Crystal structure of the NADP(H)-dependent ketose reductase from Bemisia argentifolii at 2.3 angstrom resolution

Polyhydric alcohols are widely found in nature and can be accumulated to hi gh concentrations as a protection against a variety of environmental stress es. It is only recently, however, that these molecules have been shown to b e active in protection against heat stress, specifically in the use of sorb itol by the silverleaf whitefly, Bemisia argentifolii. We have determined t he structure of the enzyme responsible for production of sorbitol in Bemisi a argentifolii, NADP(H)-dependent ketose reductase (BaKR), to 2.3 Angstrom resolution. The structure was solved by multiwavelength anomalous diffracti on (MAD) using the anomalous scattering from two zinc atoms bound in the st ructure, and was refined to an R factor of 21.9 % (R-free = 25.1%). BaKR be longs to the medium-chain dehydrogenase family and its structure is the fir st for the sorbitol dehydrogenase branch of this family. The enzyme is tetr americ, with the monomer having a very similar fold to the alcohol dehydrog enases (ADHs). Although the structure determined is for the apo form, a pho sphate ion in the active site marks the likely position for the adenyl phos phate of NADP(H). The catalytic zinc ion is tetrahedrally coordinated to Cy s41, His66, Glu67 and a water molecule, in a modification of the zinc site usually found in ADHs. This modified zinc site seems likely to be a conserv ed feature of the sorbitol dehydrogenase sub-family. Comparisons with other members of the ADH family have also enabled us to model a ternary complex of the enzyme, and suggest how structural differences may influence coenzym e binding and substrate specificity in the reduction of fructose to sorbito l. (C) 2001 Academic Press.