The Effects of pH and Redox Potential on the Hydrogen Production Activity of the Hydrogenase from Megasphaera elsdenii

Abstract

The effects of temperature on the ionization constant (pK') and apparent midpoint potential (EB) of the unprotonated species of Megasphaera elsdenii flavodoxin hydroquinone shows that, above 15 degrees C, delta pK' . K-1 = -9.7 X 10(-3) and delta EB . K-1 = -0.6 mV. The effects of pH and redox potential on the hydrogen production activity with fixed concentrations of methyl viologen semiquinone (0.3 mM; artificial donor) and M. elsdenii flavodoxin hydroquinone (50 microM; natural donor) show that with decreasing pH the activity increases. Irrespective of the pH and electron donor, at increasing redox potential, a redox-potential-independent production activity is followed by a redox-potential-dependent production activity. This redox-potential-dependent behaviour of the hydrogen production activity represents an n = 2-type of redox titration curve with an 'apparent midpoint potential' which corresponds with the potential of the hydrogen electrode at that pH. The effect of pH on the manometrically determined hydrogen production activity (direct) is in good agreement with that determined spectrophotometrically (indirect; see preceding paper), with both electron donors tested. In contrast to predictions from the models for hydrogenase activity [van Dijk et al. (1980) Eur. J. Biochem.102, 317--330], a double-reciprocal plot of the kinetic data for M. elsdenii flavodoxin hydroquinone at pH 5.5 is non-linear. A slightly adapted kinetic model based on a similar mathematical formulation of its rate equation, to explain the effects of redox potential, proton and electron (donor) concentration on the hydrogenase activity is proposed. This model also explains, on a theoretical basis, the effects of pH and redox potential on the hydrogen production activity. The effect of pH on the hydrogen oxidation activity with methyl viologen and benzyl viologen as electron acceptors shows for both dyes an optimum at pH 9.7. The ratio of the activities with both viologens is constant over the ph range tested.