Biosynthesis of Isoprenoids: Crystal Structure of the [4Fe-4S] Cluster Protein IspG

IspG protein serves as the penultimate enzyme of the recently discovered non-mevalonate pathway for the biosynthesis of the universal isoprenoid precursors, isopentenyl diphosphate and dimethylallyl diphosphate. The enzyme catalyzes the reductive ring opening of 2C-methyl-D-erythritol 2,4-cyclodiphosphate, which affords 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate. The protein was crystallized under anaerobic conditions, and its three-dimensional structure was determined to a resolution of 2.7 angstrom. Each subunit of the c(2) symmetric homodimer folds into two domains connected by a short linker sequence. The N-terminal domain (N domain) is an eight-stranded beta barrel that belongs to the large TIM-barrel superfamily. The C-terminal domain (C domain) consists of a beta sheet that is flanked on both sides by helices. One glutamate and three cysteine residues of the C domain coordinate a [4Fe-4S] cluster. Homodimer formation involves an extended contact area (about 1100 angstrom(2)) between helices 8 and 9 of each respective beta barrel. Moreover, each C domain contacts the N domain of the partner subunit, but the interface regions are small (about 430 angstrom(2)). We propose that the enzyme substrate binds to the positively charged surface area at the C-terminal pole of the beta barrel. The C domain carrying the iron sulfur cluster could then move over to form a closed conformation where the substrate is sandwiched between the N domain and the C domain. This article completes the set of three-dimensional structures of the non-mevalonate pathway enzymes, which are of specific interest as potential targets for tuberculostatic and antimalarial drugs. (C) 2010 Published by Elsevier Ltd.