Diffusion Processes with Hidden States from ... - FU Berlin, FB MI

2 Visual Transductionthe special case of this general signal transduction process, which leads to the conversion of singlephoton signals to amplified electrical signals. This special case will be denoted by the termRhodopsin-Transducin Process. The molecular reactions underlying the activation stages of visualtransduction in retinal rod discs are described below [7] and schematically shown in Figure 2.4.For a more detailed description of the whole process see [7].1. After the absorption of a photon of light, the rhodopsin molecule is transformed into anenzymatic active state (R*).Absorption of a photon by the bent 11-cis retinal isomer within rhodopsin has a high probability(∼ 0,67) of triggering the cis to trans isomerization of the retinoid (see figure 2.6) .Apparently, the straighter all-trans isomer no longer fits neatly into the hydrophobic pocketso that the rhodopsin molecule is strained internally, undergoing a series of molecular rearrangementsthat lead to its activation (as R*).2. R* in turn catalyzes the activation of the G-protein transducin (to G*).Upon binding of the transducin G to R*, the Gα subunit releases its bound molecule ofguanosine diphosphate (GDP) and takes up a molecule of guanosine triphosphate (GTP)from the cytoplasm (see Figure 2.4). The GTP-bound form of Gα represents the active entity,denoted as G*, which carries forward the activation signal. The activated rhodopsin R*separates from the activated transducin, and unaltered by the interaction, is free to interactwith additional molecules of the G-protein, thereby catalyzing the activation of many G*during its active lifetime.3. Transducin again activates the effector protein phosphodiesterase (PDE) to PDE*.Figure 2.4: Scheme of the visual transduction process in the retinal rod discs (from [7]). For detailssee text. The scheme of the 7T M− or serpentine-receptor rhodopsin (R or its activatedform R ∗ ) is shown in Figure 2.5.4. The activated PDE hydrolyzes the diffusive messenger cyclic guanosine mono-phosphate(cGMP) so that the cytoplasmic concentration of cGMP decreases, leading to closure ofcGMP-gated ion channels in the plasma membrane. Closure of these channels has the dual8