GAP are heavily linked to the G-protein linked receptor family. The activity of G proteins comes from their ability to bind guanosine triphosphate (GTP). Binding of GTP inherently changes the activity of the G proteins and increases their activity, through the loss of inhibitory subunits.In this more active state, G proteins can bind other proteins and turn on downstream signalling targets. this whole process is regulated by GAPs, which can down regulate the activity of G proteins.G proteins can weakly hydrolyse GTP, breaking a phosphate bond to make GDP.In the GDP-bound state, the G proteins are subsequently inactivated and can no longer bind their targets.This hydrolysis reaction, however, occurs very slowly, meaning G proteins have a built-in timer for their activity. G proteins have a window of activity followed by slow hydrolysis, which turns them off. GAP accelerates this G protein timer by increasing the hydrolytic GTPase activity of the G proteins, hence the name GTPase-activating protein.