Membrane guanylyl cyclases are a type of enzyme found in the cell membrane that convert GTP (guanosine triphosphate) into cGMP (cyclic guanosine monophosphate). This process plays a crucial role in various signaling pathways, influencing functions such as blood vessel dilation, neurotransmission, and cell growth. These enzymes are activated by specific ligands, including natriuretic peptides and nitric oxide. Once activated, they increase the levels of cGMP, which acts as a secondary messenger in cells, helping to regulate physiological processes like smooth muscle relaxation and the response to stress.