Isolation of a hydrogenase-cytochrome b complex from cytoplasmic membranes of Xanthobacter autotrophicus GZ 29

The aerobic nitrogen-fixing hydrogen bacterium Xanthobacter autotrophicus strain GZ29 [1] can grow autotrophically with hydrogen, oxygen, carbon dioxide, and molecular nitrogen as sole sources of electrons, energy, carbon, and nitrogen. Carbon dioxide is fixed via the Calvin cycle, and for activation of hydrogen only a membrane-bound hydrogenase activity was detected which does not reduce NAD [2]. This is true also for many other aerobic hydrogen bacteria [2,3], and it is assumed that in these organisms the membrane-bound hydrogenase has to provide electrons for NAD reduction in a reversed electron transport as well as for energy conservation. In X. autotrophicus the hydrogenase beyond it has to supply electrons for reduction of nitrogen by nitrogenase at the redox potential of ferredoxin (around 4 2 0 mV). For this reason the hydrogenase of X. autotrophicus should differ from the type of membrane-bound hydrogenase found in Alcaligenes eutrophus which reduces electron acceptors only at the redox potential of quinones [4]. It was also considered that even two different hydrogenases might be present, one reducing ferredoxin and one providing electrons for the respiratory chain. The involvement of ferredoxin in the nitrogen fixation process of X. autotrophicus GZ29 has recently been shown by electron transfer and regulation studies [5]. In the present study the membrane-bound hydrogenase system of S. autotrophicus is shown to be associated with a cytochrome b complex, and catalytic as well as molecular properties are described.