Receptor protein that receives chemical signals from outside the cell, cause a cellular/tissue response when bound to chemical-signals. Hormone a chemical substance produced in the body that controls and regulates the activity of certain cells or organs. Endocrine gland secrete hormones into the blood. Protein kinase enzyme that modifies other proteins by phosphorylation. Protein phosphatase enzyme that removes a phosphate group from a protein. G-protein bind to guanine nucleotides gdp and gtp; heterotrimers; associated w/ inner surface of pm and transmembrane receptors of hormones. Gdi guanine dissociation inhibitors form soluble complexes w/ small gtpases by shielding their lipid. Gef proteins involved in the activation of small gtpases. Gap accelerating proteins are a family of regulatory proteins whose members can bind to activated g proteins & stimulate their gtpase activity w/ the result of terminating the signaling event. Adenylyl cyclase enzyme that synthesizes cyclic adenosine monophosphate or cyclic amp from atp.