PHARMACOLOGY

 TYPE II : G-PROTEIN COUPLED RECEPTORS ::

These are membrane receptors coupled to G-proteins After stimulation of these receptors action is produced within seconds Muscarinic receptors and adrenergic receptors (a and b) are the examples of type II receptors 

                These receptors consist of a single polypeptide chain of 400-500 residues They possess seven transmembrane helices similar to those of the channel linked receptors Both the extracellular amino terminus and the intracellular carboxy terminus vary greatly in length and sequence The intracellular part contains G-protein coupling domain whereas between 2 and 3 transmembrane helices lie ligand binding domain 

TYPE III : KINASE-LINKED RECEPTORS ::

These are the membrane receptors that incorporate an intracellular protein kinase domain (usually tyrosine kinase ) within their structure These are the receptors for various hormones eg. insulin growth factors and cytokines Atrial Natriuretic Peptide (ANP) receptors which are linked to guanylyl cyclase also belong to this type of receptors Activation of these receptors may take about a few minutes to produce the action 

        The first step following agonist bluding is dimerisation which leads to autophosphorylation of the intracellular domain of each receptor SH2-domian proteins then bind to the phosphorylated receptor and the themselves phosphorylated Two well characterise pathways are shown [A] The growth factor (Ras/Raf) pathway : [B] The cytokine (jakstat) pathway