Relation between anaerobic inactivation and oxygen tolerance in a large series of NiFe hydrogenase mutants

Nickel-containing hydrogenases, the biological catalysts of H-2 oxidation and production, reversibly inactivate under anaerobic, oxidizing conditions. We aim at understanding the mechanism of (in)activation and what determines its kinetics, because there is a correlation between fast reductive reactivation and oxygen tolerance, a property of some hydrogenases that is very desirable from the point of view of biotechnology. Direct electrochemistry is potentially very useful for learning about the redox-dependent conversions between active and inactive forms of hydrogenase, but the voltammetric signals are complex and often misread. Here we describe simple analytical models that we used to characterize and compare 16 mutants, obtained by substituting the position-74 valine of the O-2-sensitive NiFe hydrogenase from Desulfovibrio fructosovorans. We observed that this substitution can accelerate reactivation up to 1,000-fold, depending on the polarity of the position 74 amino acid side chain. In terms of kinetics of anaerobic (in) activation and oxygen tolerance, the valine-to-histidine mutation has the most spectacular effect: The V74H mutant compares favorably with the O-2-tolerant hydrogenase from Aquifex aeolicus, which we use here as a benchmark.