The Structure of Hydrogenase-2 from Escherichia coli: Implications for H₂ -Driven Proton Pumping

Under anaerobic conditions Escherichia coli is able to metabolize molecular hydrogen via the action of several [NiFe]-hydrogenase enzymes. Hydrogenase-2, which is typically present in cells at low levels during anaerobic respiration, is a periplasmic-facing membrane-bound complex that functions as a proton pump to convert energy from H2 oxidation into a proton gradient; consequently, its structure is of great interest. Empirically, the complex consists of a tightlybound core catalytic module, comprising large (HybC) and small (HybO) subunits, which is attached to an Fe-S protein (HybA) and an integral membrane protein, HybB. To date, efforts to gain a more detailed picture have been thwarted by low native expression levels of hydrogenase-2 and the labile interaction between HybOC and HybA/HybB subunits. In this paper we describe a new over-expression system that has facilitated determination of highresolution crystal structures of HybOC and, hence, a prediction of the quaternary structure of the HybOCAB complex.