Reaction of nitric oxide with heme proteins and model compounds of hemoglobin

Abstract

Rates for the reaction of nitric oxide with several ferric heme proteins and model compounds have been measured. The NO combination rates are markedly affected by the presence or absence of distal histidine. Elephant myoglobin in which the E7 distal histidine has been replaced by glutamine reacts with NO 500-1000 times faster than do the native hemoglobins or myoglobins. By contrast, there is no difference in the CO combination rate constants of sperm whale and elephant myoglobins. Studies on ferric model compounds for the R and T states of hemoglobin indicate that their NO combination rate constants are similar to those observed for the combination of CO with the corresponding ferro derivatives. The last observation suggests that the presence of an axial water molecule at the ligand binding site of ferric hemoglobin A prevents it from exhibiting significant cooperativity in its reactions with NO.

Routes to S -nitroso-hemoglobin formation with heme redox and preferential reactivity in the β subunits

“…The dominance of reaction 3 at low NO levels implies a substantial subunit inequivalence in the binding of NO to form the ferric nitrosyl heme intermediate and͞or the subsequent reaction of that intermediate to form the products. Previous studies of the NO binding to met-Hb (13,24,25) show that equilibrium favors the ␤ subunit only by a factor of Ϸ1.5. Although the subunit dependence of Fe(III)NO reaction rates has not been examined in detail, it has been noted that the reductive nitrosylation occurs ''considerably faster'' with isolated ␤ chains (␤ subunit tetrameric protein) than with ␣ chains (25).…”

“…Previous studies of the NO binding to met-Hb (13,24,25) show that equilibrium favors the ␤ subunit only by a factor of Ϸ1.5. Although the subunit dependence of Fe(III)NO reaction rates has not been examined in detail, it has been noted that the reductive nitrosylation occurs ''considerably faster'' with isolated ␤ chains (␤ subunit tetrameric protein) than with ␣ chains (25). In these previous studies, with NO present in excess, the reaction would presumably involve Hb (Fe(III)NO) 4 The observed chemistry rationalizes a number of observations made in previous studies of this reaction.…”

“…It has been appreciated, however, that the situation with tetrameric hemoglobins, composed of distinct ␣ and ␤ subunits, is more complex (24,25). Although subunit inequivalence has been reported in the uptake of NO (13,24,25), such effects in the latter steps of the reaction and their impact on the nature of distribution of the final reaction products have not been previously investigated.…”

“…Although subunit inequivalence has been reported in the uptake of NO (13,24,25), such effects in the latter steps of the reaction and their impact on the nature of distribution of the final reaction products have not been previously investigated. Moreover, this reaction has not been previously studied in situations where the NO is limiting, as in the biological situation.…”

Spectroscopic and kinetic aspects of Elephas maximus hemoglobin

“…With regard to the general mechanism of the autoreduction, it is clear that Hb(III)(OH,) initially reacts with NO to form Hb(I1I)NO; for myoglobins, these adducts have formation constants of the order of lo3 -lo5 [29], or greater in the absence of distal histidine. In the rate-determining step, the Hb(II1)NO may then react with another molecule of NO as summarized in D) is controlled by the degree of tension in the proximal imidazole -iron bond and presumably by the environment provided by the amino acid residues in the heme pocket [3].…”