Krebs cycle

step 1

a-CoA (2C acetyl group) is transferred or synthesized with/to oxaloacetate(4C), forming citrate (6C)

• oxaloacetate, is already present in mitochondria

• condensation reaction 

• enzyme- citrate synthase 

step 2/3 

citrate (6C) is rearranged into its isomer isocitrate (6C) 

• isomerisation

• enzyme- aconitase 

step 4

isocitrate (6C) is oxidized to form alpha-ketoglutarate (5C), one carbon is removed and released as CO2 and NAD+ is reduced to NADH + H+

• first oxidation 

• enzyme- isocitrate dehydrogenase 

step 5 

alpha ketoglutarate (5C) is oxidized to form succinyl CoA (4C), one carbon is removed and released as CO2 and NAD+ is reduced to NADH + H+

• second oxidation 

• enzyme- alpha ketoglutarate dehydrogenase 

step 6

release of CoA produces succinate (4C), the energy released converts GDP to GTP which is a coupled reaction with the substrate level phosphorylation of APD→ ATP

• substrate level phosphorylation

• enzyme- succinyl CoA synthethase

step 7

succinate is oxidized to form fumarate(4C), 2 electrons and 2 protons are removed and transferred to FAD to form FADH2

• third oxidation

• enzyme- succinate dehydrogenase

step 8

fumarate is converted into malate by the addition of H2O 

• third oxidation reaction 

• enzyme- fumarase 

step 9

malate is oxidized to produce oxaloacetate, reducing NAD+ →NADH + H+

• fourth oxidation and regeneration of oxaloacetate

• oxaloacete can react with acetyl CoA to re enter cycle

• enzyme- malate dehydrogenase