Six- to five-coordinate heme-nitrosyl conversion in cytochrome c′ and its relevance to guanylate cyclase

The 5-coordinate ferrous heme of Alcaligenes xylosoxidans cytochrome c' reacts with NO to form a 6-coordinate nitrosyl intermediate (lambda(Soret) at 415 nm) which subsequently converts to a 5-coordinate nitrosyl end product (lambda(Soret) at 395 nm) in a rate-determining step. Stopped-flow measurements at pH 8.9, 25 degreesC, yield a rate constant for the formation of the 6-coordinate nitrosyl adduct, k(on) = (4.4 +/- 0.5) x 10(4) M-1 s(-1), which is 3-4 orders of magnitude lower than the values for other pentacoordinate ferrous hemes and is consistent with NO binding within the sterically crowded distal heme pocket. Resonance Raman measurements of the freeze-trapped 6-coordinate nitrosyl intermediate reveal an unusually high Fe-NO stretching frequency of 579 cm(-1), suggesting a distorted Fe-N-O coordination geometry. The rate of to 5-coordinate heme nitrosyl conversion is also dependent upon NO concentration, with a rate constant, k(6-5) = (8.1 +/- 0.7) x 10(3) M-1 s(-1), implying that an additional molecule of NO is required to form the 5c-NO adduct. Since crystallographic studies have shown that the 5-coordinate nitrosyl complex of cytochrome c' binds NO to the proximal (rather than distal) face of the heme, the NO dependence of the 6- to 5-coordinate NO conversion supports a mechanism in which the weakened His ligand, as well as the distally bound NO, is displaced by a second NO molecule which attacks and is retained in the proximal coordination position. The fact that a dependent 6- to 5-coordinate nitrosyl conversion has been previously reported for soluble guanylate cyclase suggests that the mechanism of Fe-His bond cleavage may be similar to that of cytochrome c' and strengthens the recent proposal that both proteins exhibit proximal NO binding in their 5-coordinate nitrosyl adducts.