🚀 go-pugleaf

I'm not a "pretty impressive biochemist" so I'd appreciate hearing the
criticisms *before* I try and suggest anything...:)

But anyway, my view is that if you are given a starting material and a
desired product then there are only so many ways to achieve the specific
conversion.  There aren't too many ways to oxidize -CH2-CH2- to -C(O)-CH2-.
So there *may* be structural homology between the FA and TCA enzymes due
to common ancestry (divergent evolution), but there *may not* (i.e. 
convergent evolution of enzymes to do similar reactions in similar ways).

    That said, FAD-using and NAD-using enzymes would be expected to share
structural similarity in the cofactor-binding portions of the enzymes; the
CoA binding in the FA metabolic enzymes obviously is unnecessary in the
TCA enzymes however.  So we would expect similarities *and* differences, 
and this is actually what we find when comparing any two enzymes that 
perform similar but non-identical reactions.

   As to *why* always FAD and NAD, this seems to be a case of divergent 
evolution.  (Convergent evolution has happened for other kinds of enzymes.)

   As to *why* CH2-CH2 ----> C(O)-CH2?  I would guess that this reliance on
acetyl group formation (and acetyl CoA) allows a "common currency" of 
metabolism so that the TCA and the FA metabolic pathways ultimately 
respond to each other.  This is useful since fat is used as an energy
storage --- you stockpile it when you can and don't use it until you have to.

That's my attempt.  Seriously I am curious as to what objections have been
raised and what stances they were raised against!!

Ed Young