Predict which one of the five steps of the \alpha-ketoglutarate dehydrogenase complex reaction is metabolically irreversible under physiological conditions.
Step 1. Decarboxylation.
Step 2. Oxidation of 4-carbon group, reduction of lipoamide disulfide.
Step 3. Transacylation.
Step 4. Dihydrolipoyl dehydrogenase.
Step 5. FADH2 enzymatically reoxidized by NAD+ to form NADH .
