DXP reductoisomerase: Reaction of the substrate in pieces reveals a catalytic role for the nonreacting phosphodianion group

The role of the nonreacting phosphodianion group of 1-deoxy-d-xylulose-5- phosphate (DXP) in catalysis by DXP reductoisomerase (DXR) was investigated for the reaction of the "substrate in pieces". The truncated substrate 1-deoxy-l-erythrulose is converted by DXR to 2-C-methylglycerol with a k _cat/K_m that is 10⁶-fold lower than that for DXP. Phosphite dianion was found to be a nonessential activator, providing 3.2 kcal/mol of transition state stabilization for the truncated substrate. These results implicate a phosphate-driven conformational change involving loop closure over the DXR active site to generate an environment poised for catalysis.