Hydrogenases catalyze
hydrogen/proton interconversion that is normally
electrochemically reversible (having minimal overpotential requirement),
a special property otherwise almost exclusive to platinum metals.
The mechanism of [NiFe]-hydrogenases includes a long-range proton-coupled
electron-transfer process involving a specific Ni-coordinated cysteine
and the carboxylate of a nearby glutamate. A variant in which this
cysteine has been exchanged for selenocysteine displays two distinct
changes in electrocatalytic properties, as determined by protein film
voltammetry. First, proton reduction, even in the presence of H2 (a strong product inhibitor), is greatly enhanced relative
to H2 oxidation: this result parallels a characteristic
of natural [NiFeSe]-hydrogenases which are superior H2 production
catalysts. Second, an inflection (an S-shaped “twist”
in the trace) appears around the formal potential, the small overpotentials
introduced in each direction (oxidation and reduction) signaling a
departure from electrocatalytic reversibility. Concerted proton–electron
transfer offers a lower energy pathway compared to stepwise transfers.
Given the much lower proton affinity of Se compared to that of S,
the inflection provides compelling evidence that concerted proton–electron
transfer is important in determining why [NiFe]-hydrogenases are reversible
electrocatalysts.