Questions of the Week (2 Feb)

Where does the CO2 in the Krebs cycle come from? Does the Krebs cycle need O2?
As noted in lecture, O2 is not involved in Krebs cycle but only in oxidative phosphorylation as the final electron acceptor.
The 2 oxygen molecules come directly from the compounds involved in the Krebs cycle. There are 3 carboxylic acid groups in the citrate as noted in the diagram below which showcases the cycle in more details.

In Krebs cycle, ATP is produced by substrate-level phosphorylation.
Does the alpha-ketoglutarate have a phosphate group? If not, where did the phosphate group come from?



(Extra) With reference to the diagrams above and below, an inorganic phosphate group is taken from the matrix and added to succinyl CoA by an enzyme before removal in forming succinate. Since the phosphate group (in transition) comes from succinyl CoA, it is considered a substrate-level phosphorylation.









































What triggers off the anaerobic pathway?

Under aerobic condition, pyruvate is converted to acetyl-CoA by an enzyme pyruvate dehydrogenase. The enzyme is allosterically regulated and will be inhibited by high levels of NADH and CoA which will accumulate in the mitochondrial matrix if oxygen is absent and oxidative phosphorylation is inhibited.
Thus pyruvate can only undergo the anaerobic pathway to form ethanol or lactate to continue generating ATP under glycolysis.