Direct electrochemistry of Megasphaera elsdenii iron hydrogenase. Definition of the enzyme's catalytic operating potential and quantitation of the catalytic behaviour over a continuous potential range.

The Fe-hydrogenase from Megasphaera elsdenii undergoes direct electron exchange with glassy carbon electrodes. Cyclic voltammetry defines the catalytic-performance of the enzyme over a continuous but precisely defined range of potentials. In the presence of H2 and protons the bias of the enzyme towards H2 production is readily visualised. Variation of the response with pH indicates that protein ionisations with pK of approximately 6.7 and 8.3 regulate the catalytic activity. Possible origins for these observations in the chemistry of the H2-activating site are discussed. The mid-wave potential of the catalytic response, Emid, is defined as the catalytic operating potential of the enzyme. Under an atmosphere of hydrogen Emid = -421 +/- 10 mV, pH 7 with a variation of -21 +/- 4 mV pH-1, 22 degrees C. Deviation of Emid from the thermodynamic potential of the hydrogen/proton couple reflects the enzyme's influence over the catalysed reaction. Emid is the reduction potential of the H2-activating centre (H-cluster) in the absence of kinetic bottle-necks at other steps in the reaction mechanism.