145trisphosphate

1,4,5-trisphosphate, often abbreviated as IP3 or InsP3, is a ubiquitous second messenger in eukaryotic cells. It is a phosphorylated derivative of inositol, specifically inositol 1,4,5-trisphosphate. IP3 is generated through the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2), a membrane phospholipid, catalyzed by the enzyme phospholipase C (PLC). This hydrolysis is typically triggered by extracellular signals, such as hormones or neurotransmitters, binding to G protein-coupled receptors or receptor tyrosine kinases.

Once produced in the cytoplasm, IP3 diffuses rapidly and binds to specific IP3 receptors (IP3Rs) located on the surface of the endoplasmic reticulum and sarcoplasmic reticulum. These receptors are ligand-gated calcium channels. The binding of IP3 to these receptors causes a conformational change, opening the channels and leading to the release of stored calcium ions (Ca2+) from these intracellular organelles into the cytosol. This transient increase in cytosolic calcium concentration is a critical signaling event.

The elevated calcium levels then activate a wide range of downstream cellular processes. These include muscle contraction, neurotransmitter release, gene expression, cell proliferation, and apoptosis, depending on the cell type and the specific signaling context. The precise role of IP3 and the resulting calcium signaling varies significantly across different biological functions and organisms. The dephosphorylation of IP3 by specific phosphatases, such as inositol polyphosphate 5-phosphatase, terminates the signal, allowing for precise temporal control of cellular responses.