Comparison and Contrasts between the Active Site PKs of Mn-Superoxide Dismutase and Those of Fe-Superoxide Dismutase

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• 作者：James Maliekal ; Anush Karapetian ; Carrie Vance ; Emine Yikilmaz ; Qiang Wu ; Timothy Jackson ; Thomas C. Brunold ; Thomas G. Spiro ; and Anne-Frances Miller
• 刊名：Journal of the American Chemical Society
• 出版年：2002
• 出版时间：December 18, 2002
• 年：2002
• 卷：124
• 期：50
• 页码：15064 - 15075
• 全文大小：257K
• 年卷期：v.124,no.50(December 18, 2002)
• ISSN：1520-5126

文摘

The Fe- and Mn-containing superoxide dismutases catalize the same reaction and have almostsuperimposable active sites. Therefore, the details of their mechanisms have been assumed to be similar.However, we now show that the pH dependence of Escherichia coli MnSOD activity reflects a differentactive site proton equilibrium in (oxidized) Mn³⁺SOD than the event that affects the active site pK of oxidizedFeSOD. We find that the universally conserved Tyr34 that has a pK above 11.5 in Fe³⁺SOD is responsiblefor the pK near 9.5 of Mn³⁺SOD and, thus, that the oxidized state pK of Mn³⁺SOD corresponds to anouter-sphere event whereas that of Fe³⁺SOD corresponds to an inner sphere event [Bull, C.; Fee, J. A. J.Am. Chem. Soc. 1985, 107, 3295-3304]. We also present the first description of a reduced-state pK forMnSOD. Mn²⁺SOD's pK involves deprotonation of Tyr34, as does Fe²⁺SOD's pK [Sorkin, D. L.; MillerA.-F. Biochemistry 1997, 36, 4916-4924]. However, the values of the pKs, 10.5 and 8.5 respectively, arequite different and Mn²⁺SOD's pK affects the coordination geometry of Mn²⁺, most likely via polarization ofthe conserved Gln146 that hydrogen bonds to axially coordinated H₂O. Our findings are consistent withthe different electronic configurations of Mn^2+/3+ vs Fe^2+/3+, such as the stronger hydrogen bonding betweenGln146 and coordinated solvent in MnSOD than that between the analogous Gln69 and coordinated solventin FeSOD, and the existence of weakly localized H₂O near the sixth coordination site of Mn²⁺ in Mn²⁺SOD[Borgstahl et al. J. Mol. Biol. 2000, 296, 951-959].