David Moss david.moss at ifia.fzk.de
Thu Nov 28 14:48:45 EST 1996

Melissa M <mirage at sympatico.ca> schrieb im Beitrag
<329D0764.7E23 at sympatico.ca>...
> There is something I have come along  in my studies that I just cannot 
> seem to grasp. If you can help me if would be appreciated.
> I do not understand co-factors and co-enzymes. I understand the substrate
> and the active site. I also do not understand how ATP and NAD or NADH
> play a role in this. If you could explain this to me in simple terms it
> would be appreciated

You're probably getting confused by some irrelevant, historical
A coenzyme (e.g. ATP, NAD) is simply a second substrate. 
Enzymes don't only catalyze reactions involving one substrate, they can
also catalyze bimolecular reactions, i.e. those involving two substrates.
Some of these "2nd substrates" are used by so many different enzymes
that the special designation "co-enzyme" seems approriate.
For example, there are a huge number of NAD-linked dehydrogenases that
catalyze the oxidation of various substrates: all of these actually
the bimolecular oxidation of one substrate by another, the latter being NAD
in all cases.
Cofactors (e.g. haem, flavin) are different: these are permanently attached
to the enzyme (usually covalently), are involved in catalysis and like any
behaved catalyst are not consumed during the reaction. They are just as
a part of the enzyme as any one of its amino acids. Sometimes the cofactor
can be seperated from the rest of the protein by chemical treatment: the
free enzyme is referred to as the "apoprotein", and has no enzymatic

Hope that helps,
David Moss

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