Crystal structure of SQD1, an enzyme involved in the biosynthesis of the plant sulfolipid headgroup donor UDP-sulfoquinovose

The SQD1 enzyme is believed to be involved in the biosynthesis of the sulfo quinovosyl headgroup of plant sulfolipids, catalyzing the transfer of SO3- to UDP-glucose. We have determined the structure of the complex of SQD1 fro m Arabidopsis thaliana with NAD(+) and the putative substrate UDP-glucose a t 1.6-Angstrom resolution. Both bound ligands are completely buried within the binding cleft, along with an internal solvent cavity which is the likel y binding site for the, as yet, unidentified sulfur-donor substrate. SQD1 i s a member of the short-chain dehydrogenase/reductase (SDR) family of enzym es, and its structure shows a conservation of the SDR catalytic residues. A mong several highly conserved catalytic residues, Thr-145 forms unusually s hort hydrogen bonds with both susceptible hydroxyls of UDP-glucose. A His s ide chain may also be catalytically important in the sulfonation.