G protein‐coupled receptors (GPCRs) were initially regarded to adopt an inactive and an active conformation and to activate a single type of G protein. Studies with recombinant cell systems have led to a more complex picture. First, GPCRs can activate distinct G protein species. Second, GPCR multistate models have been invoked to explain their complex behaviour in the presence of agonists, antagonists and other binding partners. The occurrence of intermediate receptor conformational states during GPCR activation and antagonist binding is suggested by fluorescence measurements and studies with constitutively active receptor mutants and insurmountable antagonists. Different agonists may trigger distinct effector pathways through a single receptor by dictating its preference for certain G proteins (i.e. ‘agonist trafficking’). Structural modification and exogenous and endogenous (e.g. other cellular proteins, lipids) allosteric modulators also affect ligand–GPCR interaction and receptor activation. These new developments in GPCR research could lead to the development of more selective therapeutic drugs.
Binding kinetics are the rates of association and dissociation of a drug-protein complex and are important molecular descriptors for the optimization of drug binding to G-protein coupled receptors (GPCRs). There are now many examples of binding kinetics in GPCR drug discovery. In this report, the first principles and examples of binding kinetics in GPCR drug discovery are reviewed. Addressed are the influence of binding kinetics on the translation of binding to the therapeutic window in the context of the equilibrium state of the system and molecular mechanisms of slow binding including induced fit, displacement of water, rebinding and heterovalency.
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