Takashima S
Department of Bioengineering, University of Pennsylvania, Philadelphia 19104-6392.
Biophys J. 1993 May;64(5):1550-8. doi: 10.1016/S0006-3495(93)81524-8.
Previously, we discussed the calculation of the dipole moments of small proteins using the three-dimensional protein data-base. Our results demonstrate that the calculated dipole moments are in acceptable agreement with measured values. We, however, noted the difficulty of the calculation with larger proteins, in particular those consisting of several subunits. Hemoglobin (Hb) is a protein having a molecular weight of 64,000 that consists of four subunits, a typical case where the computation was found to be difficult. To circumvent the difficulties, we calculated the dipole moment of each subunit separately. The dipole moment of the whole protein was calculated by the vectorial summation of subunit moments. With this method, the calculated net dipole moment is in good agreement with the experimental value. Our calculation shows that the dipole moment vectors of subunits are, by and large, antiparallel in tetramers causing partial cancellation of the net dipole moment. In addition to normal HbA, the dipole moment of abnormal HbS was calculated using an approximate computational technique. Because of the loss of two negative changes as a result of the replacement of glutamic acid with valine in beta-chains, the dipole moment of HbS was found, experimentally and theoretically, to be significantly smaller than that of HbA.
此前，我们讨论了利用三维蛋白质数据库计算小蛋白质的偶极矩。我们的结果表明，计算得到的偶极矩与测量值吻合良好。然而，我们注意到对于较大的蛋白质，尤其是由几个亚基组成的蛋白质，计算存在困难。血红蛋白（Hb）是一种分子量为64,000的蛋白质，由四个亚基组成，这是一个计算困难的典型例子。为了克服这些困难，我们分别计算了每个亚基的偶极矩。整个蛋白质的偶极矩通过亚基矩的矢量求和来计算。用这种方法，计算得到的净偶极矩与实验值吻合良好。我们的计算表明，在四聚体中，亚基的偶极矩矢量大体上是反平行的，从而导致净偶极矩部分抵消。除了正常的HbA，我们还使用一种近似计算技术计算了异常HbS的偶极矩。由于β链中缬氨酸取代谷氨酸导致两个负电荷丢失，实验和理论上都发现HbS的偶极矩明显小于HbA的偶极矩。
