The oxygen tolerance
of the [NiFe]-hydrogenase fromH.
thermoluteoluswas recently assigned to originate from an
unusual coordination sphere of the active site nickel atom (Shomura
et al.Science2017,357, 928–932, 10.1126/science.aan4497). In the oxidized state,
a terminal cysteine residue is displaced by a bidentate coordinating
nearby Glu32 and thus moves to occupy a third μ-cysteine bridging
position. Spectral features of the oxidized state were assigned to
originate from a closed-shell Ni(IV)/Fe(II) state (Kulka-Peschke et
al.J. Am. Chem. Soc.2022,144, 17022–17032, 10.1021/jacs.2c06400). Such a high-valent
nickel oxidation state is unprecedented in biological systems. The
spectral properties and the coordination sphere of that [NiFe]-hydrogenase
can, however, also be rationalized by an energetically lower broken-symmetry
Ni(III)/Fe(III) state of the active site which was not considered.
In this open-shell singlet, the ligand-mediated antiferromagnetic
spin-coupling leads to an overallS= 0 spin state
with evenly distributed spin densities over the metal atoms. Experiments
are suggested that may clarify the final assignment of redox states.