Cooperative coupling in allosteric systems

Allosterism of the Monod type applies only to systems with more than one binding site and two (noncooperative) states with intrinsic binding constants. Allosterism is then defined by an interconversion constant Lo (greater than 1) and a ratio of intrinsic binding constants, c (less than 1). The value of c determines whether weak or strong cooperativity among binding sites prevails. Cooperativity is weak, if 1 greater than c greater than 0.1, and strong, when c less than or equal to 0.1. Cooperativity is the stronger, the smaller c. Cooperativity may exist only between a restricted number of binding sites. The binding of Ca2+ to calmodulin shows this behavior under certain conditions. An (internal) indicator for binding may signal binding to both states or to only one. The results would be quite different with the extent of the difference determined by the extent of cooperativity (c in relation to the particular Li near 1). The size of Lo cannot be ignored in reference to the size of c. Effectors external to the ligand could alter Lo to shift cooperative behavior. Effectors could also make Lo too small or too large for any allosteric behavior to appear.