Question

Why are amino acids that can only deliver their carbon skeletons to the citric acid cycle in the form of acetyl-CoA not glucogenic? In other words, why can their carbon skeletons not form glucose?

Answer 1

In order to make glucose acetyl-CoA that is produced from the amino acids breakdown must be get converted into pyruvate and then pyruvate can make glucose by gluconeogenesis. But the reaction that make acetyl-CoA from pyruvate (catalyzed by pyruvate dehydrogenase) is irreversible, means only pyruvate can be converted into acetyl-CoA but acetyl-CoA can not get converted back to pyruvate. Animal cells do not have any other way to convert the acetyl-CoA form upon breakdown of amino acids into pyruvate, hence they can not make glucose.