title: Metal substitution in the active site of nitrogenase MFe(7)S(9) (M = Mo(4+), V(3+), Fe(3+)).
abstract: The unifying view that molybdenum is the essential component in nitrogenase has changed over the past few years with the discovery of a vanadium-containing nitrogenase and an iron-only nitrogenase. The principal question that has arisen for the alternative nitrogenases concerns the structures of their corresponding cofactors and their metal-ion valence assignments and whether there are significant differences with that of the more widely known molybdenum-iron cofactor (FeMoco). Spin-polarized broken-symmetry (BS) density functional theory (DFT) calculations are used to assess which of the two possible metal-ion valence assignments (4Fe(2+)4Fe(3+) or 6Fe(2+)2Fe(3+)) for the iron-only cofactor (FeFeco) best represents the resting state. For the 6Fe(2+)2Fe(3+) oxidation state, the spin coupling pattern for several spin state alignments compatible with S = 0 were generated and assessed by energy criteria. The most likely BS spin state is composed of a 4Fe cluster with spin S(a) = (7)/(2) antiferromagnetically coupled to a 4Fe' cluster with spin S(b) = (7)/(2). This state has the lowest DFT energy for the isolated FeFeco cluster and displays calculated Mössbauer isomer shifts consistent with experiment. Although the S = 0 resting state of FeFeco has recently been proposed to have metal-ion valencies of 4Fe(2+)4Fe(3+) (derived from experimental Mössbauer isomer shifts), our isomer shift calculations for the 4Fe(2+)4Fe(3+) oxidation state are in poorer agreement with experiment. Using the Mo(4+)6Fe(2+)Fe(3+) oxidation level of the cofactor as a starting point, the structural consequences of replacement of molybdenum (Mo(4+)) with vanadium (V(3+)) or iron (Fe(3+)) in the cofactor have been investigated. The size of the cofactor cluster shows a dependency on the nature of the heterometal and increases in the order FeMoco < FeVco < FeFeco.
authors.affiliation: Department of Molecular Biology TPC-15, The Scripps Research Institute, La Jolla, CA 92037, USA. tlovell@scripps.edu
authors.forename: Timothy
authors.initials: T
authors.lastname: Lovell
authors.forename: Rhonda A
authors.initials: RA
authors.lastname: Torres
authors.forename: Wen-Ge
authors.initials: WG
authors.lastname: Han
authors.forename: Tiqing
authors.initials: T
authors.lastname: Liu
authors.forename: David A
authors.initials: DA
authors.lastname: Case
authors.forename: Louis
authors.initials: L
authors.lastname: Noodleman
mesh_terms: D015502:Absorptiometry, Photon; D016948:Azotobacter vinelandii / Q000201:enzymology; D003035:Cobalt / Q000737:chemistry; D018360:Crystallography, X-Ray; D007501:Iron / Q000737:chemistry; D007536:Isomerism; D008667:Metalloproteins / Q000737:chemistry*; D008982:Molybdenum / Q000737:chemistry*; D009591:Nitrogenase / Q000737:chemistry*; D010084:Oxidation-Reduction; D011506:Proteins / Q000737:chemistry; D014639:Vanadium / Q000737:chemistry
chemical_list: D008667:Metalloproteins; D011506:Proteins; D014639:Vanadium; D003035:Cobalt; D008982:Molybdenum; D007501:Iron; D009591:Nitrogenase