A Microscopic Study of the DeoxyHemoglobin-Catalyzed Generation of Nitric Oxide from Nitrite Anion

Abstract

There is recent evidence suggesting that nitrite anion (NO2-) represents the major intravascularNO storage molecule whose transduction to NO is facilitated by a reduction mechanism catalyzed bydeoxygenated hemoglobin (deoxy-Hb). In this work, we provide a detailed microscopic study of deoxy-Hb nitrite reductase (NIR) activity by combining classical molecular dynamics and hybrid quantummechanical-molecular mechanical simulations. Our results point out that two alternative mechanismscould be operative and suggest that the most energetic barriers should stem from either reprotonation ofthe distal histidine or NO dissociation from the ferric heme. In the first proposed mechanism, which issimilar to that proposed for bacterial NIRs, nitrite anion or nitrous acid coordinates to the heme throughthe N atom. This pathway involves HisE7 in a one or two proton transfer process, depending on whetherthe active species is nitrite anion or nitrous acid, to yield an intermediate Fe(III)NO species which eventuallydissociates leading to NO and methemoglobin. In the second mechanism, the nitrite anion coordinates tothe heme through the O atom. This pathway requires only one proton transfer from HisE7 and leadsdirectly to the formation of a hydroxo Fe(III) complex and NO.