Eukaryotes and most prokaryotes require isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) as biosynthetic precursors of terpenes. Whereas animals generate these essential metabolites via the mevalonate pathway,[1] many human pathogens including Plasmodium falciparum and Mycobacterium tuberculosis are known to use the more recently identified non-mevalonate pathway, which is a potential target for drug development.[2–4] The final step of this pathway is catalyzed by IspH protein, which generates a mixture of IPP and DMAPP by reductive dehydration of 1- hydroxy-2-methyl-2-(E)-butenyl-4-diphosphate (HMBPP, Figure 1a).[5–11] Recently, Rekittke et al. described the first X-ray structure of IspH protein from the hyperthermophilic eubacterium Aquifex aeolicus in its open state.[12] Herein, we report the crystal structure of the IspH protein from Escherichia coli[11] in its closed conformation, which serves as basis for a detailed discussion of the catalytic pathway