Final answer:
G-proteins affect the activity of adenylate cyclase by activating this enzyme, which in turn catalyzes the conversion of ATP to cAMP. cAMP then activates protein kinases such as PKA, leading to various cellular responses.
Step-by-step explanation:
G-proteins play a crucial role in the signal transduction pathway by affecting the activity of enzymes such as adenylate cyclase. When a signaling molecule, like a hormone, binds to a receptor on the cell surface, it can trigger the activation of a G-protein. Once activated by the exchange of GDP for GTP on its alpha subunit, the G-protein then interacts with adenylate cyclase, stimulating its activity. This enzyme catalyzes the conversion of ATP to cyclic AMP (cAMP), which functions as a second messenger in the cell.
The increase in cAMP levels leads to the activation of protein kinase A (PKA), which then phosphorylates various substrates resulting in changes in their activity and thus affecting a range of cellular processes. This amplification of the signal allows a single hormone-receptor interaction to have a large and widespread effect within the cell. Moreover, G-proteins can also stimulate other pathways, such as the one leading to the release of calcium ions or the activation of protein kinase C (PKC), illustrating the diversity of the cellular responses to G-protein activation.
To ensure that the signal is properly regulated and does not lead to overstimulation, cAMP is deactivated by phosphodiesterase (PDE), which breaks down cAMP and thereby controls the activity of the hormone and prevents excessive cellular responses. This deactivation is just as crucial as the activation process for maintaining cell homeostasis.