Biosynthesis of terpenoids: 1-deoxy- D-xylulose-5-phosphate reductoisomerase from Escherichia coli is a class B dehydrogenase

1-Deoxy- D-xylulose-5-phosphate is converted into 2- C-methyl- D-erythritol-4-phosphate by the catalytic action of 1-deoxy- D-xylulose-5-phosphate reductoisomerase (Dxr protein) using NADPH as cofactor. The stereochemical features of this reaction were investigated in in vitro experiments with the recombinant Dxr protein of Escherichia coli using (4 R)- or (4 S)-[4- 2H 1]NADPH as coenzyme. The enzymatically formed 2- C-methyl- D-erythritol-4-phosphate was isolated and converted into 1,2:3,4-di- O-isopropylidene-2- C-methyl- D-erythritol; NMR spectroscopic investigation of this derivative indicated that only (4 S)-[4- 2H 1]NADPH affords 2- C-methyl- D-erythritol-4-phosphate labelled exclusively in the H Re position of C-1. Stereospecific transfer of H Si from C-4 of the cofactor identifies the Dxr protein of E. coli as a class B dehydrogenase.