SynopsisPeptide agonists covalently attached to tobacco mosaic virus exhibit such unusual properties as superpotency, superaffinity, enhanced resistance towards enzymic degradation, and prolonged action at the target cell. These properties can be exploited for the isolation by density-gradient centrifugation of membrane vesicles bearing specific receptors for the peptides and for radioactive and fluorescent labeling of cell-surface receptors. Our observations can be explained by cooperative-affinity phenomena caused by the deployment in space of the agonist molecules.
DEPLOYMENT OF AGONIST MOLECULES AS A FACTOR IN BIOLOGICAL ACTIVITYSince early work on the conformation of cyclic peptides and on the organization of information in linear peptide agonists (for reviews, see Refs. 1-3), it has become increasingly clear that there exist causal relationships between the amino acid sequences of peptides, their conformational possibilities, their interactions with receptors, and their biological activity. In this review, we would like to point out the importance of another factor governing receptor interactions and biological activity, namely the deployment in space of groups of individual agonist molecules.
Cooperative AffinityThe strong affinity of antibodies for membrane-bound antigens has been explained by the simultaneous binding of the two recognition sites on the antibody molecule to two antigen molecules connected through the membrane.4.5 Cooperative affinity effects of this type should prevail in any situation in which ligand and acceptor each possess more than one mutual recognition or binding site, provided that these sites are deployed in such a manner that simultaneous interaction is stereochemically feasible. A classical example of almost perfect complementarity is the DNA double helix, in which one single strand can be regarded as the ligand and the other as the acceptor molecule, both containing favorably deployed binding sites: the purine and pyrimidine bases.