Chemistry Reference
In-Depth Information
giving rise to a hydroxide that attacks the scissile carbonyl carbon, with formation of a tetrahedral intermediate.
The latter interacts in a bidentate manner with the zinc ion, with one of its hydroxyls occupying the position of the
catalytic solvent in the substrate-depleted enzyme. The glutamate subsequently acts as a general acid catalyst,
delivering the proton captured from the solvent to the scissile-bond nitrogen, which becomes a secondary
ammonium function.
Alcohol Dehydrogenases
Alcohol dehydrogenases are a class of zinc enzymes which catalyse the oxidation of primary and secondary alcohols
to the corresponding aldehyde or ketone by the transfer of a hydride anion to NAD
þ
with release of a proton.
They are part of a very large family of short- and medium-chain dehydrogenases/reductases, which account for
about 82 and 25 genes, respectively, in the human genome. The diversity of reactions catalysed by members of the
SDR family are summarised in
Figure 12.9
.
By far, the most extensively studied alcohol dehydrogenases are those
of mammalian liver. They are dimeric proteins, with each subunit binding two Zn
2
þ
ions, only one of which is
catalytically active. This catalytic Zn
2
þ
ion has distorted tetrahedral geometry, coordinated to one histidine and
two cysteine residues. The noncatalytic zinc plays a structural role and is coordinated tetrahedrally to four cysteine
residues. The essential features of the catalytic cycle are summarised in
Figure 12.10
.
After binding of NAD
þ
, the
water molecule is displaced from the zinc atom by the incoming alcohol substrate. Deprotonation of the
coordinated alcohol yields a zinc alkoxide intermediate, which then undergoes hydride transfer to NAD
þ
to give
the zinc-bound aldehyde and NADH. A water molecule then displaces the aldehyde to regenerate the original
catalytic zinc centre, and finally NADH is released to complete the catalytic cycle.
Thus, the role of zinc in the dehydrogenation reaction is to promote deprotonation of the alcohol, thereby
enhancing hydride transfer from the zinc alkoxide intermediate. Conversely, in the reverse hydrogenation reaction,
its role is to enhance the electrophilicity of the carbonyl carbon atom. Alcohol dehydrogenases are exquisitely
stereospecific and, by binding their substrate via a three-point attachment site (
Figure 12.11
), they can distinguish
between the two methylene protons of the prochiral ethanol molecule.
Other Mononuclear Zinc Enzymes
We have already seen the diversity of function in the lyases, hydrolases, and oxidoreductases. Several other types
of zinc coordination are found in a number of other enzymes, illustrated in
Figure 12.12
. These include enzymes
with the coordination motif [(His)
2
(Cys) Zn
2
þ
-OH
2
], found in the lysozyme of bacteriophage T7, or [(Cys)
3
Zn
2
þ
-
OH
2
] which occurs in 5-aminolaevulinate dehydratase (or porphobilinogen synthase). This latter enzyme catal-
yses the condensation of two molecules of 5-aminolaevulinate to form the pyrrole precursor of the porphyrins
(haem, chlorophyll, and cobalamines), and its inhibition by Pb
2
þ
is the cause of lead poisoning (saturnism),
frequently observed among inner city children (Chapter 1).
Tetrahedral structural sites typically only involve coordination by the protein, frequently by cysteine residues,
as illustrated by the structural [Cys
4
Zn
II
] site in liver alcohol dehydrogenase. However, a class of zinc proteins and
enzymes with tetrahedral “non-aqua” functional zinc sites have emerged in which the activity centres upon the
reactivity of a zinc thiolate linkage rather than of a zinc-bound water molecule. The first to be discovered was the
Ada DNA repair protein (
Figure 12.12
) which has a [(Cys)
4
Zn] motif, and whose function is to repair damage to
DNA due to methylation. The Ada protein achieves the repair by undergoing sacrificial alkylation of one of its zinc
cysteine thiolate ligands (
Figure 12.13
). Thus, Ada does not act as an enzyme, but rather as a reagent (hence its
