[Mike Dacre]

I think I am little confused about what a coenzyme is I think. My understanding is the definition of a coenzyme is a compound that helps an enzyme carry out its function. However the function of pyruvate dehydrogenase is to decarboxylate pyruvate, attach it to coenzyme A, and reduce NAD in the process.

I am a little confused about the definition though, because even though coenzyme A is recycled, from the perspective of PDH it is a substrate. The reaction has the (simplified) form of A + B  C + D, or pyruvate + CoA acetyl-CoA + CO₂. Could someone explain what makes this a coenzyme instead of a substrate?

Thanks!

[Babcock_Hall]

We had a thread on this some time ago, called cosubstrate:  http://www.chemicalforums.com/index.php?topic=77497.msg282809#msg282809

My present view is that your initial definition is fine.  Some coenzymes are catalytic (pyridoxal phosphate and biotin come to mind).  Some coenzymes are cosubstrates that are recycled (NAD and coenzyme A come to mind).  And to confuse matters further, a few enzymes have tightly bound NAD that does not dissociate with each catalytic cycle.