + ADP or GDP + Pi + 2 H20
2 CO2 + 3NADH + 3H+ + FADH2 + ATP or GTP + CoA
15. ENERGY PRODUCTION
• The NADH and FADH2 (produced by the cycle)
donate electrons to the electron transport chain.
For each mole of NADH, about 2.5moles of ATP are
generated, and for each mole of FADH2, about 1.5
moles of ATP are generated by the passage of
electrons to oxygen.
• Total energy generated by one round of the cycle
is about 10 moles of ATP.