Morris R J, Gibson Q H
J Biol Chem. 1980 Sep 10;255(17):8050-3.
The Root effect hemoglobin from Thunnus thynnus, the Atlantic bluefin tuna, remains in the T state at acid pH even when liganded. There are two components in the binding of CO to the T state with rates of 0.01 and 1.0 X 10(6) M-1 S-1 at 20 degrees C, 0.1 M KPi, pH 6.0, and two components in binding to the R state with rates of 1.7 and 8.5 X 10(6) M-1 S-1 as measured by partial flash photolysis at 20 degrees C, 0.05 M Tris-HCl, pH 8.0. A single rate of NO binding of 1.8 X 10(7) M-1 S-1 is found at both pH 6 and pH 8. The rate of binding of NO to dimethyldeuterohemedisulfonate is much higher still at 5 X 10(8) M-1 S-1. The observed rate of NO binding to the protein is not affected by chain differences or by the R to T transition. It may measure the rate of diffusion of ligand to the vicinity of the heme site. The T state components have different NO dissociation velocity constants of 0.004 and 0.07 S-1. The apparent quantum yields for the T state components are approximately 0.035 and 0.0008; the high affinity component has the lower yield. A two-step scheme is used to explain the results. The presence of chain differences and allosteric states is reflected in the intrinsic binding rates of both NO and CO to the heme group.
大西洋蓝鳍金枪鱼（Thunnus thynnus）的根效应血红蛋白即使在与配体结合时，在酸性pH条件下仍保持T态。在20℃、0.1M磷酸钾、pH 6.0条件下，CO与T态的结合有两个组分，速率分别为0.01和1.0×10⁶ M⁻¹ s⁻¹；在20℃、0.05M Tris-HCl、pH 8.0条件下，通过部分闪光光解测定，CO与R态的结合也有两个组分，速率分别为1.7和8.5×10⁶ M⁻¹ s⁻¹。在pH 6和pH 8时，均发现NO的单一结合速率为1.8×10⁷ M⁻¹ s⁻¹。NO与二甲基氘代血红素二磺酸盐的结合速率更高，为5×10⁸ M⁻¹ s⁻¹。观察到的NO与蛋白质的结合速率不受链差异或R态到T态转变的影响。它可能测量了配体扩散到血红素位点附近的速率。T态组分具有不同的NO解离速度常数，分别为0.004和0.07 s⁻¹。T态组分的表观量子产率约为0.035和0.0008；高亲和力组分的产率较低。采用两步机制来解释这些结果。链差异和别构状态的存在反映在NO和CO与血红素基团的内在结合速率上。
