Electronic Structure of the Perturbed Blue Copper Site in Nitrite Reductase: Spectroscopic Properties, Bonding, and Implications for the Entatic/Rack State
详细信息 查看全文
- 作者：Louis B. LaCroix ; Susan E. Shadle ; Yaning Wang ; Bruce A. Averill ; Britt Hedman ; Keith O. Hodgson ; and Edward I. Solomon
- 刊名：Journal of the American Chemical Society
- 出版年：1996
- 出版时间：August 21, 1996
- 年：1996
- 卷：118
- 期：33
- 页码：7755 - 7768
- 全文大小：365K
- 年卷期：v.118,no.33(August 21, 1996)
- ISSN：1520-5126
文摘
Low-temperature optical absorption, circular dichroism, magneticcircular dichroism, and sulfur K-edgeX-ray absorption spectra have been measured for the green "blue"copper center (type 1) in Achromobactercycloclastesnitrite reductase. Combined with density functional calculations,the results of these spectroscopies have been usedto define the extremely "perturbed" electronic structure of thissite relative to that of the prototypical "classic"sitefound in plastocyanin. Experimentally calibrated densityfunctional calculations have been further used todeterminethe specific geometric distortions which generate the perturbedelectronic structure. These studies indicate that theprincipal electronic structure changes in nitrite reductase, relativeto plastocyanin, are a rotation of the Cudx2-y2half-filled, highest occupied molecular orbital (HOMO) and an increasein the ligand field strength at the Cu center.The HOMO rotation increases the pseudo-
interaction anddecreases the 
interaction of the cysteine (Cys) sulfurwith Cudx2-y2.Furthermore, significant methionine (Met) sulfur character ismixed into the HOMO due to increasedoverlap with Cudx2-y2.These changes in Cu-ligand interactions result in theredistribution of absorption intensityin the charge transfer and ligand field transitions. Additionally,the new S(Met)-Cu interaction accounts for theunexpectedly high sulfur covalency in the HOMO. The increase inligand field strength shifts all the d 
d transitionsin nitrite reductase to ~1000 cm-1 higherenergy than their counterparts in plastocyanin, which accounts fortheEPR spectral differences between the type 1 sites in these complexes.The geometric distortion primarily responsiblefor the electronic structure changes in nitrite reductase, relative toplastocyanin, is determined to involve a coupledangular movement of the Cys and Met residues toward a more flattenedtetrahedral (toward square planar) structure.This movement is consistent with a tetragonal Jahn-Tellerdistortion resulting from the shorter Cu-S(Met) bond innitrite reductase relative to plastocyanin. This increasedJahn-Teller distortion implies that the type 1 site is"lessentatic" than that in plastocyanin.
interaction anddecreases the interaction of the cysteine (Cys) sulfurwith Cudx2-y2.Furthermore, significant methionine (Met) sulfur character ismixed into the HOMO due to increasedoverlap with Cudx2-y2.These changes in Cu-ligand interactions result in theredistribution of absorption intensityin the charge transfer and ligand field transitions. Additionally,the new S(Met)-Cu interaction accounts for theunexpectedly high sulfur covalency in the HOMO. The increase inligand field strength shifts all the d d transitionsin nitrite reductase to ~1000 cm-1 higherenergy than their counterparts in plastocyanin, which accounts fortheEPR spectral differences between the type 1 sites in these complexes.The geometric distortion primarily responsiblefor the electronic structure changes in nitrite reductase, relative toplastocyanin, is determined to involve a coupledangular movement of the Cys and Met residues toward a more flattenedtetrahedral (toward square planar) structure.This movement is consistent with a tetragonal Jahn-Tellerdistortion resulting from the shorter Cu-S(Met) bond innitrite reductase relative to plastocyanin. This increasedJahn-Teller distortion implies that the type 1 site is"lessentatic" than that in plastocyanin.