Biology of Information Flow, Receptors and Signaling
- Pp. 599-640 (42)Robert E. Smith
The most common second messenger is Ca<sup>2+</sup>, which is stored in the type of endoplasmic reticulum (ER). Some of the other important second messengers include IP<sub>3</sub> and diacyl glycerol (produced by the hydrolysis of phosphoinositides), arachidonic acid (produced by the hydrolysis of phospholipids that have arachidonoyl on carbon number 2 of the glycerol backbone), ceramide, eicosanoids, lysophosphatidic acid, NO (nitric oxide), cAMP and cGMP. The most common type of receptor is the G proteincoupled receptor (GPCR). Protein kinases can catalyze the addition of a phosphate to another protein or even to themselves and affect signal transduction. Another class of G proteins is called the small GTPases. Nuclear receptors (NRs) play key roles in growth, development and homeostasis. Lipohilic natural hormones diffuse past the cell membrane and bind to receptors located in either the cytosol (type I NR) or the nucleus (type II NR). The IP<sub>3</sub> receptor, or IP3R is a membrane-bound complex of glycoproteins. It is a Ca<sup>2+</sup> channel that is activated by IP<sub>3</sub>, which is a secondary intra-cellular messenger. Inter-and intracellular communication can be considered as a network that contains many items (nodes) that have about one to thousands of connections. The most widely connected nodes are called hubs. Probably, the major genetic hub in human and many other mammalian cells is the gene TP53 which codes for the protein p53. About 50% of all human cancers have one or more mutations in p53 that alter DNA transcription.