<prompt> Which of the following in not a second messenger-cAMPNOCOProtein kinase  <response>: Ans. is d i.e., Protein kinase Second messengers are molecules that relay signals from the membrane receptors to target molecules inside the cells. In above described receptors second messengers are cAMP, cGMP, Phosphatidylinositol, diacylgleceral, IP3, Ca2. Some gaseous molecule can diffuse both through cytosol and across cellular membranes and act as second messenger. These are nitric oxide NO, Carbon monoxide CO and hydrogen sulphate H2S. 