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Hemoglobin as a Nitrite Anhydrase: Modeling Methemoglobin ‐ Mediated N 2 O 3 Formation
Author(s) -
Hopmann Kathrin H.,
Cardey Bruno,
Gladwin Mark T.,
KimShapiro Daniel B.,
Ghosh Abhik
Publication year - 2011
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201003578
Subject(s) - methemoglobin , nitrite , chemistry , hemoglobin , heme , ferrous , inorganic chemistry , stereochemistry , biochemistry , medicinal chemistry , enzyme , organic chemistry , nitrate
Nitrite has recently been recognized as a storage form of NO in blood and as playing a key role in hypoxic vasodilation. The nitrite ion is readily reduced to NO by hemoglobin in red blood cells, which, as it happens, also presents a conundrum. Given NO’s enormous affinity for ferrous heme, a key question concerns how it escapes capture by hemoglobin as it diffuses out of the red cells and to the endothelium, where vasodilation takes place. Dinitrogen trioxide (N 2 O 3 ) has been proposed as a vehicle that transports NO to the endothelium, where it dissociates to NO and NO 2 . Although N 2 O 3 formation might be readily explained by the reaction Hb‐Fe 3+ +NO 2 − +NO⇌Hb‐Fe 2+ +N 2 O 3 , the exact manner in which methemoglobin (Hb‐Fe 3+ ), nitrite and NO interact with one another is unclear. Both an “Hb‐Fe 3+ ‐NO 2 − +NO” pathway and an “Hb‐Fe 3+ ‐NO+NO 2 − ” pathway have been proposed. Neither pathway has been established experimentally. Nor has there been any attempt until now to theoretically model N 2 O 3 formation, the so‐called nitrite anhydrase reaction. Both pathways have been examined here in a detailed density functional theory (DFT, B3LYP/TZP) study and both have been found to be feasible based on energetics criteria. Modeling the “Hb‐Fe 3+ ‐NO 2 − +NO” pathway proved complex. Not only are multiple linkage‐isomeric ( N ‐ and O ‐coordinated) structures conceivable for methemoglobin–nitrite, multiple isomeric forms are also possible for N 2 O 3 (the lowest‐energy state has an NN‐bonded nitronitrosyl structure, O 2 NNO). We considered multiple spin states of methemoglobin–nitrite as well as ferromagnetic and antiferromagnetic coupling of the Fe 3+ and NO spins. Together, the isomerism and spin variables result in a diabolically complex combinatorial space of reaction pathways. Fortunately, transition states could be successfully calculated for the vast majority of these reaction channels, both M S =0 and M S =1. For a six‐coordinate Fe 3+ ‐ O ‐nitrito starting geometry, which is plausible for methemoglobin–nitrite, we found that N 2 O 3 formation entails barriers of about 17–20 kcal mol −1 , which is reasonable for a physiologically relevant reaction. For the “Hb‐Fe 3+ ‐NO+NO 2 − ” pathway, which was also found to be energetically reasonable, our calculations indicate a two‐step mechanism. The first step involves transfer of an electron from NO 2 − to the Fe 3+ –heme–NO center ({FeNO} 6 ) , resulting in formation of nitrogen dioxide and an Fe 2+ –heme–NO center ({FeNO} 7 ). Subsequent formation of N 2 O 3 entails a barrier of only 8.1 kcal mol −1 . From an energetics point of view, the nitrite anhydrase reaction thus is a reasonable proposition. Although it is tempting to interpret our results as favoring the “{FeNO} 6 +NO 2 − ” pathway over the “Fe 3+ ‐nitrite+NO” pathway, both pathways should be considered energetically reasonable for a biological reaction and it seems inadvisable to favor a unique reaction channel based solely on quantum chemical modeling.