G protein-coupled receptors are integral membrane proteins that contain seven transmembrane helices. The extracellular part consists of loops where there are two highly conserved cysteine residues forming a disulfide bond, which stabilizes the structure of the receptor. G-protein coupled receptors are activated by an external signal in the form of a ligand. This creates conformational changes in the receptor, causing activation of the G-protein. A further effect depends on the type of G protein. For example, adenylate cyclase is one of the cellular proteins that can be regulated by G-protein and its activating subunit Gs. Adenylate cyclase activation begins when it binds to the subunit of the activated G-protein, and ends when the G-protein hydrolyzes GTP and returns to the GDP-bound state, in which all its subunits are connected into a single molecule with a quaternary structure. GPCRs act as activators of the intracellular signal transduction pathways, ultimately leading to a cellular response.