FIG 1.

Three segments encompassing the four cysteine ligands (in red) involved in H-cluster iron binding in the three [FeFe]-hydrogenases of R. albus. The consensus sequence in the three segments is based on the review by Lubitz et al. (28). Underlined letters indicate fully conserved residues; an “x” indicates that more than four different residues are found at that position. HydA is subunit A of the electron-bifurcating ferredoxin- and NAD-dependent [FeFe]-hydrogenase HydABC; HydA2 is the ferredoxin-dependent [FeFe]-hydrogenase; HydS is the putative H₂-sensing [FeFe]-hydrogenase. HydA2 has 27% (83% coverage) and HydS 26% (52% coverage) sequence identity to HydA, and HydA has 42% sequence identity to the [FeFe]-hydrogenase I from C. pasteurianum.