Sanfratello V, Boffi A, Cupane A, Leone M
Istituto Nazionale di Fisica della Materia and Department of Physical and Astronomical Sciences, University of Palermo, Italy.
Biopolymers. 2000;57(5):291-305. doi: 10.1002/1097-0282(2000)57:5<291::AID-BIP60>3.0.CO;2-O.
We report the visible and Soret absorption bands, down to cryogenic temperatures, of the ferrous nicotinate adducts of native and deuteroheme reconstituted horse heart myoglobin in comparison with soybean leghemoglobin-a. The band profile in the visible region is analyzed in terms of vibronic coupling of the heme normal modes to the electronic transition in the framework of the Herzberg-Teller approximation. This theoretical approach makes use of the crude Born-Oppenheimer states and therefore neglects the mixing between electronic and vibrational coordinates; however, it takes into account the vibronic nature of the visible absorption bands and allows an estimate of the vibronic side bands for both Condon and non-Condon vibrational modes. In this framework, an x-y splitting of the Q transition for native and deuteroheme reconstituted horse myoglobin is clearly assessed and attributed to electronic perturbations that, in turn, are caused by a reduction of the typical D(4h) symmetry of the system due to heme distortions of B(1g)-type symmetry and/or to an x-y asymmetric position of the nicotinate ring; in deuteroheme reconstituted horse myoglobin the asymmetric heme peripheral substituents add to the above effect(s). On the contrary, in leghemoglobin-a no spectral splitting upon nicotinate binding is observed, pointing to a planar heme configuration in which only distortions of A(1g)-type symmetry are effective and to which the nicotinate ring is bound in an x - y symmetric position. The local dynamic properties of the heme pocket of the three proteins are investigated through the temperature dependence of spectral line broadening. Leghemoglobin-a behaves as a softer matrix with respect to horse myoglobin, thus validating the hypothesis of a looser heme pocket conformation in the former protein, which allows a nondistorted heme configuration and a symmetric binding of the bulky nicotinate ligand.
我们报告了天然和氘代血红素重构的马心肌红蛋白与大豆豆血红蛋白-a的亚铁烟酸加合物在低温下的可见吸收带和索雷特吸收带。在赫兹伯格-泰勒近似框架下，根据血红素正常模式与电子跃迁的振动耦合来分析可见区域的能带轮廓。这种理论方法利用了粗略的玻恩-奥本海默态，因此忽略了电子和振动坐标之间的混合；然而，它考虑了可见吸收带的振动电子性质，并允许估计康登和非康登振动模式的振动电子边带。在此框架下，清楚地评估了天然和氘代血红素重构的马肌红蛋白的Q跃迁的x-y分裂，并将其归因于电子扰动，而这种电子扰动又是由于B(1g)型对称性的血红素畸变和/或烟酸环的x-y不对称位置导致系统典型的D(4h)对称性降低引起的；在氘代血红素重构的马肌红蛋白中，不对称的血红素外围取代基增加了上述效应。相反，在豆血红蛋白-a中，未观察到烟酸结合后的光谱分裂，这表明血红素呈平面构型，其中只有A(1g)型对称性的畸变是有效的，且烟酸环以x - y对称位置结合。通过谱线展宽的温度依赖性研究了这三种蛋白质血红素口袋的局部动力学性质。相对于马肌红蛋白，豆血红蛋白-a表现为更柔软的基质，从而验证了前一种蛋白质中血红素口袋构象较松散的假设，这种构象允许血红素构型无畸变且大体积的烟酸配体对称结合。
