What are the main types of cell surface receptors?

The three main types of cell surface receptors are G protein coupled receptors (GPCRs), ion channel receptors, and enzyme-linked receptors. GPCRs bind a ligand and activate a membrane G-protein. The activated form of the G-protein then interacts with an enzyme or ion channel in the membrane; G-protein-linked receptors have seven specific transmembrane domains with their own binding site. During cell signaling, the inactive G-protein binds to the receptor specific site and the shape of the protein changes, becoming activated; GDP is released and becomes GTP. The subunits of G-protein separate into the alpha and beta subunit; one or both of the subunits can then activate other proteins. The GTP on the active alpha subunit is hydrolyzed to GDP and the beta subunit is deactivated, allowing for the cell cycle to start again. 

Enzyme linked receptors are cell surface receptors with domains that are connected with an enzyme. Sometimes the domain of the receptor itself is an enzyme or the enzyme-linked receptor has a domain that directly attaches to the enzyme. When a ligand binds to the domain, the enzyme becomes activated and leads to a response. The tyrosine kinase receptor is an example. The tyrosine kinase receptor transfers phosphate groups to tyrosine molecules. The signaling molecules then bind to the domain of the two tyrosine kinase receptors and dimerize. Phosphates then become added to tyrosine residues on the domain and the signal can then be transmitted to the next messenger cell.

Ion channel-linked receptors bind to a ligand and open a channel through the membrane that lets specific ions to pass through. When a ligand binds to the extracellular region of the channel, there is a conformational change in the protein structure that allows ions like calcium, sodium and magnesium to pass through.