文摘
Resonance Raman (RR) studies have been conducted on Alcaligenes xylosoxidans cytochromec', a mono-His ligated hemoprotein which reversibly binds NO and CO but not O2. Recent crystallographiccharacterization of this protein has revealed the first example of a hemoprotein which can utilize bothsides of its heme (distal and proximal) for binding exogenous ligands to its Fe center. The present RRinvestigation of the Fe coordination and heme pocket environments of ferrous, carbonyl, and nitrosylforms of cytochrome c' in solution fully supports the structures determined by X-ray crystallography andoffers insights into mechanisms of ligand discrimination in heme-based sensors. Ferrous cytochrome c'reacts with CO to form a six-coordinate heme-CO complex, whereas reaction with NO results in cleavageof the proximal linkage to give a five-coordinate heme-NO adduct, despite the relatively high stretchingfrequency (231 cm-1) of the ferrous Fe-N(His) bond. RR spectra of the six-coordinate CO adduct indicatethat CO binds to the Fe in a nonpolar environment in line with its location in the hydrophobic distal hemepocket. On the other hand, RR data for the five-coordinate NO adduct suggest a positively polarizedenvironment for the NO ligand, consistent with its binding close to Arg 124 on the opposite (proximal)side of the heme. Parallels between certain physicochemical properties of cytochrome c' and those ofheme-based sensor proteins raise the possibility that the latter may also utilize both sides of their hemesto discriminate between NO and CO binding.