Beta subunit of a hydrogen-evolving hydrogenase that utilizes protons both as a substrate for hydrogen production and proton translocation. Acts by coupling the redox reaction via ferredoxin and iron-sulfur (Fe-S) clusters to proton translocation across the membrane thereby conserving the redox energy in a proton gradient. H2 + 2 oxidized [2Fe-2S]-[ferredoxin] = 2 H(+) + 2 reduced [2Fe-2S]-[ferredoxin] Binds 1 nickel ion per mole of protein. Inhibited by 0.1 mM Cu(2+). Measured for the whole complex. Optimum pH is 7.0. Active between pH 6.0-8.5. Optimum temperature is 90 degrees Celsius for membrane-bound enzyme but 80 degrees Celsius for the purified enzyme. Membrane-bound enzyme has a half-life of 2h at 100 degrees Celsius whereas the half-life of the purified enzyme is 30 minutes at 100 degrees Celsius. The membrane-bound hydrogenase complex is composed of MbhK and MbhL, and may also contain MbhJ. Belongs to the complex I 30 kDa subunit family. The subunit composition of membrane-bound hydrogenase complex is currently unclear. It has been shown to be a heterodimer of MbhK and MbhL (PubMed:10852873). Other studies have shown it to contain MbhJ in addition to MbhK and MbhL (PubMed:11054105). There is conflicting N-terminal sequencing data for this protein. The mature protein may start from Ser-2 (PubMed:10852873) or Lys-3 (PubMed:11054105).