DIRECT ELECTROCHEMISTRY OF MEGASPHAERA-ELSDENII IRON HYDROGENASE - DEFINITION OF THE ENZYMES CATALYTIC OPERATING POTENTIAL AND QUANTITATIONOF THE CATALYTIC BEHAVIOR OVER A CONTINUOUS POTENTIAL RANGE

The Fe-hydrogenase from Megasphaera elsdenii undergoes direct electron exchange with glassy carbon electrodes. Cyclic voltammetry defines th e catalytic performance of the enzyme over a continuous but precisely defined range of potentials. In the presence of H-2 and protons the bi as of the enzyme towards H-2 production is readily visualised. Variati on 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 H-2-activating site are discussed. The mid-wave potential of the catalytic response, E(mid), is defined as the catalytic operating potential of the enzyme . Under an atmosphere of hydrogen E(mid) = -421+/-10 mV, pH 7 with a v ariation of -21+/-4 mV pH(-1), 22 degrees C. Deviation of E(mid) from the thermodynamic potential of the hydrogen/proton couple reflects the enzyme's influence over the catalysed reaction. E(mid) is the reducti on potential of the H-2-activating centre (H-cluster) in the absence o f kinetic bottle-necks at other steps in the reaction mechanism.