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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?

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    $\begingroup$ It would help to get examples of such specific questions, where you got it right or wrong, to understand the point of confusion. $\endgroup$ Commented Apr 1, 2022 at 22:53
  • $\begingroup$ I have rewritten your question in a format that is more precise scientifically and focusses more on the scientific question, rather than your person difficulties interpreting your teachers' questions. If after reading and understanding my answer you still are puzzled by your marks you should ask your teachers why you were marked incorrect in particular cases. I see from an internet search that the term "link reaction" is used at an elementary level. This is imprecise and not used by biochemists, which is why I have substituted pyruvate dehydrogenase reaction. $\endgroup$
    – David
    Commented Apr 2, 2022 at 10:59

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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:

TCA cycle and inputs

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