Bucci E, Fronticelli C, Gryczynski Z
Department of Biochemistry, University of Maryland, Baltimore 21201.
Biochemistry. 1991 Apr 2;30(13):3195-9. doi: 10.1021/bi00227a006.
We have measured the temperature dependence of the oxygen-binding isotherms of human and bovine hemoglobin at pH 9.0 in 0.1 M borate buffer. In both hemoglobins the ionization of the Bohr protons is finished at this pH; therefore, their heat does not interfere with the measurements. Two sets of curves have been obtained, which have been analyzed by either singular or global procedures for estimating the enthalpy changes of subsequent steps of oxygenation. The data indicate that in human hemoglobin the reaction with oxygen is enthalpy driven for steps 1, 2, and 4 while it is entropy driven for step 3. In bovine hemoglobin this phenomenon is even more evident: steps 2 and 4 are enthalpy driven while steps 1 and 3 are entropy driven. The discontinuous distribution of heat at subsequent steps of oxygenation suggests that the T to R transition in hemoglobin is not a monotonic process and involves conformations with novel characteristics.
我们在0.1 M硼酸盐缓冲液中，于pH 9.0条件下测量了人血红蛋白和牛血红蛋白氧结合等温线的温度依赖性。在这两种血红蛋白中，玻尔质子的电离在该pH时已完成；因此，它们的热量不会干扰测量。我们获得了两组曲线，已通过奇异或全局程序对其进行分析，以估计后续氧合步骤的焓变。数据表明，在人血红蛋白中，与氧的反应在步骤1、2和4中是由焓驱动的，而在步骤3中是由熵驱动的。在牛血红蛋白中，这种现象更为明显：步骤2和4是由焓驱动的，而步骤1和3是由熵驱动的。氧合后续步骤中热量的不连续分布表明，血红蛋白中从T态到R态的转变不是一个单调过程，并且涉及具有新特性的构象。
