title: Accessibility of the distal heme face, rather than Fe-His bond strength, determines the heme-nitrosyl coordination number of cytochromes c': evidence from spectroscopic studies.
abstract: The heme coordination chemistry and spectroscopic properties of Rhodobacter capsulatus cytochrome c' (RCCP) have been compared to data from Alcaligenes xylosoxidans (AXCP), with the aim of understanding the basis for their different reactivities with nitric oxide (NO). Whereas ferrous AXCP reacts with NO to form a predominantly five-coordinate heme-nitrosyl complex via a six-coordinate intermediate, RCCP forms an equilibrium mixture of six-coordinate and five-coordinate heme-nitrosyl species in approximately equal proportions. Ferrous RCCP and AXCP both exhibit high Fe-His stretching frequencies (227 and 231 cm(-)(1), respectively), suggesting that factors other than the Fe-His bond strength account for their differences in heme-nitrosyl coordination number. Resonance Raman spectra of ferrous-nitrosyl RCCP confirm the presence of both five-coordinate and six-coordinate heme-NO complexes. The six-coordinate heme-nitrosyl of RCCP exhibits a fairly typical Fe-NO stretching frequency (569 cm(-)(1)), in contrast to the relatively high value (579 cm(-)(1)) of the AXCP six-coordinate heme-nitrosyl intermediate. It is proposed that NO experiences greater steric hindrance in binding to the distal face of AXCP, as compared to RCCP, leading to a more distorted Fe-N-O geometry and an elevated Fe-NO stretching frequency. Evidence that RCCP has a more accessible distal coordination site than in AXCP stems from the fact that ferric RCCP readily forms a heme complex with exogenous imidazole, whereas AXCP does not. A model is proposed in which distal heme-face accessibility, rather than the proximal Fe-His bond strength, determines the heme-nitrosyl coordination number in cytochromes c'.
authors.affiliation: Department of Chemistry & Biochemistry, Eastern Oregon University, La Grande, Oregon 97850-2899, USA. candrew@eou.edu
authors.forename: Colin R
authors.initials: CR
authors.lastname: Andrew
authors.forename: Lenord J
authors.initials: LJ
authors.lastname: Kemper
authors.forename: Tammy L
authors.initials: TL
authors.lastname: Busche
authors.forename: Arianne M
authors.initials: AM
authors.lastname: Tiwari
authors.forename: Michael C
authors.initials: MC
authors.lastname: Kecskes
authors.forename: James M
authors.initials: JM
authors.lastname: Stafford
authors.forename: Lea C
authors.initials: LC
authors.lastname: Croft
authors.forename: Shen
authors.initials: S
authors.lastname: Lu
authors.forename: Pierre
authors.initials: P
authors.lastname: Moënne-Loccoz
authors.forename: Willa
authors.initials: W
authors.lastname: Huston
authors.forename: James W B
authors.initials: JW
authors.lastname: Moir
authors.forename: Robert R
authors.initials: RR
authors.lastname: Eady
mesh_terms: D001665:Binding Sites; D003575:Cytochromes c1 / Q000737:chemistry* / Q000378:metabolism*; D006418:Heme / Q000378:metabolism; D007501:Iron; D008958:Models, Molecular; D011487:Protein Conformation; D016354:Rhodobacter capsulatus / Q000201:enzymology*; D013053:Spectrophotometry; D013816:Thermodynamics
chemical_list: D006418:Heme; D003575:Cytochromes c1; D007501:Iron