Final answer:
Monomeric GTPases are enzymes acting as molecular switches in cells, using GTP hydrolysis to toggle between active and inactive states, thus regulating various cellular processes such as signal transduction via GPCRs and endocytosis.
Step-by-step explanation:
Monomeric GTPases are a family of enzymes that function as molecular switches within cells, regulating a variety of biological processes including cell growth, differentiation, movement, and various cytoskeletal components. They operate by alternating between an active form, bound to guanosine triphosphate (GTP), and an inactive form, bound to guanosine diphosphate (GDP). The hydrolysis of GTP to GDP is critical for turning off the signal and is catalyzed by the inherent enzymatic activity of the GTPases themselves.
For example, in signal transduction pathways involving G-protein-coupled receptors (GPCRs), the alpha subunit of the heterotrimeric G protein will exchange GDP for GTP upon activation by a signaling molecule (ligand). This exchange leads to the dissociation of the alpha subunit from the beta and gamma subunits, thereby triggering a cellular response. Eventually, the intrinsic GTPase activity of the alpha subunit catalyzes the hydrolysis of GTP back to GDP, which allows the subunits to reassociate, turning the signal off and resetting the protein for another cycle.
Moreover, monomeric GTPases like dynamin are involved in cellular processes such as the formation and pinching off of coated vesicles during endocytosis, where hydrolysis of GTP is necessary for the final steps of vesicle formation.