TY - JOUR
T1 - CO2 Reduction Catalyzed by Nitrogenase
T2 - Pathways to Formate, Carbon Monoxide, and Methane
AU - Khadka, Nimesh
AU - Dean, Dennis R.
AU - Smith, Dayle
AU - Hoffman, Brian M.
AU - Raugei, Simone
AU - Seefeldt, Lance C.
N1 - Funding Information:
This work is based upon work supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (DE-SC0010687 and DE-SC0010834; L.C.S. and D.R.D.), the Division of Chemical Sciences, Geosciences, and Bio-Sciences (S.R.), and the NSF (MCB 1515981; B.M.H.).
PY - 2016/9/6
Y1 - 2016/9/6
N2 - The reduction of N2 to NH3 by Mo-dependent nitrogenase at its active-site metal cluster FeMo-cofactor utilizes reductive elimination 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 two or eight electrons/protons to carbon monoxide (CO) and methane (CH4) at low rates. Here, it is demonstrated that nitrogenase preferentially reduces CO2 by two electrons/protons to formate (HCOO-) at rates >10 times higher than rates of CO2 reduction to CO and CH4. Quantum mechanical calculations on the doubly reduced FeMo-cofactor with a Fe-bound hydride and S-bound proton (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 (α-70Val→Ala, α-195His→Gln) are found to significantly alter the distribution of products between formate and CO/CH4.
AB - The reduction of N2 to NH3 by Mo-dependent nitrogenase at its active-site metal cluster FeMo-cofactor utilizes reductive elimination 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 two or eight electrons/protons to carbon monoxide (CO) and methane (CH4) at low rates. Here, it is demonstrated that nitrogenase preferentially reduces CO2 by two electrons/protons to formate (HCOO-) at rates >10 times higher than rates of CO2 reduction to CO and CH4. Quantum mechanical calculations on the doubly reduced FeMo-cofactor with a Fe-bound hydride and S-bound proton (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 (α-70Val→Ala, α-195His→Gln) are found to significantly alter the distribution of products between formate and CO/CH4.
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U2 - 10.1021/acs.inorgchem.6b00388
DO - 10.1021/acs.inorgchem.6b00388
M3 - Article
C2 - 27500789
AN - SCOPUS:84986211401
VL - 55
SP - 8321
EP - 8330
JO - Inorganic Chemistry
JF - Inorganic Chemistry
SN - 0020-1669
IS - 17
ER -