Question

2. Name the three main classes of cell-surface receptors and describe mechanism of action.

Answer 1

Cell surface receptors are the receptors that are found spanning the cell membrane and perform signal transduction, convert the extracellular signal into the intracellular signal. They bind with the ligand , so the ligand need not to enter the cell.

The receptor has three main components - extracellular ligand binding domain, hydrophobic membrane spanning region, intracellular domain.

Three types of cell surface receptors-

1. Ion channel linked receptors- these bind to the ligand , afterwards ion channel gets open spanning through membrane through which specific ion enters. When a ligand binds to extracellular domain, there is conformational change in the protein structure (of channel) and due to this ion flows.

2. G protein coupled receptors- when they bind to ligand, it activates G protein ( transmembrane) . The activated G protein either interacts with the ion channel or an enzyme for further transduction of signal. Heterotrimeri G proteins have three subunits: α, β, and γ. When a signaling molecule binds to a G-protein-coupled receptor in the plasma membrane, a GDP molecule associated with the α subunit is exchanged for GTP. The β and γ subunits dissociate from the α subunit, and a cellular response is triggered either by the α subunit or the dissociated β pair. Hydrolysis of GTP to GDP terminates the signal.

3. Enzyme linked receptors- Their intracellular domains are associated with enzymes or sometimes domain is an enzyme itself. When the ligand binds to the extracellular domain , signal is transferred through membrane and it activates the enzyme , which sets off a chain of events within the cell that eventually leads to a response. An example of this type of enzyme-linked receptor is the tyrosine kinase receptor.