Mechanisms of Ferriheme Reduction by Nitric Oxide:  Nitrite and General Base Catalysis¹

Abstract

The reductive nitrosylation (Fe(III)(P) + 2NO + H(2)O = Fe(II)(P)(NO) + NO(2)(-) + 2H(+)) of the ferriheme model Fe(III)(TPPS) (TPPS = tetra(4-sulfonatophenyl)porphyrinato) has been investigated in moderately acidic solution. In the absence of added or adventitious nitrite, this reaction displays general base catalysis with several buffers in aqueous solutions. It was also found that the nitrite ion, NO(2)(-), is a catalyst for this reaction. Similar nitrite catalysis was demonstrated for another ferriheme model system Fe(III)(TMPy) (TMPy = meso-tetrakis(N-methyl-4-pyridyl)porphyrinato), and for ferriheme proteins met-hemoglobin (metHb) and met-myoglobin (metMb) in aqueous buffer solutions. Thus, it appears that such catalysis is a general mechanistic route to the reductive nitrosylation products. Two nitrite catalysis mechanisms are proposed. In the first, NO(2)(-) is visualized as operating via nucleophilic addition to the Fe(III)-coordinated NO in a manner similar to the reactions proposed for Fe(III) reduction promoted by other nucleophiles. This would give a labile N(2)O(3) ligand that hydrolyzes to nitrous acid, regenerating the original nitrite. The other proposal is that Fe(III) reduction is effected by direct outer-sphere electron transfer from NO(2)(-) to Fe(III)(P)(NO) to give nitrogen dioxide plus the ferrous nitrosyl complex Fe(II)(P)(NO). The NO(2) thus generated would be trapped by excess NO to give N(2)O(3) and, subsequently, nitrite. It is found that the nitrite catalysis rates are markedly sensitive to the respective Fe(III)(P)(NO) reduction potentials, which is consistent with the behavior expected for an outer-sphere electron-transfer mechanism. Nitrite is the product of NO autoxidation in aqueous solution and is a ubiquitous impurity in experiments where aqueous NO is added to an aerobic system to study biological effects. The present results demonstrate that such an impurity should not be assumed to be innocuous, especially in the context of recent reports that endogenous nitrite may play physiological roles relevant to the interactions of NO and ferriheme proteins.

Generating S-Nitrosothiols from Hemoglobin

“…Reductive Nitrosylation Versus Intermediate Formation, Conformation Dependence-The above results are consistent with preferential binding to ferric heme of NO relative to nitrite under conditions where both are at low concentrations. Ferric NOHb can undergo reductive nitrosylation (13,67,69,89) or, if there is nitrite present, formation of the intermediate that we attribute to heme-bound N 2 O 3 . Because both can be relatively rapid, the question arises as to whether there is a conformational dependence associated with the partitioning of two reactions.…”

Mechanisms of Nitric Oxide Reactions Mediated by Biologically Relevant Metal Centers

“…More recent studies by Fernandez et al demonstrated that the reductive nitrosylations of Fe III (TPPS) [127] and of metHb and metMb [128] are promoted by general base catalysis and by other nucleophiles, including nitrite ion (Scheme 6). In the case of the latter, the catalytic role of nitrite in promoting reductive nitrosylation of these ferric heme models and proteins was first discovered in an attempt to pin down experimental anomalies that were eventually attributed to the ubiquitous NO 2 -impurities in aqueous NO solutions.…”

“…Since the Cu(phen) 2 (H 2 O) 2+ analog (phen ¼ 1,10-phenanthroline) is a much weaker oxidant (0.18 V), this property can be attributed to the steric Scheme 6 Proposed mechanism for the nitrite catalysis of the reductive nitrosylation of Fe III (TPPS), metHb, and metMb [127] 122 P.C. Ford et al repulsion between the methyl groups of the respective dmp ligands that favors the tetrahedral coordination of Cu(I) over the tetragonal pyramidal structure of Cu(II).…”

Structure and Reactivity of Hexacoordinate Hemoglobins

“…The possibility of a similar function has been tested in several hxHbs. In all cases, ferrous hxHbs will bind reversibly to NO [15], the ferric forms will react with NO [74] in some cases showing slow reduction [15,[75][76][77][78], and the oxy-ferrous hxHbs will scavenge NO resulting in their oxidation (Table 3) [75,[79][80][81][82][83]. In spite of the fact that the efficiency of these reactions is at best on-par with Mb and red blood cell Hb, and is certainly limited in vivo by as-of-yet unknown mechanisms for rereduction of the heme iron [4], observation of the NOD reaction has been proposed as support for an NO scavenging function in nsHbs [80,84], Ngbs [79], and Cgbs [85].…”