IP3 molecules bind to IP3 receptors and open up the calcium channels on the endoplasmic reticulum. I am wondering what happens to IP3 molecules after they have been released from the IP3 receptor? Do they remain as IP3 molecules, or do they break down? Do they have any purpose after they have been released, and perhaps after been broken down?
All biomolecules are eventually broken down. This process is called turnover. Since inositol is a signalling molecule it is necessary to remove in order to terminate the signal, despite adaptive mechanism in the ER calcium channels.
IP3 is generally dephosphorylated by a family of phosphatase enzymes called inositol polyphosphate phosphatases. After dephosphorylation, the inositol can be recycled into the membrane by phosphatidyl-inositol synthase.
1 I'm citing this website instead of a research article because apart from citing the original research articles, it also has other informative links about the enzymes and the substrate molecules. It is also from a very reliable source.