Final answer:
cyclic AMP (cAMP) is a second messenger activated by adenylyl cyclase following the binding of a hormone to a receptor, which triggers a signaling cascade affecting cellular processes. Calcium ions can also act as second messengers in a similar signaling pathway initiated by phospholipase C.
Step-by-step explanation:
An example of a second messenger is cyclic AMP (cAMP), which is activated by the enzyme adenylyl cyclase when a hormone binds to its receptor on the cell surface. This receptor is associated with a G-protein, and the binding of the hormone to the receptor results in the activation of the G-protein, which subsequently activates adenylyl cyclase. The adenylyl cyclase then converts ATP into cAMP.
cAMP's main role within the cell is to bind to and activate cAMP-dependent kinases, or A-kinase, leading to the phosphorylation of various target proteins, thereby regulating vital metabolic pathways. This cascade is an essential part of signal transduction within cells, affecting numerous processes such as metabolic regulation, gene expression, and even learning and memory in neurons.
Additionally, calcium ions (Ca2+) serve as another second messenger system where G proteins activate the enzyme phospholipase C (PLC), leading to the production of inositol triphosphate (IP3) and diacylglycerol (DAG). IP3 stimulates the release of Ca2+ from internal storage sites, and these calcium ions then act as second messengers, directly affecting enzymatic activity or binding to proteins like calmodulin, which modulates protein kinase activity within the cell. This system is important for various hormonal responses, including those that regulate blood pressure and stimulate growth hormone release.