PNNL

Abstract

The reduction of N2 to NH3 by Mo-dependent nitrogenase at its active-site metal cluster FeMo-cofactor utilizes reductive elimination (re) of Fe-bound hydrides with obligatory loss of H2 to activate the enzyme for binding/reduction of N2. Earlier work showed that wild type nitrogenase and a nitrogenase with amino acid substitutions in the MoFe protein near FeMo-cofactor can catalytically reduce CO2 by 2 or 8 electrons/protons to carbon monoxide (CO) and methane (CH4) at low rates. Here, it is demonstrated that nitrogenase preferentially reduces CO2 by 2 electrons/protons to formate (HCOO-) at rates >10 times higher than rates of CO2 reduction to CO and CH4. The failure of H2 to inhibit CO2 reduction rules out a re mechanism in this case. Quantum mechanical (QM) calculations on the activated FeMo-cofactor with a bound hydride (E2(2H) state) favor a direct reaction of CO2 with the hydride (‘direct hydride transfer’ reaction pathway), with facile hydride transfer to CO2 yielding formate. In contrast, a significant barrier is observed for reaction of Fe-bound CO2 with the hydride (‘associative’ reaction pathway), which leads to CO and CH4. Remarkably, in the direct hydride transfer pathway, the Fe-H behaves as a hydridic hydrogen, whereas in the associative pathway it acts as a protic hydrogen. MoFe proteins with amino acid substitutions near FeMo-cofactor (a-70Val?Ala, a -195His?Gln) are found to significantly alter the distribution of products between formate and CO/CH4. This work is based upon work supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences (LCS and DRD), the Division of Chemical Sciences, Geosciences, and Bio-Sciences (SR) and the NIH (GM 111097; BMH).