KINETICS OF LIGAND-BINDING TO A CLUSTER OF MEMBRANE-ASSOCIATED RECEPTORS

The process of ligand binding to a cluster of membrane-associated rece ptors is examined theoretically. The theoretical model proposed involv es the diffusion of ligands from the solution to the disc-like cluster of receptors on the surface of the spherical cell. When the ligand hi ts the internal part of the disc-like cluster, it begins to move later ally until it leaves the disc through its outer surface or is bound by one of the receptors inside the disc. If the ligand leaves the cluste r, it returns to the solution and hits the disc again after a certain period, etc. According to our model the transition from a diffusion-li mited to a reaction-limited process of binding is determined by the di mensionless parameter lambda equivalent to ($) over tilde Dt(c)/a(2), where D is the lateral diffusion coefficient, t(c) is the characterist ic time of reaction, and a is the radius of the disc-like cluster. The forward rate constant k(f) turns out to be a function of lambda. Comp aring the results of our calculations of k(f) with some experimental d ata we found that agreement is achieved at high lambda, i.e. the proce ss of ligand binding by clustered receptors is predominantly reaction- limited.