Glutamate Gated Proton-Coupled Electron Transfer Activity of a [NiFe]-Hydrogenase.

[NiFe] hydrogenases are metalloenzymes that catalyze the reversible oxidation of H2. While electron transfer to and from the active site is understood to occur through iron-sulfur clusters, the mechanism of proton transfer is still debated. Two mechanisms for proton exchange with the active site have been proposed that involve distinct and conserved ionizable amino acid residues, one a glutamate, and the other an arginine. To examine the potential role of the conserved glutamate on active site acid-base chemistry, we mutated the putative proton donor E17 to Q in the soluble hydrogenase I from Pyrococcus furiosus using site directed mutagenesis. FTIR spectroscopy, sensitive to the CO and CN ligands of the active site, reveals catalytically active species generated upon reduction with H2, including absorption features consistent with the Nia-C intermediate. Time-resolved IR spectroscopy, which probes active site dynamics after hydride photolysis from Nia-C, indicates the E17Q mutation does not interfere with the hydride photolysis process generating known intermediates Nia-I1 and Nia-I2. Strikingly, the E17Q mutation disrupts obligatory proton-coupled electron transfer from the Nia-I1 state, thereby preventing formation of Nia-S. These results directly establish E17 as a proton donor/acceptor in the Nia-S ↔ Nia-C equilibrium.