Question

6. Describe the sources of acetyl groups that enter the Citric Acid Cycle. 7. Show and discuss that reaction that involves citrate synthase. List the substrates and product(s), 8. Show and discuss that reaction that involves aconitase. List the substrates and product(s). 9. Show and discuss that reaction that involves isocitrate dehydrogenase. List the substrates and product(s).

Answer 1

Answer 1. The primary sources of acetyl-CoA is from the breakdown of sugars by glycolysis which yield pyruvate that in turn is decarboxylated by the pyruvate dehydrogenase complex generating acetyl-CoA.

Answer 2. Citrate synthase catalyzes the condensation reaction of the two-carbon acetate residue from acetyl coenzyme A and a molecule of four-carbon oxaloacetate to form the six-carbon citrate.

acetyl-CoA + oxaloacetate + H₂O → citrate + CoA-SH.

Oxaloacetate is regenerated after the completion of one round of the Krebs cycle which is the first substrate to bind to the enzyme. This induces the enzyme to change its conformation, and creates a binding site for the acetyl-CoA.

Answer 3. Aconitase is an enzyme that confers the ability to catalyze following equilibrium.

Citrate = isocitrate in the presence of aconitase and water.

This reaction is also part of Kreb cycle.

Answer 4. Isocitrate dehydrogenase is an enzyme that catalyzes the oxidative decarboxylation of isocitrate that produce alpha ketoglutarate + CO2.

This is accomplished in two step

1. Oxidation of isocitrate to oxalosuccinate
2. Decarboxylation of carboxyl group beta to keto and forming Alpha-ketoglutarate.