|
The aldo-keto reductases metabolize a wide range of substrates and are potential
drug targets. This protein superfamily includes aldose reductases, aldehyde
reductases, hydroxysteroid dehydrogenases and dihydrodiol dehydrogenases. By
combining multiple sequence alignments with known three-dimensional structures
and the results of site-directed mutagenesis studies, we have developed a
structure/function analysis of this superfamily. Our studies suggest that the
(alpha/beta)8-barrel fold provides a common scaffold for an NAD(P)(H)-dependent
catalytic activity, with substrate specificity determined by variation of loops
on the C-terminal side of the barrel. All the aldo-keto reductases are dependent
on nicotinamide cofactors for catalysis and retain a similar cofactor binding
site, even among proteins with less than 30% amino acid sequence identity.
Likewise, the aldo-keto reductase active site is highly conserved. However, our
alignments indicate that variation ofa single residue in the active site may
alter the reaction mechanism from carbonyl oxidoreduction to carbon-carbon
double-bond reduction, as in the 3-oxo-5beta-steroid 4-dehydrogenases
(Delta4-3-ketosteroid 5beta-reductases) of the superfamily. Comparison of the
proposed substrate binding pocket suggests residues 54 and 118, near the active
site, as possible discriminators between sugar and steroid substrates. In
addition, sequence alignment and subsequent homology modelling of mouse liver
17beta-hydroxysteroid dehydrogenase and rat ovary 20alpha-hydroxysteroid
dehydrogenase indicate that three loops on the C-terminal side of the barrel
play potential roles in determining the positional and stereo-specificity of the
hydroxysteroid dehydrogenases. Finally, we propose that the aldo-keto reductase
superfamily may represent an example of divergent evolution from an ancestral
multifunctional oxidoreductase and an example of convergent evolution to the
same active-site constellation as the short-chain dehydrogenase/reductase
superfamily.
|
