Prediction of the Active-Site Structure and NAD+ Binding in SQD1, a Protein Essential for Sulfolipid Biosynthesis in Arabidopsis

Sulfolipids of photosynthetic bacteria and plants are characterized by their unique sulfoquinovose headgroup, a derivative of glucose in which the 6-hydroxyl group is replaced by a sulfonate group. These sulfolipids have been discussed as promising anti-tumor and anti-HIV therapeutics based on their inhibition of DNA polymerase and reverse transcriptase. To study sulfolipid biosynthesis, in particular the formation of UDP-sulfoquinovose, we have combined computational modeling with biochemical methods. A database search was performed employing the derived amino acid sequence from SQD1, a gene involved in sulfolipid biosynthesis of Arabidopsis thaliana. This sequence shows high similarity to other sulfolipid biosynthetic proteins of different organisms and also to sugar nucleotide modifying enzymes, including UDP-glucose epimerase and dTDP-glucose dehydratase. Additional biochemical data on the purified SQD1 protein suggest that it is involved in the formation of UDP-sulfoquinovose, the first step of sulfolipid biosynthesis. To understand which aspects of epimerase catalysis may be shared by SQD1, we built a three-dimensional model of SQD1 using the 1.8 إ crystallographic structure of UDP-glucose 4-epimerase as a template. This model predicted an NAD+ binding site, and the binding of NAD+ was subsequently confirmed by enzymatic assay and mass spectrometry. The active-site interactions together with biochemical data provide the basis for proposing a reaction mechanism for UDP-sulfoquinovose formation.