Identification of HcgC as a SAM-Dependent Pyridinol Methyltransferase in [Fe]-Hydrogenase Cofactor Biosynthesis

Previous retrosynthetic and isotope-labeling studies have indicated that biosynthesis of the iron guanylylpyridinol (FeGP) cofactor of [Fe]-hydrogenase requires a methyltransferase. This hypothetical enzyme covalently attaches the methyl group at the 3-position of the pyridinol ring. We describe the identification of HcgC, a gene product of the hcgA-G cluster responsible for FeGP cofactor biosynthesis. It acts as an S-adenosylmethionine (SAM)-dependent methyltransferase, based on the crystal structures of HcgC and the HcgC/SAM and HcgC/S-adenosylhomocysteine (SAH) complexes. The pyridinol substrate, 6-carboxymethyl-5-methyl-4-hydroxy-2-pyridinol, was predicted based on properties of the conserved binding pocket and substrate docking simulations. For verification, the assumed substrate was synthesized and used in a kinetic assay. Mass spectrometry and NMR analysis revealed 6-carboxymethyl-3,5-dimethyl-4-hydroxy-2-pyridinol as the reaction product, which confirmed the function of HcgC.

CuI and H2 O2 Inactivate and FeII Inhibits [Fe]-Hydrogenase at Very Low Concentrations

[Fe]-Hydrogenase (Hmd) catalyzes reversible hydride transfer from H₂ . It harbors an iron-guanylylpyridinol as a cofactor with an Fe^II that is ligated to one thiolate, two COs, one acyl-C, one pyridinol-N, and solvent. Here, we report that Cu^I and H₂ O₂ inactivate Hmd (half-maximal rates at 1 μM Cu^I and 20 μM H₂ O₂ ) and that Fe^II inhibits the enzyme with very high affinity (K_i =40 nM). Infrared and EPR studies together with competitive inhibition studies with isocyanide indicated that Cu^I exerts its inhibitory effect most probably by binding to the active site iron-thiolate ligand. Using the same methods, it was found that H₂ O₂ binds to the active-site iron at the solvent-binding site and oxidizes Fe^II to Fe^III . Also it was shown that Fe^II reversibly binds away from the active site iron, with binding being competitive to the organic hydride acceptor; this inhibition is specific for Fe^II and is reminiscent of that for the [FeFe]-hydrogenase second iron, which specifically interacts with H₂ .

Toward functional type III [Fe]-hydrogenase biomimics for H2 activation: insights from computation

The chemistry of [Fe]-hydrogenase has attracted significant interest due to its ability to activate molecular hydrogen. The intriguing properties of this enzyme have prompted the synthesis of numerous small molecule mimics aimed at activating H2. Despite considerable effort, a majority of these compounds remain nonfunctional for hydrogenation reactions. By using a recently synthesized model as an entry point, seven biomimetic complexes have been examined through DFT computations to probe the influence of ligand environment on the ability of a mimic to bind and split H2. One mimic, featuring a bidentate diphosphine group incorporating an internal nitrogen base, was found to have particularly attractive energetics, prompting a study of the role played by the proton/hydride acceptor necessary to complete the catalytic cycle. Computations revealed an experimentally accessible energetic pathway involving a benzaldehyde proton/hydride acceptor and the most promising catalyst.