What is the usual second messenger for protein hormones?

The usual second messenger for protein hormones is cyclic AMP (cAMP). Protein hormones, which include many important hormones such as insulin and glucagon, typically bind to receptors on the cell membrane, activating a series of intracellular processes. This binding often results in the conversion of ATP to cAMP, a process catalyzed by the enzyme adenylate cyclase.

Cyclic AMP then acts as a second messenger, triggering various cellular responses, including changes in enzyme activity, modulation of gene expression, and alterations in ion channel activity. This amplifies the signal of the hormone and leads to the desired physiological effects.

Understanding the role of cAMP as a second messenger is crucial in the context of cell signaling and hormonal regulation, as many physiological processes depend on the proper functioning of this pathway.