Which of the following in not a second messenger-

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, Ca+2.* 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).