Dixon H B, Tipton K F
Biochem J. 1973 Aug;133(4):837-42. doi: 10.1042/bj1330837.
Simple systems are considered in which the binding of a ligand at a single site exhibits a doubly sigmoid curve when saturation is plotted against the logarithm of ligand concentration, i.e. where a fraction of the site exhibits one dissociation constant and the rest exhibits another. The condition for this to occur is that the ligand should also combine at another site on the binding molecule with comparable affinity and that the binding at one site should markedly lower affinity at the other. The protonation of simple compounds such as cysteine and 3-hydroxypyridine is taken as an example of this process and the equations derived are also applied to the binding of substrates to enzymes.
我们考虑这样一些简单系统，其中当以配体浓度的对数为横坐标绘制饱和度曲线时，单个位点上配体的结合呈现出双S形曲线，也就是说，一部分位点表现出一个解离常数，其余位点表现出另一个解离常数。发生这种情况的条件是，配体还应以相当的亲和力结合在结合分子的另一个位点上，并且一个位点上的结合应显著降低另一个位点的亲和力。以半胱氨酸和3-羟基吡啶等简单化合物的质子化作为该过程的一个例子，推导得到的方程也适用于底物与酶的结合。
