[Fe]-Hydrogenase (Hmd): Insights From Enzyme Structure, Spectroscopy and Synthetic Models

The enzyme N5,N10-methylenetetrahydromethanopterin (H4MPT) dehydrogenase (Hmd, EC 1.12.99; aka [Fe]-hydrogenase, or mono-iron hydrogenase) from Methanobacter marburgensis was discovered in 1990 by Thauer and co-workers at MPI Terrestrial Biology in Manheim. Against the backdrop of constitutive [NiFe] expression by methanogens, the Hmd enzyme and its metabolic counterparts (e.g., CO2-aminofuran transferase, H4MPT reductase) and its associated biogenesis machinery are all upregulated in the absence of nickel. Upon its discovery, Hmd was determined to catalyze the reversible hydrogenation (reduction) of the C1 carrier H4MPT+ using molecular dihydrogen (H2) as the reductive source. Unlike the majority of methanogenic enzymatic machinery, Hmd's catalytic activity was not dependent on other cofactors (e.g. flavins, F420) or nickel ions. Hmd has undergone a rich history of discovery and re-discovery, wherein it has been successively known as the "iron-sulfur-cluster free hydrogenase," the "metal-free hydrogenase" and—ultimately—the mono-iron hydrogenase or [Fe]-hydrogenase). Ultimately, the methods of inorganic spectroscopy (Mössbauer, XAS, XAFS, NRVS, IR) were brought to bear to discern the presence of a single, low-spin Fe(II) ion in the active site. The novelty of this result was amplified by the ultimate discovery of a unique organometallic Fe–Cacyl bond present as a stable coordination motif in the active site. In the last decade, synthetic modeling studies have demonstrated the surprising stability of organometallic units bound to the {Fe(CO)2}2+ motif. Scaffold-based ligand designs with a designed facial-CNS donor chelate have provided particular advances in understanding the critical role of the organometallic carbanion in promoting Kubas-like H2 binding. Computational work (DFT and QM/MM) has provided a framework for understanding the surprisingly thermoneutral binding and heterolytic splitting of H2, as well as the endergonic hydride transfer to H4MPT+. Computational studies have implicated the O-atom of the iron-ligated pyridinol/pyridone as the pendant base (i.e., not the thiolato-S as in [NiFe] hydrogenase) that drastically lowers the kinetic barrier to H2 heterolysis. However, synthetic model studies have yet to fully evaluate this hypothesis. An outlook summary describes such "missing links" in the synthetic modeling work, and briefly describes the present knowledge of active site biogenesis.