To respond to environmental changes and regulate the cellular metabolism and physiological activities, cells develop a complicated signal transduction network to receive and process signals that originate outside their borders. Within this network, various molecules, such as growth factors, hormones, neurotransmitters, and extracellular matrix components, carry the signal, binds to the all kinds of receptors and regulate the corresponding pathways locally or in a long distance in response to environmental changes. Receptors are generally transmembrane proteins, which bind to signaling molecules outside the cell and subsequently transmit the signal through a sequence of molecular switches to internal signaling pathways. There are three classes of membrane-anchored receptors: G-protein-coupled receptors, ion channel receptors, and enzyme-linked receptors.
Once a receptor protein receives a signal, it undergoes a conformational change, launches a series of signal transduction cascades, amplify the message, and produce multiple intracellular signals for each bound receptor. Activation of receptors can trigger the synthesis of small molecules called second messengers, such as cyclic AMP (cAMP), these cAMP molecules activate the enzyme protein kinase A (PKA), which then phosphorylates multiple protein substrates by attaching phosphate groups to them. Each step in the cascade further amplifies the initial signal,and the phosphorylation reactions mediate both short- and long-term responses in the cell. Other second messengers include diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3), which activate another enzyme called protein kinase C (PKC). Protein kinases such as PKA and PKC catalyze the transfer of phosphate groups from ATP molecules to protein molecules. These phosphorylation reactions control the activity of many enzymes involved in intracellular signaling pathways. Through this network of signaling pathways, the cell is constantly integrating all the information it receives from its external environment.
Cell signal Products (Link)
Pathways of Intracellular Signal Transduction. In The Cell: A Molecular Approach. 2nd edition. Geoffrey M Cooper, Sinauer Associates; 2000. ISBN-10: 0-87893-106-6.