Providing a Chemical Basis toward Understanding the Histidine Base-On Motif of Methylcobalamin-Dependent Methionine Synthase: An Improved Purification of Methylcobinamide, plus Thermodynamic Studies o
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- 作者：Jeanne Sirovatka Dorweiler ; Rowena G. Matthews ; and Richard G. Finke
- 刊名：Inorganic Chemistry
- 出版年：2002
- 出版时间：December 2, 2002
- 年：2002
- 卷：41
- 期：24
- 页码：6217 - 6224
- 全文大小：129K
- 年卷期：v.41,no.24(December 2, 2002)
- ISSN：1520-510X
文摘
Reported herein are the synthesis and improved purification of MeCbi+·BF4- leading to 95% pure product. Theavailability of this higher purity MeCbi+·BF4- has, in turn, allowed a study of the Kassoc, 
H, and S for exogenousimidazole and pyridine bases binding to MeCbi+ in ethylene glycol and buffered aqueous solution. The resultsshow that (1) the bases studied have larger Kassoc values (where measurable) when binding to MeCbi+ than whenbinding to AdoCbi+ under analogous conditions; (2) comparison of the thermodynamic binding parameters for pyand N-MeIm show that these bases bind similarly, within experimental error to MeCbi+, contrary to what was seenearlier with AdoCbi+; (3) the bases follow the expected trend, with the base with the highest pKa of those studied,4-Me2Npy, exhibiting the highest Kassoc value (Kassoc(25 
C) = 18.0 ± 0.3 M-1) and the base of lowest pKa, py,exhibiting the lowest detectable Kassoc value (Kassoc (25 C) = 6.2 ± 0.4 M-1); (4) there is no detectable binding(Kassoc = 0.07 M-1) for 2-Mepy or 2,6-Me2py with MeCbi+; and (5) the base that is closest to the biologicallyrelevant axial His759 residue in methionine synthase, N-MeIm, exhibits an unusual H value for the formation ofMeCbi+·N-MeIm, results interpreted as offering further support for the presence of 
plus 
effects when imidazolebases bind to alkylcobinamides. The results of these studies allow the percentage of base-on methylcobinamide,MeCbi+·base, to be calculated as a function of temperature and added base. As such, they provide necessarybackground information for RS- + MeCbi+·base and other methionine synthase chemical precedent studies.
H, and S for exogenousimidazole and pyridine bases binding to MeCbi+ in ethylene glycol and buffered aqueous solution. The resultsshow that (1) the bases studied have larger Kassoc values (where measurable) when binding to MeCbi+ than whenbinding to AdoCbi+ under analogous conditions; (2) comparison of the thermodynamic binding parameters for pyand N-MeIm show that these bases bind similarly, within experimental error to MeCbi+, contrary to what was seenearlier with AdoCbi+; (3) the bases follow the expected trend, with the base with the highest pKa of those studied,4-Me2Npy, exhibiting the highest Kassoc value (Kassoc(25 C) = 18.0 ± 0.3 M-1) and the base of lowest pKa, py,exhibiting the lowest detectable Kassoc value (Kassoc (25 C) = 6.2 ± 0.4 M-1); (4) there is no detectable binding(Kassoc = 0.07 M-1) for 2-Mepy or 2,6-Me2py with MeCbi+; and (5) the base that is closest to the biologicallyrelevant axial His759 residue in methionine synthase, N-MeIm, exhibits an unusual H value for the formation ofMeCbi+·N-MeIm, results interpreted as offering further support for the presence of plus effects when imidazolebases bind to alkylcobinamides. The results of these studies allow the percentage of base-on methylcobinamide,MeCbi+·base, to be calculated as a function of temperature and added base. As such, they provide necessarybackground information for RS- + MeCbi+·base and other methionine synthase chemical precedent studies.