Perutz M F, Kilmartin J V, Nagai K, Szabo A, Simon S R
Biochemistry. 1976 Jan 27;15(2):378-87. doi: 10.1021/bi00647a022.
Studies of high spin ferrous and ferric derivatives led us to conclude that in the quaternary R structure the state of the hemes is similar to that in the free alpha and beta subunits, but in the T structure a tension acts on the hemes which tries to pull the iron and the proximal histidine further from the plane of the porphyrin. We have now studied the effect of inositol hexaphosphate (IHP) on the three low spin ferrous compounds of hemoglobin with O2, CO, and NO. IHP failed to switch the quaternary structure of carbonmonoxy- and oxyhemoglobin A to the T state, but merely caused a transition to an as yet undefined modification of the R structure. IHP is known to cause a switch to the T structure in hemoglobin Kansas. We have found that this switch induces red shifts of the visible alpha and beta absorption bands and the appearance of a shoulder on the red side of the alpha band; these changes are very weak in carbonmonoxy- and slightly stronger in oxyhemoglobin Kansas. As already noted by previous authors, addition of IHP to nitrosylhemoglobin A induces all the changes in uv absorption and CD spectra, sulfhydryl reactivities, and exchangeable proton resonances normally associated with the R leads to T transition, and is accompanied by large changes in the Soret and visible absorption bands. Experiments with nitrosyl hybrids show that these changes in absorption are caused predominantly by the hemes in the alpha subunits. In the accompanying paper Maxwell and Caughey (J. C. Maxwell and W. S. Caughey (1976), Biochemistry, following paper in this issue) report that the NO in nitrosylhemoglobin without IHP gives a single ir stretching frequency characteristic for six-coordinated nitrosyl hemes; addition of IHP causes the appearance of a second ir band, of intensity equal to that of the first, which is characteristic for five-coordinated nitrosyl hemes. Taken together, these results show that the R leads to T transition causes either a rupture or at least a very dramatic stretching of the bond from the iron to the heme-linked histidine, such that an equilibrium is set up between five- and six-coordinated hemes, biased toward five-coordinated hemes in the alpha and six-coordinated ones in the beta subunits. The reason why IHP can switch nitrosyl-, but not carbonmonoxy- or oxyhemoglobin A, from the R to the T structure is to be found in the weakening of the iron-histidine bond by the unpaired NO electron and by the very short Fe-NO bond length.
对高自旋亚铁和高铁衍生物的研究使我们得出结论，在四级R结构中，血红素的状态类似于游离的α和β亚基中的状态，但在T结构中，一种张力作用于血红素，试图将铁和近端组氨酸拉离卟啉平面更远。我们现在研究了肌醇六磷酸（IHP）对血红蛋白与O2、CO和NO形成的三种低自旋亚铁化合物的影响。IHP未能将碳氧血红蛋白A和氧合血红蛋白A的四级结构转变为T态，而只是导致向一种尚未明确的R结构修饰转变。已知IHP会使堪萨斯血红蛋白转变为T结构。我们发现这种转变会导致可见的α和β吸收带发生红移，并在α带的红侧出现一个肩峰；这些变化在碳氧血红蛋白中非常微弱，在堪萨斯氧合血红蛋白中稍强。如先前作者所指出的，向亚硝基血红蛋白A中添加IHP会引起紫外吸收和圆二色光谱、巯基反应性以及可交换质子共振的所有变化，这些变化通常与R向T的转变相关，并伴随着Soret带和可见吸收带的巨大变化。对亚硝基杂合体的实验表明，这些吸收变化主要由α亚基中的血红素引起。在随附的论文中，麦克斯韦和考伊（J.C.麦克斯韦和W.S.考伊（1976年），《生物化学》，本期后续论文）报告说，没有IHP的亚硝基血红蛋白中的NO给出了六配位亚硝基血红素特有的单一红外伸缩频率；添加IHP会导致出现第二个红外带，其强度与第一个相等，这是五配位亚硝基血红素的特征。综上所述，这些结果表明，R向T的转变导致从铁到与血红素相连的组氨酸的键断裂或至少非常显著地拉伸，从而在五配位和六配位血红素之间建立平衡，在α亚基中偏向五配位血红素，在β亚基中偏向六配位血红素。IHP能够将亚硝基血红蛋白，而不是碳氧血红蛋白或氧合血红蛋白A从R结构转变为T结构的原因在于未成对的NO电子和非常短的Fe-NO键长削弱了铁-组氨酸键。
