Submitted to: Journal of Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 13, 2000
Publication Date: N/A
Interpretive Summary: The silverleaf whitefly (Bemisia argentifolii) is a major agronomic pest on a variety of crop species including cotton. Whiteflies are particularly destructive in warmer climates and they thrive even in such hot arid regions as the desert southwest of the United States. Whiteflies feed on plant sap to obtain nutrients including sugars and amino acids. Previously, we showed that, when exposed to high temperatures, whiteflies synthesize and accumulate sorbitol a compound known to protect organisms against temperature extremes and desiccation. Production of sorbitol in whiteflies requires a specific enzyme called ketose reductase (KR). In this paper, we report the atomic structure of whitefly KR, determined by X-ray crystallography. The structure of KR shows the exact position of most of the atoms of the molecule. The positions of these atoms indicates how this enzyme carries out its function of making sorbitol. This knowledge can be used to design specific inhibitors of KR. By targeting the sorbitol- making enzyme, these inhibitors could be used as a novel strategy for controlling whiteflies.
Technical Abstract: Polyols can be accumulated to high concentrations as a protection against a variety environmental stresses, including heat stress by the silverleaf whitefly, Bemisia argentifolii. We have determined the structure of the enzyme responsible for production of sorbitol in Bemisia argentifolii, NADP(H)-dependent ketose reductase (BaKR), to 2.3 A resolution. The structure was solved by multiwavelength anomalous diffraction (MAD) using the anomalous scattering from two zinc atoms bound in the structure, and was refined to an R factor of 21.9% (Rfree = 25.1%). BaKR belongs to the medium-chain dehydrogenase family and its structure is the first for the sorbitol dehydrogenase branch of this family. The enzyme is tetrameric, with the monomer having a very similar fold to the alcohol dehydrogenases (ADHs). Although the structure determined is for the apo form, a phosphate ion in the active site marks the likely position for the adenyl phosphate of NADP(H). The catalytic zinc ion is tetrahedrally coordinated, to Cys41, His66, Glu67 and a water molecule, in a modification of the zinc site usually found in ADHs. Structural comparisons with other members of the ADH family have enabled us to model a ternary complex of the enzyme, and suggest how the differences may affect coenzyme binding and substrate specificity in the reduction of fructose to sorbitol.