Distal Histidine Modulates the Unusual O-Binding of Nitrite to Myoglobin: Evidence from the Quantum Chemical Analysis of EPR Parameters.

Nitrite ligand can coordinate with the transition metal through either N- or O-, which is known as linkage isomerism and is believed to occur in metalloproteins. In contrast to the commonly found N-binding motif of nitrite to iron in synthetic models, the less commonly observed O-binding of nitrite to myoglobin ( Copeland , D. M. ; Soares , A. S. ; West , A. H. ; Richter-Addo , G. B. J. Inorg. Biol. Chem. 2006, 100 , 1413 - 1425 ) and hemoglobin ( Yi , J. ; Safo , M. K. ; Richter-Addo , G. Biochemistry , 2008 , 47 , 8247 - 8249 ) reported by Richter-Addo and co-workers is intriguing. On the basis of site-directed mutagenesis studies, it was argued that the distal histidine modulates this unique binding. However, EPR measurements on nitrite binding to methemoglobin could not rule out the possibility of N-bound species to low spin ferric iron. Given to the very similar active sites, there exists a controversy within the two powerful experimental techniques in identifying the coordination motif of nitrite to myoglobin, which is central to understanding the denitrification mechanism. Herein, we report the computation of spin Hamiltonian EPR parameters of different linkage isomers of nitrite bound myoglobin using wave function based "ab initio" and density functional theories to shed light on the binding motif of nitrite to ferric iron. Our predicted spin Hamiltonian parameters agree closely with the experimental EPR data, which provides strong support for the crystallographically implied O-binding to the low-spin ferric heme. This unique O-binding of nitrite to iron is modulated by the distal histidine whose contributions to the active site electronic structure have been successfully quantified. Our quantum chemical insights on the electronic structure of this intermediate are crucial for understanding the structure-function relationship of other metal-nitrite species found in various metalloenzymes.