Anaerobic Formate and Hydrogen Metabolism

Abstract

Numerous recent developments in the biochemistry, molecular biology, and physiology of formate and H2 metabolism and of the [NiFe]-hydrogenase (Hyd) cofactor biosynthetic machinery are highlighted. Formate export and import by the aquaporin-like pentameric formate channel FocA is governed by interaction with pyruvate formate-lyase, the enzyme that generates formate. Formate is disproportionated by the reversible formate hydrogenlyase (FHL) complex, which has been isolated, allowing biochemical dissection of evolutionary parallels with complex I of the respiratory chain. A recently identified sulfido-ligand attached to Mo in the active site of formate dehydrogenases led to the proposal of a modified catalytic mechanism. Structural analysis of the homologous, H2-oxidizing Hyd-1 and Hyd-5 identified a novel proximal [4Fe-3S] cluster in the small subunit involved in conferring oxygen tolerance to the enzymes. Synthesis of Salmonella Typhimurium Hyd-5 occurs aerobically, which is novel for an enterobacterial Hyd. The O2-sensitive Hyd-2 enzyme has been shown to be reversible: it presumably acts as a conformational proton pump in the H2-oxidizing mode and is capable of coupling reverse electron transport to drive H2 release. The structural characterization of all the Hyp maturation proteins has given new impulse to studies on the biosynthesis of the Fe(CN)2CO moiety of the [NiFe] cofactor. It is synthesized on a Hyp-scaffold complex, mainly comprising HypC and HypD, before insertion into the apo-large subunit. Finally, clear evidence now exists indicating that Escherichia coli can mature Hyd enzymes differentially, depending on metal ion availability and the prevailing metabolic state. Notably, Hyd-3 of the FHL complex takes precedence over the H2-oxidizing enzymes.

The dual-function chaperone HycH improves assembly of the formate hydrogenlyase complex

“…In the processed HycE model structure, this Arg537 is buried within the protein, indicating a possible structural rearrangement after the proteolytic processing, as has been proposed for Hyd-2 [7,39]. The C-terminal extension of HycE itself has a total of 30% basic amino acids and HycH is predicted to be relatively acidic, however, with low conservation of these residues; thus, it is unlikely that the C-terminal extension is the interaction interface with HycH.…”

“…The FHL complex is very similar to Complex I whose crystal structure has been resolved [31]. An FHL model based on that structure showed not only an interaction interface of HycE and HycG but also a predicted interaction between the C-terminal domain of HycF and HycE, whereby this domain wrapped around HycE [7]. Therefore, to test whether HycF might facilitate the interaction, plasmid pHycEHI was co-transformed with pHycBFG, which encodes all three iron-sulphur subunits of the FHL complex.…”

“…The HycE protein undergoes a maturation process before it can interact with the electron transfer subunits [7]. These steps are summarised in Figure 4A and can be broken down into cofactor delivery, nickel insertion, proteolytic processing and interaction with the remainder of the FHL subunits.…”

“…Recently, isolation of affinity-tagged HycE from a genetic background devoid of HycG identified a potential HycE-HycH and HycE-HypC complex [6] although HycH and HypC are not part of the final structural FHL complex [2,4]. It is predicted that in the soluble domain of the FHL complex, HycE interacts directly with its small subunit HycG and the electron transfer subunit HycF, while the FDH-H (FdhF) and its small subunit HycB interact indirectly via HycF [4,7]. The interaction of unprocessed HycE with HypC was established earlier [8,9].…”

“…The fdhF gene, encoding the FDH-H component of the FHL complex, is not part of the hycA-I operon [7].…”