The H2 sensor of Ralstonia eutropha: biochemical and spectroscopic analysis of mutant proteins modified at a conserved glutamine residue close to the [NiFe] active site

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Abstract
[NiFe] hydrogenases contain a highly conserved histidine residue close to the [NiFe] active site which is altered by a glutamine residue in the H2-sensing [NiFe] hydrogenases. In this study, we exchanged the respective glutamine residue of the H2 sensor (RH) of Ralstonia eutropha, Q67 of the RH large subunit HoxC, by histidine, asparagine and glutamate. The replacement by histidine and asparagine resulted in slightly unstable RH proteins which were hardly affected in their regulatory and enzymatic properties. The exchange to glutamate led to a completely unstable RH protein. The purified wild-type RH and the mutant protein with the Gln/His exchange were analysed by continuous-wave and pulsed electron paramagnetic resonance (EPR) techniques. We observed a coupling of a nitrogen nucleus with the [NiFe] active site for the mutant protein which was absent in the spectrum of the wild-type RH. A combination of theoretical calculations with the experimental data provided an explanation for the observed coupling. It is shown that the coupling is due to the formation of a weak hydrogen bond between the protonated N(ε) nucleus of the histidine with the sulfur of a conserved cysteine residue which coordinates the metal atoms of the [NiFe] active site as a bridging ligand. The effect of this hydrogen bond on the local structure of the [NiFe] active site is discussed.

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