Electrical conduction of hemoprotein in the solid phase: Anhydrous cytochrome c3 film

Cytochrome c3 can be reduced with molecular hydrogen under the action of hydrogenase [hydrogen: ferricytochrome c3 oxidoreductase, EC 1.12.2.1] even in the solid state. The electrical conductivity of a cytochrome c3 anhydrous film containing a trace amount of hydrogenase was measured at physiological temperatures as a function of temperature and hydrogen pressure. Ferricytochrome c3 (oxidized form) and ferrocytochrome c3 (reduced form) equilibrate at a given temperature and pressure by a catalytic action of hydrogenase. Under these conditions, the conductivity of cytochrome c3 showed an unusual temperature dependence: The activation energy was positive under higher hydrogen pressure, but was negative under lower pressure. These findings are interpreted as a thermal equilibration between the ferri- and ferro- forms using the Hill equation for the reduction ratio and applying the theory of semiconduction to the electrical conductivity. The theory predicted that the activation energy of conductivity would coincide with the free energy difference between ferri- and ferrocytochrome c3, which is confirmed by measurement of the temperature dependence of the equilibrium constant. The temperature and hydrogen pressure dependences of the conductivity were reproduced well by the above theory.