Final answer:
G-proteins are trimeric proteins that transduce signals from G-protein-coupled receptors on the cell surface to the cell's interior, influencing various cellular processes. Upon activation by a signaling molecule, the G-protein undergoes conformational changes, leading to the activation of target proteins and generation of cellular responses. G-proteins are key in signal transduction pathways involving hormones like glucagon.
Step-by-step explanation:
G-Proteins: Function and Mechanism
G-proteins or guanyl nucleotide-binding regulatory proteins are a family of proteins that serve as molecular switches inside cells. They play a pivotal role in transmitting signals from receptors on the cell surface to the cell's interior, influencing various biological processes. G-proteins are trimeric, consisting of α (alpha), β (beta), and γ (gamma) subunits.
When a hormone or another signaling molecule binds to a G-protein-coupled receptor (GPCR) on the plasma membrane, an inactive G-protein binds to the receptor, causing a conformational change. This leads to the exchange of guanosine diphosphate (GDP) for guanosine triphosphate (GTP) on the α subunit. The GTP-bound α subunit separates from the βγ pair and can then activate other target proteins, such as enzymes or ion channels, initiating a cellular response. Eventually, GTP is hydrolyzed to GDP, and the subunits reassociate, returning the G-protein to its inactive state.
For example, when the hormone glucagon binds its receptor, the associated G-protein activates adenylate cyclase, increasing cyclic AMP (cAMP) production and activating Protein Kinase A (PKA). This cascade can lead to the regulatory phosphorylation of various enzymes. G-proteins are classified as stimulatory (Gs) or inhibitory (Gi) based on their effect on adenylate cyclase.