Final answer:
The two categories of G-proteins are monomeric and trimeric. Monomeric G-proteins consist of a single subunit, while trimeric G-proteins are composed of three distinct subunits: alpha, beta, and gamma. These trimeric G-proteins play a critical role in cell signaling via GPCRs.
Step-by-step explanation:
The two categories of G-proteins are monomeric and trimeric. Monomeric G-proteins, also known as small GTPases, consist of a single subunit that can bind to guanosine triphosphate (GTP). On the other hand, trimeric G-proteins are composed of three subunits: alpha (α), beta (β), and gamma (γ).
When a signaling molecule binds to a G-protein-coupled receptor (GPCR) on the plasma membrane, the alpha subunit exchanges a guanosine diphosphate (GDP) molecule for a GTP molecule. The beta and gamma subunits then dissociate from the alpha subunit, triggering a cellular response. The signal is terminated when GTP is hydrolyzed back to GDP on the alpha subunit.