Novel NADPH-binding domain revealed by the crystal structure of aldose reductase

ALDOSE reductase is the first enzyme in the polyol pathway and catalyses the NADPH-dependent reduction of D-glucose to D-sorbitol. Under normal physiological conditions aldose reductase participates in osmoregulation 1 , but under hyperglycaemic conditions it contributes to the onset and development of severe complications in diabetes 2 . Here we present the crystal structure of pig lens aldose reductase refined to an R -factor of 0.232 at 2.5-Å resolution. It exhibits a single domain folded in an eight-stranded parallel α/β barrel, similar to that in triose phosphate isomerase 3 and a score of other enzymes. Hence, aldose reductase does not possess the expected canonical dinucleotide-binding domain 4 . Crystallographic analysis of the binding of 2′-monophospho-adenosine-5′-diphosphoribose, which competitively inhibits NADPH binding reveals that it binds into a cleft located at the C-terminal end of the strands of the α/β barrel. This represents a new type of binding for nicotinamide adenine dinucleotide coenzymes.