Originally Posted by ajdelange
As yeast's main source if reducing power is NADH I'd guess (but I do not know - I don't know that much biochem in general) that NADH is the reducing agent for conversion of diacetyl to Acetoin:
2(CH3)(CHO)(CHO)(CH3) + H+ NADH --> 2(CH3)(CHOH)(CHO)(CH3) + NAD+
and for the conversion of acetoin to 2,3 butane diol:
2(CH3)(CHOH)(CHO)(CH3) + H+ NADH --> 2(CH3)(CHOH)(CHOH)(CH3) + NAD+
Seems to me that if the H were supplied by the enzyme, it would become oxidized and unable to catalyze further reduction.
The reaction you wrote above is an acid/base reaction. The ultimate
source of the H+ (the acid) is some redox reaction far earlier in
the sequence, but ultimately it is used in an aqueous acid base reaction.
For a mechanism involving histidine as an acid/base, see the diagram
labeled "Serine Protease Mechanism" on this page:
Your molecular structure and stoichiometry above is incorrect. You've
2(CH3)(CHO)(CHO)(CH3) + H+ NADH --> 2(CH3)(CHO)(CHO)(CH3) + H+ NADH --> 2(CH3)(CHOH)(CHO)(CH3) + NAD+ + NAD+
But the diketone on the left should be
(CH3)(CO)(CO)(CH3) + 2H+ + 2NADH - - > (CH3)(CHOH)(CHOH)(CH3)
Each carbonyl requires 2 moles of H, one formally H+ and one
formally H- (minus), the NADH supplies the H-, the H+ is
supplied by the enzyme usually through some acidic source such
as a protonated histidine or a protonated serine or protonated
tryptophan. A reverse reaction would be the conversion of
ethanol to ethanal:
CH3CH2OH + NAD+ ---> CH3CH=O + NADH + H+
Here the NAD+ picks up an H- and the H+ is picked
up by some part of the enzyme (a histidine or serine residue
or other etc).
But all of this is overkill for most of the members of this
list. All the op needs to know is that the yeast are chemical
factories that absorb things from solution and convert them
to something else, one thing in particular being O2.
I know you are trying to help by explaining, but if you are
going to be very technical, you have to at least be correct,
otherwise you are just creating more confusion. Your comment
about Lewis structures involving nothing but acid/base
chemistry is also not correct, but I'm not going to get into
that here because I don't think too many here would care.