Modeling of the Electrostatic Interaction and Catalytic Activity of [NiFe] Hydrogenases on a Planar Electrode

Hydrogenases are
a group of enzymes that have caught the interest
of researchers in renewable energies, due to their ability to catalyze
the redox reaction of hydrogen. The exploitation of hydrogenases in
electrochemical devices requires their immobilization on the surface
of suitable electrodes, such as graphite. The orientation of the enzyme
on the electrode is important to ensure a good flux of electrons to
the catalytic center, through an array of iron–sulfur clusters.
Here we present a computational approach to determine the possible
orientations of a [NiFe] hydrogenase (PDB 1e3d) on a planar electrode, as a function
of pH, salinity, and electrode potential. The calculations are based
on the solution of the linearized Poisson–Boltzmann equation,
using the PyGBe software. The results reveal that electrostatic interactions
do not truly immobilize the enzyme on the surface of the electrode,
but there is instead a dynamic equilibrium between different orientations.
Nonetheless, after averaging over all thermally accessible orientations,
we find significant differences related to the solution’s salinity
and pH, while the effect of the electrode potential is relatively
weak. We also combine models for the protein adsoption–desorption
equilibria and for the electron transfer between the proteins and
the electrode to arrive at a prediction of the electrode’s
activity as a function of the enzyme concentration.