Mutation in the flexible loop of 1-deoxy-d-xylulose 5-phosphate reductoisomerase broadens substrate utilization

The second enzyme in the methylerythritol phosphate pathway to isoprenoids, 1-deoxy-d-xylulose 5-phosphate reductoisomerase (DXR; EC 1.1.1.267) mediates the transformation of 1-deoxy-d-xylulose 5-phosphate (DXP) into 2-C-methyl-d-erythritol 4-phosphate. Several DXR mutants have been prepared to study amino acid residues important in binding or catalysis, but in-depth studies of many conserved residues in the flexible loop portion of the enzyme have not been conducted. In the course of our studies of this enzyme, an analog of DXP, 1,2-dideoxy-d-threo-3-hexulose 6-phosphate (1-methyl-DXP), was found to be a weak competitive inhibitor. Using the X-ray crystal structures of DXR as a guide, a highly conserved tryptophan residue in the flexible loop was identified that potentially blocks the use of this analog as a substrate. To test this hypothesis, four mutants of the Synechocystis sp. PCC6803 DXR were prepared and a W204F mutant was found to utilize the analog as a substrate.