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1974 (1)
1Author    HinderkM. Emrich, Roland ReichRequires cookie*
 Title    Über Primärreaktionen beim Sehvorgang Thermodynamischer und kinetischer Einfluß des pH-Wertes auf die M etarhodopsin-I-II-Umwandlung Protonenverbrauch als Auswirkung einer Konform ationsänderung Prim ary Reactions in the Visual Process Thermodynamic and Kinetic Influence of pH on the M etarhodopsin I-II Transition Proton Consumption as an Effect of a Conformation Change  
 Abstract    In isolated bovine rod outer segments metarhodopsin I —II absorption-changes are measured using rapid flash-photometry. The rapid H f-uptake accompanying this transition is measured using phthaleine pH-indicators. 1. The meta I —II equilibrium is pH-dependent in the same manner as the proton uptake (Figs 1 ,1 4). This can be explained by the assumption that the meta II conformation is stabilized by the protonization of an amino-group, which in its protonized state cannot be re-enfolded into the hydrophobic core of the molecule, since it is too polar (Fig. 15). In the proposed model the con­ formation-regulating proton binding group is not in the chromophoric region of rhodopsin. This resolves the well known contradiction between the expected bathochromic and observed hypso-chromic shift of meta II H+. The model furthermore involves that the dissociation-constant of the protonized group differs from the measured apparent pK = 6.3. 2. The meta I —II transition is H+ (and OH")-catalysed (Figs 4, 5). The kinetics show usually a characteristic deviation from a first-order process (Fig. 2). This can be explained by some in­ homogeneity of the rhodopsin-molecules e. g. some differences in the shieldings of the catalytic centers from H+ (and OH-). 3. The activation energy decreases with increasing proton-concentration (Fig. 6). The protons seem to open a new reaction pathway with a smaller activation energy. 4. Triton X 100 and ethanol increase, sodium-desoxycholate slows down the velocity of the M-I-II-transition, whereas digitonin has only a small effect (Fig. 7). 5. The relaxation-velocity of the optical pH-signal depends on the structural state of the rhodopsin-containing substrate (Figs 10. 11). The outer membrane of the outer segment and the disc-membrane are regarded as possible H+-diffusion-barriers (Fig. 12), because the time lag between Mu-signal and pH-signal is nearly eliminated, when the membranes are destroyed by digitonin. 6. In some experiments, using successive non-saturating flashes, an initial H+-uptake of 2 — 3 H+/M n was observed (Fig. 17). After full-bleaching, how'ever, the ^ H +/^M ii-ratio is one. The hypothesis of a conformative coupling of 2 — 3 rhodopsin-molecules is presented (Fig. 18). The possible existence of an "unfolded" state of rhodopsin is discussed on the basis of a photo-regeneration-experiment, in which no H+-release was observed (Figs 19, 20). 
  Reference    (Z. Naturforsch. 29c, 577 [1974]; eingegangen am 2. Mai/6. Juni 1974) 
  Published    1974 
  Keywords    Rhodopsin-photolysis, Photoreception, pH-Change, Proton-catalysis, Conformation-change 
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 TEI-XML for    default:Reihe_C/29/ZNC-1974-29c-0577.pdf 
 Identifier    ZNC-1974-29c-0577 
 Volume    29