The side-on copper(I) nitrosyl geometry in copper nitrite reductase is due to steric interactions with isoleucine-257.
Density functional theory calculations were used to investigate the binding mode of copper(I) nitrosyl (Cu(I)-NO) in copper nitrite reductase (CuNIR). The end-on Cu(I)-NO geometry (2) was found to be the global energy minimum, while the side-on binding mode (1) corresponds to a local minimum. Isoleucine-257 severely interacts sterically with the Cu(I)-NO unit when bound end-on but not in the side-on case. In addition, the side-on geometry is also stabilized by a hydrogen bond between aspartic acid-98 and NO, estimated to be approximately 3 kcal/mol. The steric constraint of the CuNIR active site is mainly responsible for the observed side-on coordination of NO in the CuNIR crystal structure. We speculate that a small conformational change of the active site that slightly changes the position of isoleucine-257 would allow NO to bind end-on. This explains the observed end-on binding of NO to copper(I) when CuNIR is in solution.
Merkle
Anna C
AC
Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109-1055, USA.
Lehnert
Nicolai
N
D003300:Copper / Q000737:chemistry*; D018360:Crystallography, X-Ray; D007532:Isoleucine / Q000737:chemistry* / Q000378:metabolism; D008956:Models, Chemical; D008958:Models, Molecular; D009572:Nitrite Reductases / Q000737:chemistry* / Q000378:metabolism; D009603:Nitroso Compounds / Q000737:chemistry*; D013237:Stereoisomerism
D009603:Nitroso Compounds; D007532:Isoleucine; D003300:Copper; D009572:Nitrite Reductases; C052695:nitrite reductase, copper-containing