Citric Acid Cycle

TCA cycle steps, reactions, and descriptions

Conversion of pyruvate to acetyl CoA

Precursor step to TCA cycle

1. Removal of hydroxyl group, hydroxyethyl is bound to pyruvate dehydrogenase

2. Hydroxyethyl is oxidized to acetyl, reducing NAD to NADH

3. The acetyl group is transferred to Coenzyme A, producing acetyl CoA

Citrate synthase

Step 1

Condensation of acetyl-CoA with oxaloacetate to form citrate, catalyzed by citrate synthase

1. oxaloacetate is joined with acetyl-CoA

2. water molecule attacks the acetyl

3. Coenzyme A is released from the complex

Acontinase

Step 2

Citrate becomes isomer, isocitrate by removal of water from 3’ and addition of water to 4’

Isocitrate dehydrogenase

Step 3

oxidative decarboxylation of isocitrate to form α-ketoglutarate

1. oxidation of -OH group at 4’ (NAD to NADH)

2. intermediate has CO₂ removed to yield alpha-ketoglutarate

Alpha-ketoglutarate dehydrogenase

Step 4

1. Alpha-ketoglutarate is oxidized, NAD is reduced to NADH

2. CO₂ is removed and CoA is added to form the 4-carbon succinyl-CoA

Succinyl-CoA synthase

Step 5

CoA is removed from succinyl-CoA to produce succinate

GDP and free phosphate is used to make GTP through substrate-level phosphorlyation

GTP is converted to ATP

Succinate dehydrogenase

Step 6

Succinate is oxidized to fumarate, enzyme catalyzes removal of 2 hydrogens

FAD is reduced to FADH₂

Fumarase (fumarate hydratase)

Step 7

reversible hydration of fumarate to L-malate by adding H₂O back to the substrate

Malate dehydrogenase

Step 8

Malate is oxidized to produce oxaloacetate

NAD is reduced to NADH

TCA cycle net equation