title: Kinetics, binding constant, and activation energy of the 48-kDa protein-rhodopsin complex by extra-metarhodopsin II.
abstract: We have found that the 48-kDa protein (or S-antigen 48k) of the rod photoreceptor enhances the light-induced formation of the photoproduct metarhodopsin II (MII) from prephosphorylated rhodopsin. The effect is analogous to the known enhancement of MII (extra-MII) that results from selective interaction of MII with G-protein. We have determined some parameters of the MII-48k interaction by measuring the extra-MII absorption change induced by the 48-kDa protein. The amplitude saturation yields a dissociation constant for the MII-48k complex on the order of 50 nM. At the technical limit of these measurements, 13.7 degrees C and 12 microM 48-kDa protein, we find a rate of 2.3 s-1 for formation of the 48k-MII complex. Extrapolation of these values to cellular conditions yields an occupation time of phosphorylated MII by 48k less than 200 ms. This is short compared to estimated rates of phosphorylation. The temperature dependence of the MII-48k formation rate is very high (Q10 for 5 degrees C/15 degrees C = 9-10). The related Arrhenius activation energy (165 kJ mol-1) is correspondingly high and indicates a considerable transient chemical change during the binding process.
authors.affiliation: Institut für Biophysik und Strahlenbiologie, Universität Freiburg, Federal Republic of Germany.
authors.forename: A
authors.initials: A
authors.lastname: Schleicher
authors.forename: H
authors.initials: H
authors.lastname: Kühn
authors.forename: K P
authors.initials: KP
authors.lastname: Hofmann
mesh_terms: D000818:Animals; D000941:Antigens; D019393:Arrestin; D001324:Autoantigens; D002417:Cattle; D005136:Eye Proteins; D007700:Kinetics; D008027:Light; D008433:Mathematics; D008565:Membrane Proteins / Q000378:metabolism*; D010782:Photolysis; D010786:Photoreceptor Cells / Q000378:metabolism*; D012160:Retina / Q000378:metabolism*; D012168:Retinal Pigments / Q000378:metabolism*; D012243:Rhodopsin / Q000031:analogs & derivatives / Q000378:metabolism*; D012374:Rod Cell Outer Segment / Q000378:metabolism*; D012542:Scattering, Radiation; D013056:Spectrophotometry, Ultraviolet
chemical_list: D000941:Antigens; D019393:Arrestin; D001324:Autoantigens; D005136:Eye Proteins; D008565:Membrane Proteins; D012168:Retinal Pigments; C013110:metarhodopsins; D012243:Rhodopsin