During catalysis
by liver alcohol dehydrogenase (ADH), a water
bound to the catalytic zinc is replaced by the oxygen of the substrates.
The mechanism might involve a pentacoordinated zinc or a double-displacement
reaction with participation by a nearby glutamate residue, as suggested
by studies of human ADH3, yeast ADH1, and some other tetrameric ADHs.
Zinc coordination and participation of water in the enzyme mechanism
were investigated by X-ray crystallography. The apoenzyme and its
complex with adenosine 5′-diphosphoribose have an open protein
conformation with the catalytic zinc in one position, tetracoordinated
by Cys-46, His-67, Cys-174, and a water molecule. The bidentate chelators
2,2′-bipyridine and 1,10-phenanthroline displace the water
and form a pentacoordinated zinc. The enzyme–NADH complex has
a closed conformation similar to that of ternary complexes with coenzyme
and substrate analogues; the coordination of the catalytic zinc is
similar to that found in the apoenzyme, except that a minor, alternative
position for the catalytic zinc is ∼1.3 Å from the major
position and closer to Glu-68, which could form the alternative coordination
to the catalytic zinc. Complexes with NADH and N-1-methylhexylformamide
or N-benzylformamide (or with NAD+ and
fluoro alcohols) have the classical tetracoordinated zinc, and no
water is bound to the zinc or the nicotinamide rings. The major forms
of the enzyme in the mechanism have a tetracoordinated zinc, where
the carboxylate group of Glu-68 could participate in the exchange
of water and substrates on the zinc. Hydride transfer in the Michaelis
complexes does not involve a nearby water.
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