Filiaci M, Nienhaus G U
Department of Physics, University of Illinois at Urbana-Champaign 61801-3080, USA.
Eur Biophys J. 1997;26(3):209-14. doi: 10.1007/s002490050073.
Using stopped-flow rapid mixing and flash photolysis techniques, the dissociation rate coefficients of horse carbonmonoxy myoglobin (hMbCO) and oxygenated myoglobin (hMbO2) in aqueous solution have been determined as a function of temperature between 274 and 342 K. From the Arrhenius plot, an activation enthalpy for dissociation of 74 kJ/mol was obtained for both ligands. The pronounced kinetic differences arise from markedly different pre-exponentials. We compare the Arrhenius parameters with those of the association reaction, as measured at cryogenic temperatures. In our analysis we conclude that the entropy loss upon binding of O2 is twice as large as that for CO. Taking reasonable estimates for the frequency factor, the transition state entropy in hMbO2 is located roughly half way in between the entropies of the bound and unbound states. By contrast, the entropy of the transition state in hMbCO appears to be identical to that of the bound state. Possible structural reasons for the different behavior are discussed.
利用停流快速混合和闪光光解技术，测定了马一氧化碳肌红蛋白（hMbCO）和氧合肌红蛋白（hMbO2）在水溶液中的解离速率系数与温度（274至342K）的函数关系。从阿伦尼乌斯图中，两种配体解离的活化焓均为74kJ/mol。显著的动力学差异源于明显不同的指前因子。我们将阿伦尼乌斯参数与在低温下测得的缔合反应参数进行了比较。在我们的分析中，我们得出结论，O2结合时的熵损失是CO结合时的两倍。对频率因子进行合理估计后，hMbO2中的过渡态熵大致位于结合态和未结合态熵的中间位置。相比之下，hMbCO中过渡态的熵似乎与结合态的熵相同。讨论了不同行为的可能结构原因。
