Question

Which one of the following best explains why the PDH cofactor thiamine pyrophosphate (TPP) is considered a catalytic cofactor rather than a stoichiometric cofactor?

a.) TPP is covalently linked to the PDH complex.
b.) TPP is an enzyme and can serve as a catalyst on its own.
c.) TPP is a metal-containing cofactor.
d.) TPP does not serve as a substrate for the PDH complex.

Answer 1

The correct option is (d), as thiamine pyrophosphate (TPP) functions as a reusable, non-substrate catalytic cofactor in the pyruvate dehydrogenase (PDH) complex, which is essential for mitochondrial energy production.

Step-by-step explanation:

Among the options provided, the best explanation for why thiamine pyrophosphate (TPP) is considered a catalytic cofactor rather than a stoichiometric cofactor is that TPP does not serve as a substrate for the PDH complex. This means option (d) is correct. A catalytic cofactor is a term used to describe coenzymes or metal ions that participate in the enzyme's catalytic process but are not consumed in the reaction. Since TPP, which is a derivative of vitamin B1, can be reused again in the catalytic cycle of the reaction, it is classed as a catalytic cofactor.

TPP plays a significant role in the mechanism of the enzyme transketolase by aiding in the transfer of a two-carbon moiety during a reaction, referred to as the transketolation reaction. In the context of the pyruvate dehydrogenase (PDH) complex, TPP is involved in the oxidative decarboxylation of pyruvate into acetyl-CoA, thereby contributing to the mitochondrial energy production pathway.