PHARMACOLOGY

 MOLECULAR CLASSIFICATION OF RECEPTORS ::

Based on molecular structure signal transduction or post - receptor mechanisms receptors have been classified into four superfamilies :

Type I : Channel -linked receptors or Ionotropic receptors 

Type II: G-protein -coupled receptors or metabotropic receptors 

Type III : Kinase -linked receptors 

Type IV: Gene-transcription regulatory receptors or Nuclear receptors 

Type I,II and III are extracellular receptors or transmembrane receptors since they lie within the cell-membrane with binding domain outside the membrane Type IV receptors lie on the nuclear membrane and hence known as intracellular receptors Each transmembrane receptor contains one or more hydrophobic membrane-spanning a-helical segment linking extracellular ligand binding region of the receptor to the intracellular domain 

Type I-Channel linked receptors :: 

These are membrane receptors directly coupled to an ion channel After stimulation of these receptors action is produced in millseconds Nicotinic receptors GABAa receptors and glutamate receptors are the examples of Type I receptors Ion channels as such can be considered as proteins 

                            These receptors consist of four different types of subunits termed a,b.y.$ each of 40-58 KDa The oligomeric structure (a,b,y.$) possesses two acetylcholine binding sites each lying at the interface between one of the two a-subunits and its neighbour Each subunit spans the membrane four times so that oligomer has no less than 20 membrane spanning helices surrounding a central pore The long extracellular N-terminal tail of subunit has binding domain