G Protein Coupled Receptors (GPCRs) are a large family of seven
transmembrane (TM) helical structural motif containing proteins that
regulate cellular communication. Based on sequence similarity within the
seven TM segment in human, the major five GPCR families are rhodopsin,
secretin, glutamate, adhesion and frizzled/taste. GPCRs are principle
signal transducer that activate senses of sight, smell and test, and
responsible for many signaling disorders and diseases. Molecular
mechanisms of GPCR activation are always implicated as a therapeutic
purpose to discover new drug targets. Several molecular mechanisms and
models of GPCR activation have been proposed. Dimerization, protonation,
conformations due to intramolecular interaction and specific
conformational changes are thought to be responsible for receptor
activation. In this review, these mechanisms and proposed model systems
are discussed in detail.