SIGNAL TRANSDUCTION OLAREWAJU BLESSING OHARISI Definition Signal transduction
SIGNAL TRANSDUCTION OLAREWAJU BLESSING OHARISI
Definition: Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as series of molecular events, most commonly protein phosphorylation catalysed by protein kinases, which ultimately results in a cellular response. It is the process by which a physiological signal is transformed into a specific cellular responses. Proteins responsible for detecting stimuli are generally termed receptors, although in some cases the term sensor is used. The signal transduction pathway is as follows;
Membrane receptors transfer information from the environment to the cell’s interior. • A message also referred to as ligand is picked up by the receptor extending outside the cell from the cell membrane. The message will dock with the receptor and changes the shape of the receptor in order for the messenger to be able to enter the cell. • Transduction is the process of switching the signal message from the outside of the cell to a message within the cell. • First messengers are the signalling molecules (hormones, neurotransmitters, etc. ) that reach the cell from the extracellular fluid and bind to their specific receptors.
Secondary Messengers • Secondary messengers are the substances that enter the cytoplasm and act within the cell to trigger a response. It serves as a chemical relays from the cell membrane to the cytoplasm, thus carrying out intracellular signal transduction. During this process of signal transduction, the secondary messenger are often free to diffuse to other components of the cell like the nucleus. The signal is also amplified during this process. • An example of secondary messenger is cyclic AMP that can target cells for example cells in the liver to release glucose from glycogen.
Protein Phosphorylation • Many secondary messengers elicit responses by activating protein kinases. Protein kinase is a kinase enzyme that modifies other proteins by chemically adding phosphate groups to them. Phosphorylation usually results in a functional change of the target protein by changing enzyme activity, cellular location, or association with other proteins.
Signal Termination • Protein phosphatases are one mechanism for the termination of a signalling process. After signalling process has been initiated and the information has been transduced, the signal is terminated by the phosphatase.
Example – Liver Cell: • Epinephrine (a ligand) is the messenger given off from the adrenal gland. It binds with a receptor called a G-protein receptor on the outside of the cell and causes a conformational change to the protein for it to be able to pass through the lipid bilayer of the cell membrane. • Underside of the receptor inside the cytoplasm is three alpha subunits. • One of the alpha subunit detaches and bind to a protein that activate an enzyme that converts ATP to cyclic AMP (secondary messenger) that will spread throughout the cell and target protein kinase. • Protein kinase is activated by c. AMP and becomes phosphorylated picking up energy from ATP. They can then act as enzyme within the cell. • The phosphate becomes phosphorylase that releases glucose from glycogen within the cell.
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