Energy-converting [NiFe] hydrogenases are membrane-bound enzymes with a six-subunit core: the large and small hydrogenase subunits, plus two hydrophilic proteins and two integral membrane proteins. Their large and small subunits show little sequence similarity to other [NiFe] hydrogenases, except for key conserved residues coordinating the active site and [FeS] cluster. Energy-converting [NiFe] hydrogenases function as ion pumps, catalyzing the reduction of ferredoxin with H2 driven by the proton-motive force or the sodium-ion-motive force [1]. Eha and Ehb hydrogenases contain extra subunits in addition to those shared by other energy-converting [NiFe] hydrogenases (or [NiFe]-hydrogenase-3-type). Eha contains a 6[4Fe-4S] polyferredoxin, a 10[4F-4S] polyferredoxin, ten other predicted integral membrane proteins (EhaA, EhaB, EhaC, EhaD, EhaE, EhaF, EhaG, EhaI, EhaK, EhaL) and four hydrophobic subunits (EhaM, EhaR, EhS, EhT) [2]. Eha and Ehb catalyse the reduction of low-potential redox carriers (e.g. ferredoxins or polyferredoxins), which then might function as electron donors to oxidoreductases [3]. Based on sequence similarity and genome context analysis, other organisms such as Methanopyrus kandleri, Methanocaldococcus jannaschii, and Methanothermobacter marburgensis also encode Eha-like [NiFe]-hydrogenase-3-type complexes and have very similar eha operon structure [3]. This domain family can be found on the small membrane proteins that are predicted to be the EhaA trans-membrane subunits of multisubunit membrane-bound [NiFe]-hydrogenase Eha complexes.

1: More than 200 genes required for methane formation from H(2) and CO(2) and energy conservation are present in Methanothermobacter marburgensis and Methanothermobacter thermautotrophicus. Kaster AK, Goenrich M, Seedorf H, Liesegang H, Wollherr A, Gottschalk G, Thauer RK; Archaea. 2011;2011:973848. PMID:21559116

2: Methanobacterium thermoautotrophicum encodes two multisubunit membrane-bound [NiFe] hydrogenases. Transcription of the operons and sequence analysis of the deduced proteins. Tersteegen A, Hedderich R; Eur J Biochem. 1999;264:930-943. PMID:10491142

3: PIRSF family classification system for protein functional and evolutionary analysis. Nikolskaya AN, Arighi CN, Huang H, Barker WC, Wu CH; Evol Bioinform Online. 2007;2:197-209. PMID:19455212