Is the pyruvate dehydrogenase reaction considered part of the Krebs Tricarboxylic Acid Cycle?

Question

I’ve encountered questions where I’m asked to find out the CO₂ released, the number of oxidative carboxylations etc. in the Krebs tricarboxylic acid cycle. I always include the pyruvate dehydrogenase reaction (sometimes termed the ‘link reaction’) that converts pyruvate (from glucose) to acetyl-CoA which then feeds into the Krebs cycle. However my answers are only considered correct for certain questions, while for the others they are not.

How do I decide if a reference to the Krebs cycle includes the pyruvate dehydrogenase reaction?

Answer 1

It is outside both the remit and competence of this site to divine the intentions of university teachers in questions that they set to students. What we can do is explain the scientific points that may be at issue, in this case why:

the Krebs tricarboxylic acid cycle does not include the pyruvate dehydrogenase reaction.

• First is the obvious fact that the pyruvate dehydrogenase (PDH) reaction is not part of this cycle of linked reactions, any more than the conversion of glutamate to α-ketoglutarate (2-oxoglutarate) is.
• But even if you consider acetyl-CoA as somehow “starting” the cycle, the PDH reaction is not required to generate it. Acetyl-CoA can also be produced from the breakdown of fatty acids or certain amino acids.
• Further, acetyl-CoA produced from pyruvate need not feed into the TCA cycle: in some circumstances it is converted to malonyl-CoA and to fatty acids.

The diagram below (a fossil of my biochemistry lecturing career) illustrates some of the various possible inputs into the TCA cycle in a general sense: