Activation Barriers of Oxygen Transformation at the
Active Site of [FeFe] Hydrogenases
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Abstract
Oxygen
activation at the active sites of [FeFe] hydrogenases has
been proposed to be the initial step of irreversible oxygen-induced
inhibition of these enzymes. On the basis of a first theoretical study
into the thermodynamics of O<sub>2</sub> activation [<i>Inorg.
Chem.</i> <b>2009</b>, 48, 7127] we here investigate the
kinetics of possible reaction paths at the distal iron atom of the
active site by means of density functional theory. A sequence of steps
is proposed to either form a reactive oxygen species (ROS) or fully
reduce O<sub>2</sub> to water. In this reaction cascade, two branching
points are identified where water formation directly competes with
harmful oxygen activation reactions. The latter are water formation
by O–O bond cleavage of a hydrogen peroxide-bound intermediate
competing with H<sub>2</sub>O<sub>2</sub> dissociation and CO<sub>2</sub> formation by a putative iron-oxo species competing with protonation
of the iron-oxo species to form a hydroxyo ligand. Furthermore, we
show that proton transfer to activated oxygen is fast and that proton
supply to the active site is vital to prevent ROS dissociation. If
sufficiently many reduction equivalents are available, oxygen activation
reactions are accelerated, and oxygen reduction to water becomes possible