A Biomimetic Model for the Active Site of [Fe]-H2ase Featuring a 2-Methoxy-3,5-dimethyl-4-phosphato-6-acylmethylpyridine Ligand

Biomimetic models of [Fe]-hydrogenase featuring a 2-acylphenylthiomethyl-6-R-pyridine (R = H or OMe) ligand

A new flexible pyridine ligand (FPL)-based method is developed, by which two novel biomimetic models of [Fe]-H2ase are prepared and their enzyme-like H2/D2 activation functions are studied.

Crystal Structures of [Fe]-Hydrogenase from Methanolacinia paynteri Suggest a Path of the FeGP-Cofactor Incorporation Process

[Fe]-hydrogenase (Hmd) catalyzes the reversible heterolytic cleavage of H2, and hydride transfer to methenyl-tetrahydromethanopterin (methenyl-H4MPT+). The iron-guanylylpyridinol (FeGP) cofactor, the prosthetic group of Hmd, can be extracted from the holoenzyme and inserted back into the protein. Here, we report the crystal structure of an asymmetric homodimer of Hmd from Methanolacinia paynteri (pHmd), which was composed of one monomer in the open conformation with the FeGP cofactor (holo-form) and a second monomer in the closed conformation without the cofactor (apo-form). In addition, we report the symmetric pHmd-homodimer structure in complex with guanosine monophosphate (GMP) or guanylylpyridinol (GP), in which each ligand was bound to the protein, where the GMP moiety of the FeGP-cofactor is bound in the holo-form. Binding of GMP and GP modified the local protein structure but did not induce the open conformation. The amino-group of the Lys150 appears to interact with the 2-hydroxy group of pyridinol ring in the pHmd–GP complex, which is not the case in the structure of the pHmd–FeGP complex. Lys150Ala mutation decreased the reconstitution rate of the active enzyme with the FeGP cofactor at the physiological pH. These results suggest that Lys150 might be involved in the FeGP-cofactor incorporation into the Hmd protein in vivo.

Scaffold-Based Functional Models of [Fe]-Hydrogenase (Hmd): Building the Bridge between Biological Structure and Molecular Function

The well-known dinuclear [FeFe] and [NiFe] hydrogenase enzymes are redox-based proton reduction and H₂ oxidation catalysts. In comparison, the structural and functional aspects of the mononuclear nonredox hydrogenase, known as [Fe]-hydrogenase or Hmd, have been less explored because of the relatively recent crystallographic elucidation of the enzyme active site. Additionally, the synthetic challenges posed by the highly substituted and asymmetric coordination environment of the iron guanylylpyridinol (FeGP) cofactor have hampered functional biomimetic modeling studies to a large extent. The active site contains an octahedral low-spin Fe(II) center with the following coordination motifs: a bidentate acyl-pyridone moiety (C,N) and cysteinyl-S in a facial arrangement; two cis carbonyl ligands; and a H₂O/H₂ binding site. In [Fe]-hydrogenase, heterolytic H₂ activation putatively by the pendant pyridone/pyridonate-O base serving as a proton acceptor. Following H₂ cleavage, an intermediate Fe-H species is thought to stereoselectively transfer a hydride to the substrate methenyl-H₄MPT⁺, thus forming methylene-H₄MPT. In the past decade, chemists, inspired by the elegant organometallic chemistry inherent to the FeGP cofactor, have synthesized a number of faithful structural models. However, functional systems are still relatively limited and often rely on abiological ligands or metal centers that obfuscate a direct correlation to nature's design.Our group has developed a bioinspired suite of synthetic analogues of Hmd to better understand the effects of structure on the stability and functionality of the Hmd active site, with a special emphasis on using a scaffold-based ligand design. This systematic approach has contributed to a deeper understanding of the unique ligand array of [Fe]-hydrogenase in nature and has ultimately resulted in the first functional synthetic models without the aid of abiological ligands. This Account reviews the reactivity of the functional anthracene-scaffolded synthetic models developed by our group in the context of current mechanistic understanding drawn from both protein crystallography and computational studies. Furthermore, we introduce a novel thermodynamic framework to place the reactivity of our model systems in context and provide an outlook on the future study of [Fe]-hydrogenase synthetic models through both a structural and functional lens.

Synthesis, characterization, and some properties of two types of new [Fe]-H2ase models containing a 4-phosphatopyridine or a 4-phosphatoguanosinepyridine moiety

The novel [Fe]-H₂ase active site framework-containing model 6 was first prepared and structurally characterized.