Molecular basis for thermoprotection in Bemisia: structural differences between whitefly ketose reductase and other medium-chain dehydrogenases/reductases

The silverleaf whitefly (Bemisia argentifolii, Bellows and Perring) accumul ates sorbitol as a thermoprotectant in response to elevated temperature. So rbitol synthesis in this insect is catalyzed by an unconventional ketose re ductase (KR) that uses NADPH to reduce fructose. A cDNA encoding the NADPH- KR from adult B. argentifolii was cloned and sequenced to determine the pri mary structure of this enzyme. The cDNA encoded a protein of 352 amino acid s with a calculated molecular mass of 38.2 kDa. The deduced amino acid sequ ence of the cDNA shared 60% identity with sheep NAD(+)-dependent sorbitol d ehydrogenase (SDH). Residues in SDH involved in substrate binding were cons erved in the whitefly NADPH-KR. An important structural difference between the whitefly NADPH-KR and NAD(+)-SDHs occurred in the nucleotide-binding si te. The Agp residue that coordinates the adenosyl ribose hydroxyls in NAD()-dependent dehydrogenases (including NAD(+)-SDH), was replaced by an Ala i n the whitefly NADPH-KR. The whitefly NADPH-KR also contained two neutral t o Arg substitutions within four residues of the Asp to Ala substitution. Mo lecular modeling indicated that addition of the Arg residues and loss of th e Asp decreased the electric potential of the adenosine ribose-binding pock et, creating an environment favorable for NADPH-binding. Because of the abi lity to use NADPH, the whitefly NADPH-KR synthesizes sorbitol under physiol ogical conditions, unlike NAD+-SDHs, which function in sorbitol catabolism. (C) 1999 Elsevier Science Ltd. All rights reserved.