Final answer:
Target enzymes in cell signaling are activated by a guanine nucleotide exchange factor (GEF) that replaces GDP with GTP. The inactivation process typically involves an enzyme with GTPase activity converting GTP back to GDP.
Step-by-step explanation:
In cell signaling, target enzymes are activated by a guanine nucleotide exchange factor (GEF), which facilitates the exchange of a bound guanosine diphosphate (GDP) for GTP (guanosine triphosphate).
Once activated by GTP, these enzymes can lead to various cellular responses, such as the activation of adenylyl cyclase and subsequent production of the second messenger cAMP (cyclic adenosine monophosphate), leading to a cascade of further signaling events, including phosphorylation of substrate molecules by protein kinases.
The inactivation of these target enzymes usually involves GTP hydrolysis, wherein the GTP is converted back to GDP, aided by an enzyme with GTPase activity. This GTP hydrolysis can occur directly on the enzyme or on a separate G protein that is coupled to membrane receptors, known as G-protein-coupled receptors (GPCRs).
Overall, the process of activation and inactivation of such enzymes by GEFs and GTPases ensures precise control of cellular signaling pathways.