The Stereochemical Course and Mechanism of the IspH Reaction

Abstract

Scheme 5. Stereochemical course of the IspH reaction.Scheme 4. Synthesis of deuterated 1-deoxy-d-xylulose. IBX = 1-hydroxy-1,2-benziodoxol-3(1H)-one 1-oxide, DMSO = dimethyl sulfoxide. Angewandte Chemie 4055

Are Free Radicals Involved in IspH Catalysis? An EPR and Crystallographic Investigation

“…Second, in the IspH ferraoxetane mechanism, there is no involvement of the 3-CH 2 OH group rotation seen crystallographically 24 as well as deduced from isotope-labeling studies of the product IPP ( 2 ) C4 H E , H Z -stereochemistry, which requires such a rotation. 18 Third, C2 is protonated in the proposed ferraoxetane 19 , which will lead to the formation of only IPP, not a mixture of DMAPP and IPP. Fourth, the observation that the fluoro substrate analog 4 can react with IspH and forms the same intermediate as that seen with the natural substrate 1 is also inconsistent with an assignment to a ferraoxetane species for Intermediate III, because 4 does not have a hydroxyl group.…”

“…17 These workers favored a Birch reduction-like mechanism (Scheme 3), and ruled out the bioorganometallic mechanism, a conclusion at odds with a recent stereochemical study. 18 …”

Methylerythritol Phosphate Pathway of Isoprenoid Biosynthesis

“…Again, the DMAPP stereochemistry will depend on whether the reaction involves a C 3 -C 4 bond rotation step, and whether pyrophosphate or the water molecule generated from C 4 -OH dehydration is the C 4 proton source (Figure 7A). Recently, results of GC-MS analysis of terpenes isolated from a host fed with isotopically labeled deoxyxylulose were cited as evidence supporting the presence of C 3 -C 4 bond rotation in IspH catalysis (99). Due to the participation of IPP:DMAPP isomerase (IDI) in the transformation, labeling results from in vivo studies are complicated (8, 9, 99–102) and more detailed in vitro biochemical studies in the future will provide further evidence to this important mechanistic issue.…”

Structures of Fluoro, Amino, and Thiol Inhibitors Bound to the [Fe4S4] Protein IspH

“…One-electron reduction of the cluster results in [Fe 4 S 4 ] + with spin S=1/2, and correlates with a rotation of the ligand's hydroxymethyl group away from the cluster to form a cyclic conformation (state II) which has essential impact on the stereochemical course of the IspH reaction. [7] The transfer of two electrons from the cofactor to the substrate produces a HiPIP-type [Fe 4 S 4 ] 3+ cluster and leads to C-O bond cleavage and water release. The allyl anion (state III) then abstracts a proton from the diphosphate group, either at the ligand's C2 or C4 atom, to form IPP and DMAPP, respectively.…”