A Threonine Stabilizes the NiC and NiR Catalytic Intermediates of [NiFe]-hydrogenase [Molecular Biophysics]

February 9th, 2015 by Abou-Hamdan, A., Ceccaldi, P., Lebrette, H., Guti&eacuterrez-Sanz, O., Richaud, P., Cournac, L., Guigliarelli, B., de Lacey, A. L., L&eacuteger, C., Volbeda, A., Burlat, B., Dementin, S.

The heterodimeric [NiFe] hydrogenase from Desulfovibrio fructosovorans catalyzes the reversible oxidation of H2 into protons and electrons. The catalytic intermediates have been attributed to forms of the active site (NiSI, NiR and NiC) detected using spectroscopic methods under potentiometric but non-catalytic conditions. Here, we produced variants by replacing the conserved T18 residue in the small subunit with S, V, Q G or D, and we analysed the effects of these mutations on the kinetic (H2 oxidation, H2 production and H/D exchange), spectroscopic (IR, EPR) and structural properties of the enzyme. The mutations disrupt the H-bond network in the crystals, and have a strong effect on H2 oxidation and H2 production turnover rates. However, the absence of correlation between activity and rate of H/D exchange in the series of mutants suggests that the alcoholic group of T18 is not necessarily a proton relay. Instead, the correlation between H2 oxidation and production activity and the detection of the NiC species in reduced samples confirms that NiC is a catalytic intermediate, and suggests that T18 is essential to stabilize the local protein structure of the active site ensuring fast NiSi-NiC-NiR interconversions during H2 oxidation/production.