and marry inforrnations about R s are pouring in rapidtl Yet only some discrete date are known and unit the technöogy

receptors'(ii)Receptors for thyioid and steroid hormones are called' cytosol 'receptors because they occur either in t he cytosol or on the nucleus Affinty Agrven type of R has affinty for its own ligand Thus R for aöfenalin has afltmty for adrenalin molecules Bindring This (ie,the binding berween the R and Lmolecules)occurs by Van derWalls force (lntermoleculär dttracnon) and hydrogen bonds but not by covafcent Iinkages Recall. the bindnig by covalent linkages resulits in extremely strong oindng (inreversitble & nding) Structure An R has two domains (regions) (i) a ligand binding site . which binds with the ligand,it has alriniry for its specific L and can recognize the L (n) the öfher doman is called the actve site This site is veryi mporant for the post binding events Furthfr this site deitemes whether the L is an agonisl ür jntergoisl and acts accordingly Movement Aftfr binding with the L. the R becomeis RLC (reccptof-ligand complex) and the RLC moves laterally within the cell membtane in case of membrane nound R Without this movfment the post binding events can not occur. For this lateral movenment, cytoskeleat structres are impoitart (for cyroskeieron, see chap l sec I) Number.. life. degradation it should. be understood that since Langtey first speculaied about the existence of a necepror in 1904 and all the late 1960s,the receptors were purely hypothetical substances,,from many rndrect evidences . people inferred that there are receptors but no drect proof was available The picture has changtd now and marry inforrnations about R s are pouring in rapidtl Yet only some discrete date are known and unit the technöogy improve Stll further a comprehensve picture carnot be expected With this background, the folbwings can be noted (i) R s are proteinaceous macro moleciules (ii) in a guen cell the receptoi popularion of a grven species öf R vanes. sometnes it increases. that is, goes up (up regulation) . sometimes it goes down (down regulattion) (iii) R s have a fixed life span (for example the hatf life of insulin R is about 7 hr (iv)after the expiry of lifr the R is degraoea Details The receptor
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