Question

Questions in regards to acetyl CoA in the TCA/Kreb's Cycle.

1. How many cycles minimum would it take for an acetyl CoA carbon to be removed? (What is the first step that this removal could happen?)

2. Why can't we know for sure how many turns it takes?

Answer 1

During the process of glucose oxidation one glucose molecule can split into two acetyl CoA molecules in the glycolysis. The pyruvate generated by the glycolysis is converted into acetyl-CoA (methyl group labeled with 14C) that enters into the citric acid cycle. Citric acid cycle involves a series of reactions that occur in the presence of oxygen. Citric acid cycle generates NADH, which enters into the oxidative phosphorylation process. This cycle occurs in mitochondrial matrix and generates one ATP molecule only.

Acetyl co-A (14C at its methyl group) + 3 NAD+ + FAD+ + inorganic phosphate + GDP + 2 water molecules -----> Coenzyme A-SH + 3 NADH + 3H+ + FADH2 + 1 GTP + 2 CO2 (14-C label come out as carbon with 14C- CO2)

Carbon molecules removed from acetyl CoA:

For removing acetyl CoA carbon molecule one complete citric cycle was minimum required.

Only one cycle does it take to convert Acetyl CoA to CO2 at step 5 and step 6 and it will take one cyclerequired to release 14C- labeled acetyl coA (labeled at methyl group of acetyl coA) carbon in the form of 14C-labeled CO2 in the middle of the cycle as a metabolic waste product in the mitochondrial matrix

Citric acid cycle Step 5: Oxalosuccinate (methyl group has 14C radiolabel) -----> Alfa -Ketoglutarate + CO2 (come out as 14 C radiolabel).

Citric acid cycle Step 6: Alfa -Ketoglutarate (methyl group has 14C radiolabel)   + NAD+ + CoA-SH -----> Succinyl-CoA + NADH + H+ + CO2 (come out as 14C radiolabel)   & this takes place in the presence of Alfa -Ketoglutarate dehydrogenase.