The interconversion of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) catalyzed by E. coli and S. pombe IPP isomerase proceeds with removal of the pro-R proton at C2 of IPP and addition of a water-derived proton to the re face of the C2–C3 double bond in DMAPP; this is the same stereochemistry observed for S. cerevisiae and rat liver enzymes.