Editing Signal transduction (section)

===Nitric oxide===
Nitric oxide (NO) acts as a second messenger because it is a [[free radical]] that can diffuse through the plasma membrane and affect nearby cells. It is synthesised from [[arginine]] and oxygen by the [[Nitric oxide synthase|NO synthase]] and works through activation of [[soluble guanylyl cyclase]], which when activated produces another second messenger, cGMP. NO can also act through covalent modification of proteins or their metal co-factors; some have a redox mechanism and are reversible. It is toxic in high concentrations and causes damage during [[stroke]], but is the cause of many other functions like the relaxation of blood vessels, [[apoptosis]], and penile [[erection]]s.