title: Effect of a tridentate ligand on the structure, electronic structure, and reactivity of the copper(I) nitrite complex: role of the conserved three-histidine ligand environment of the type-2 copper site in copper-containing nitrite reductases.
abstract: It is postulated that the copper(I) nitrite complex is a key reaction intermediate of copper containing nitrite reductases (Cu-NiRs), which catalyze the reduction of nitrite to nitric oxide (NO) gas in bacterial denitrification. To investigate the structure-function relationship of Cu-NiR, we prepared five new copper(I) nitrite complexes with sterically hindered tris(4-imidazolyl)carbinols [Et-TIC = tris(1-methyl-2-ethyl-4-imidazolyl)carbinol and iPr-TIC = tris(1-methyl-2-isopropyl-4-imidazolyl)carbinol] or tris(1-pyrazolyl)methanes [Me-TPM = tris(3,5-dimethyl-1-pyrazolyl)methane; Et-TPM = tris(3,5-diethyl-1-pyrazolyl)methane; and iPr-TPM = tris(3,5-diisopropyl-1-pyrazolyl)methane]. The X-ray crystal structures of all of these copper(I) nitrite complexes were mononuclear eta(1)-N-bound nitrite complexes with a distorted tetrahedral geometry. The electronic structures of the complexes were investigated by absorption, magnetic circular dichroism (MCD), NMR, and vibrational spectroscopy. All of these complexes are good functional models of Cu-NiR that form NO and copper(II) acetate complexes well from reactions with acetic acid under anaerobic conditions. A comparison of the reactivity of these complexes, including previously reported (iPr-TACN)Cu(NO2) [iPr-TACN = 1,4,7-triisopropyl-1,4,7-triazacyclononane], clearly shows the drastic effects of the tridentate ligand on Cu-NiR activity. The copper(I) nitrite complex with the Et-TIC ligand, which is similar to the highly conserved three-histidine ((His)3) ligand environment in the catalytic site of Cu-NiR, had the highest Cu-NiR activity. This result suggests that the (His)3 ligand environment is essential for acceleration of the Cu-NiR reaction. The highest Cu-NiR activity for the Et-TIC complex can be explained by the structural and spectroscopic characterizations and the molecular orbital calculations presented in this paper. Based on these results, the functional role of the (His)3 ligand environment in Cu-NiR is discussed.
authors.affiliation: Institute for Molecular Science and Okazaki Institute for Integrative Bioscience, National Institutes of Natural Sciences, Myodaiji, Okazaki 444-8787, Japan.
authors.forename: Masato
authors.initials: M
authors.lastname: Kujime
authors.forename: Chiemi
authors.initials: C
authors.lastname: Izumi
authors.forename: Masaaki
authors.initials: M
authors.lastname: Tomura
authors.forename: Masahiko
authors.initials: M
authors.lastname: Hada
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Fujii
mesh_terms: D019342:Acetic Acid / Q000737:chemistry; D042481:Alcaligenes faecalis / Q000201:enzymology; D001665:Binding Sites; D040761:Biomimetic Materials / Q000138:chemical synthesis / Q000737:chemistry / Q000378:metabolism; D003300:Copper / Q000737:chemistry* / Q000378:metabolism; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D006639:Histidine / Q000737:chemistry* / Q000378:metabolism; D007093:Imidazoles / Q000138:chemical synthesis / Q000737:chemistry / Q000378:metabolism; D007700:Kinetics; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D000432:Methanol / Q000031:analogs & derivatives; D008958:Models, Molecular; D009572:Nitrite Reductases / Q000737:chemistry* / Q000378:metabolism; D009573:Nitrites / Q000737:chemistry* / Q000378:metabolism; D011720:Pyrazoles / Q000138:chemical synthesis / Q000737:chemistry / Q000378:metabolism; D013055:Spectrophotometry, Infrared; D013056:Spectrophotometry, Ultraviolet
chemical_list: D007093:Imidazoles; D008024:Ligands; D009573:Nitrites; D011720:Pyrazoles; D006639:Histidine; D003300:Copper; D009572:Nitrite Reductases; C052695:nitrite reductase, copper-containing; D019342:Acetic Acid; D000432:Methanol