As a receptor site for a small molecule called a ligand, exposed on the outside of the cell, the portion of the receptor on the inside of the cell can bind to the G protein, which has a guana-scene diphosphate attached to its alpha subunit. When the ligand binds to the receptor site on the outside of the cell membrane, the G protein changes confirmation, and guana-scene triphosphate replaces the guana-scene diphosphate on the alpha subunit of the G protein. The activated alpha subunit then separates from the beta and gamma subunits. This step can be repeated as long as the ligand remains bound to the receptor. When the ligand separates from the receptor site, additional G proteins are no longer activated. Inactivation of the alpha subunit occurs when its own phosphorylase activity removes a phosphate from the guana-scene triphosphate, leaving guana-scene diphosphate bound to the subunit. The G protein subunits then recombine and attach to the receptor in the cell membrane.