Kinetics and Mechanism of the Reversible Binding of Nitric Oxide to Reduced Cobalamin B12r (Cob(II)alamin)

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• 作者：Maria Wolak ; Achim Zahl ; Thorsten Schneppensieper ; Grazyna Stochel ; and Rudi van Eldik
• 刊名：Journal of the American Chemical Society
• 出版年：2001
• 出版时间：October 10, 2001
• 年：2001
• 卷：123
• 期：40
• 页码：9780 - 9791
• 全文大小：172K
• 年卷期：v.123,no.40(October 10, 2001)
• ISSN：1520-5126

文摘

The reduced form of aquacobalamin binds nitric oxide very effectively to yield a nitrosyl adduct,Cbl(II)-NO. UV-vis, ¹H-, ³¹P-, and ¹⁵N NMR data suggest that the reaction product under physiologicalconditions is a six-coordinate, "base-on" form of the vitamin with a weakly bound ges/gifchars/alpha.gif" BORDER=0>-dimethylbenzimidazolebase and a bent nitrosyl coordinated to cobalt at the ges/gifchars/beta2.gif" BORDER=0 ALIGN="middle">-site of the corrin ring. The nitrosyl adduct can formallybe described as Cbl(III)-NO^-. The kinetics of the binding and dissociation reactions was investigated bylaser flash photolysis and stopped-flow techniques, respectively. The activation parameters, ges/gifchars/Delta.gif" BORDER=0 >H^{ges/entities/thermod.gif">}, ges/gifchars/Delta.gif" BORDER=0 >S^{ges/entities/thermod.gif">}, andges/gifchars/Delta.gif" BORDER=0 >V^{ges/entities/thermod.gif">}, for the forward and reverse reactions were estimated from the effect of temperature and pressure on thekinetics of these reactions. For the "on" reaction of Cbl(II) with NO, the small positive ges/gifchars/Delta.gif" BORDER=0 >S^{ges/entities/thermod.gif">} and ges/gifchars/Delta.gif" BORDER=0 >V^{ges/entities/thermod.gif">} valuessuggest the operation of a dissociative interchange (I_d) substitution mechanism at the Co(II) center. Detailedlaser flash photolysis and ¹⁷O NMR studies provide evidence for the formation of water-bound intermediatesin the laser flash experiments and strongly support the proposed I_d mechanism. The kinetics of the "off"reaction was studied using an NO-trapping technique. The respective activation parameters are also consistentwith a dissociative interchange mechanism.