Receptors that initiate the signaling pathways are generally associated with the plasma membrane. The extracellular domain of the receptors recognizes the ligands present over the cell surface and this interaction may lead to conformational changes in the receptor. The conformational changes are associated with the recruitment of the phosphate group at its carboxy terminal on tyrosine, serine, or threonine residue. The enzyme that adds the phosphate group on amino acid residues are called protein kinases . The tyrosine is the major amino acid residue that takes part in this event (phosphorylation) hence the enzymes are referred as protein tyrosine kinases . Alternatively the enzymes which are responsible for the removal of phosphate group from amino acids are called phosphatase . In general the protein kinases can initiate while phosphatase can inhibit the signaling pathways . Several types of protein modification can also modulate the binding of an antigen to the receptor such as phosphorylation (addition of phosphate group), acetylation (addition of acetyl group), methylation (addition of methyl group), and ubiquitination (addition of ubiquitin). The ubiquitination is the event that takes place during the degradation of proteins through proteasome.

13.1 Types of cellular receptors

There are several types of cell receptors based on their signaling mechanism and biochemical pathways.

13.1.1 Receptor tyrosine kinases

They are associated with the cell membrane and are involved in the phosphorylation of tyrosine residue located in their cytoplasmic tail. The pathway begins after binding with a suitable ligand over the receptor. e.g. Insulin receptor, epidermal growth factor receptor, platelet derived growth factor receptor, and receptor involved in the process of hematopoiesis.

13.1.2 Non-receptor tyrosine kinases

They are associated with the cell membrane and are involved in the phosphorylation of proteins by a non-receptor tyrosine kinases following binding with a ligand. Immune receptors, cytokine receptors, and integrins are known to follow non-receptor tyrosine kinases signaling pathway.

13.1.3 Seven transmembrane receptors

These are the polypeptide receptors that traverse seven times in the plasma membrane and hence also named as serpentine receptor . The receptor generally binds to GTP hence also called as G protein-coupled receptors (GPCR). Binding of the ligand to GPCR activates the hetrotrimeric G protein and initiates the downstream signaling pathway. Inflammatory cytokines and cAMP are activated following binding to GPCR.