Kinetics and mechanism of reduction of sperm-whale metmyoglobin by dithionite ion

Marylin Itzkowitz, Albert Haim
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引用次数: 1

Abstract

The kinetics of reduction of sperm-whale aquometmyoglobin by excess dithionite ion was studied at 25°C, pH 6.9–9.8, and ionic strength 0.50 M (adjusted with potassium nitrate). The dependence of the pseudo first-order rate constant for the disappearance of metmyoglobin on hydrogen and dithionite ion concentrations is (k1[S2O42−] + k2[S2O42−]12)[H+]/(Ka + [H+]), where k1 = 52 ± 11 M−1 s−1, k2 = 40.9 ± 2.3 M12 s−1, and Ka (the ionization constant of the water coordinated to the iron) = (5.79 ± 0.49) × 10−10 M. The rate law is interpreted on the basis of parallel pathways for the reaction of aquometmyoglobin with S2O42− and SO2, the hydroxometmyoglobin in equilibrium with the aquo form being unreactive. From a comparison of the rate constants for anation of aquometmyoglobin and the rate constant for reduction by SO2, it is inferred that an outer-sphere redox mechanism is operative. It is postulated that the activation process for reduction of aquometmyoglobin requires considerable stretching of the Fe-OH2 bond, and this model is utilized to assign an outer-sphere mechanism to the reduction by S2O42−. The dithionite reduction of cyanometmyoglobin proceeds in two stages. The first stage proceeds at a rate that is dependent on dithionite concentration and corresponds to the outer-sphere reduction of cyanometmyoglobin. The second stage proceeds at a rate that is independent of dithionite concentration and corresponds to the dissociation of the transient cyanodeoxymyoglobin intermediate produced in the first stage. The results of the present investigation are compared with those obtained in three independent, previous studies.

二亚硫盐离子还原抹香鲸肌红蛋白的动力学及机理
在25℃、pH 6.9 ~ 9.8、离子强度0.50 M(硝酸钾调节)条件下,研究了过量二亚硝酸盐离子对抹香鲸水肌红蛋白的还原动力学。伪一阶速率常数的依赖生产者的消失在氢和亚硫酸氢离子浓度(k1 [S2O42−)+ k2 (S2O42−)12)(H +) / (Ka + [H +]), k1 = 52±11 M 1 s−−1,k2 M12 s−1 = 40.9±2.3,和Ka(水的电离常数协调铁)=(5.79±0.49)×10−10 M的法律解释的基础上平行反应的通路与S2O42 aquometmyoglobin−和二氧化硫−羟基肌红蛋白处于平衡状态,水溶液不反应。通过比较水肌红蛋白的气相化速率常数和SO2−还原速率常数,可以推断出外球氧化还原机制是有效的。假设水肌红蛋白还原的激活过程需要Fe-OH2键的相当大的拉伸,并利用该模型来分配S2O42−还原的外球机制。氰肌红蛋白的亚硫代还原分两个阶段进行。第一阶段的进行速度取决于二硫代钙的浓度,并与氰肌红蛋白的外球还原相对应。第二阶段的进行速度与二亚铁浓度无关,与第一阶段产生的瞬时氰脱氧肌红蛋白的解离相对应。本研究的结果与之前三个独立研究的结果进行了比较。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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