In Vitro Biosynthesis of the [Fe]-Hydrogenase Cofactor Verifies the Proposed Biosynthetic Precursors

In the FeGP cofactor of [Fe]-hydrogenase, low-spin FeII is in complex with two CO ligands and a pyridinol derivative; the latter ligates the iron with a 6-acylmethyl substituent and the pyridinol nitrogen. A guanylylpyridinol derivative, 6-carboxymethyl-3,5-dimethyl-4-guanylyl-2-pyridinol (3), is produced by the decomposition of the FeGP cofactor under irradiation with UV-A/blue light and is also postulated to be a precursor of FeGP cofactor biosynthesis. HcgC and HcgB catalyze consecutive biosynthesis steps leading to 3. Here, we report an in vitro biosynthesis assay of the FeGP cofactor using the cell extract of the ΔhcgBΔhcgC strain of Methanococcus maripaludis, which does not biosynthesize 3. We chemically synthesized pyridinol precursors 1 and 2, and detected the production of the FeGP cofactor from 1, 2 and 3. These results indicated that 1, 2 and 3 are the precursors of the FeGP cofactor, and the carboxy group of 3 is converted to the acyl ligand.

Heterologous Hydrogenase Overproduction Systems for Biotechnology-An Overview

Hydrogenases are complex metalloenzymes, showing tremendous potential as H2-converting redox catalysts for application in light-driven H2 production, enzymatic fuel cells and H2-driven cofactor regeneration. They catalyze the reversible oxidation of hydrogen into protons and electrons. The apo-enzymes are not active unless they are modified by a complicated post-translational maturation process that is responsible for the assembly and incorporation of the complex metal center. The catalytic center is usually easily inactivated by oxidation, and the separation and purification of the active protein is challenging. The understanding of the catalytic mechanisms progresses slowly, since the purification of the enzymes from their native hosts is often difficult, and in some case impossible. Over the past decades, only a limited number of studies report the homologous or heterologous production of high yields of hydrogenase. In this review, we emphasize recent discoveries that have greatly improved our understanding of microbial hydrogenases. We compare various heterologous hydrogenase production systems as well as in vitro hydrogenase maturation systems and discuss their perspectives for enhanced biohydrogen production. Additionally, activities of hydrogenases isolated from either recombinant organisms or in vivo/in vitro maturation approaches were systematically compared, and future perspectives for this research area are discussed.

The Bacterial [Fe]-Hydrogenase Paralog HmdII Uses Tetrahydrofolate Derivatives as Substrates

[Fe]-hydrogenase (Hmd) catalyzes the reversible hydrogenation of methenyl-tetrahydromethanopterin (methenyl-H₄ MPT⁺ ) with H₂ . H₄ MPT is a C1-carrier of methanogenic archaea. One bacterial genus, Desulfurobacterium, contains putative genes for the Hmd paralog, termed HmdII, and the HcgA-G proteins. The latter are required for the biosynthesis of the prosthetic group of Hmd, the iron-guanylylpyridinol (FeGP) cofactor. This finding is intriguing because Hmd and HmdII strictly use H₄ MPT derivatives that are absent in most bacteria. We identified the presence of the FeGP cofactor in D. thermolithotrophum. The bacterial HmdII reconstituted with the FeGP cofactor catalyzed the hydrogenation of derivatives of tetrahydrofolate, the bacterial C1-carrier, albeit with low enzymatic activities. The crystal structures show how Hmd recognizes tetrahydrofolate derivatives. These findings have an impact on future biotechnology by identifying a bacterial Hmd paralog.

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.