牛高铁血红蛋白中的链不等价性。

Chain inequivalence in bovine methemoglobin.

作者信息

Ilan Y A, Ilan Y, Chevion M, Czapski G

出版信息

Eur J Biochem. 1980 Jan;103(1):161-7. doi: 10.1111/j.1432-1033.1980.tb04300.x.

DOI:10.1111/j.1432-1033.1980.tb04300.x

Abstract

Using pulse radiolysis, a single heme in the tetramer of bovine methemoglobin was reduced within a few microseconds to the ferro state, producing a valence intermediate. The kinetics of oxygen binding to the valence intermediate as well as the re-oxidation of the ferro-heme to the ferric state were studied as a function of pH. The kinetics of the oxygenation revealed the existence of two species, characterized by high and low affinities for oxygen that are associated with two quaternary structures (R and T, respectively). A sigmoidal curve representing a transition between the two states as a function of pH was derived. Above pH 7.7 only the R state could be observed, while below pH 6.5 the T state was dominant. The reaction between the valence intermediate and ferricyanide at pH 7.75 (R state) consisted of two (about) equal contributions (k1 = 23 x 10(4) M-1 S-1; k2 = 2.1 x 10(4) M-1 S-1) attributed to the beta and alpha subunits within the tetramer, respectively. At pH 6.3 (T state) a similar phenomenon was observed (k1 = 69 x 10(4) M-1 S-1; k2 = 3.7 x 10(4) M-1 S-1), indicating chain inequivalences both in the T and the R states of methemoglobin. In the presence of inositol hexakisphosphate the T leads to R transition, as monitored by oxygenation of the valence intermediate, was shifted up to a higher pH by about 0.35. Yet similar rate constants exhibiting similar chain inequivalences have been measured.

摘要

利用脉冲辐解技术，在几微秒内将牛高铁血红蛋白四聚体中的单个血红素还原为亚铁状态，产生一个价态中间体。研究了氧与该价态中间体结合的动力学以及亚铁血红素再氧化为高铁状态的动力学与pH的关系。氧合动力学揭示了存在两种物种，其特征是对氧具有高亲和力和低亲和力，分别与两种四级结构（R和T）相关。得出了一条表示两种状态之间随pH变化的转变的S形曲线。pH高于7.7时，仅能观察到R状态，而pH低于6.5时，T状态占主导。在pH 7.75（R状态）下，价态中间体与铁氰化物之间的反应由（约）两个相等的贡献组成（k1 = 23×10⁴ M⁻¹ s⁻¹；k2 = 2.1×10⁴ M⁻¹ s⁻¹），分别归因于四聚体内的β和α亚基。在pH 6.3（T状态）下观察到类似现象（k1 = 69×10⁴ M⁻¹ s⁻¹；k2 = 3.7×10⁴ M⁻¹ s⁻¹），表明在高铁血红蛋白的T状态和R状态下链均不等价。在肌醇六磷酸存在下，通过价态中间体的氧合监测到的T向R的转变向上移动到更高的pH约0.35。然而，已测量到具有类似链不等价性的相似速率常数。