Photo transduction

Photoreceptors continually synthesize new photo transduction proteins for incorporation into newly formed outer segment discs. As new discs are formed at the base of the outer segment, older discs are phagocytosed by RPE cells at the tip of the outer segment. Each outer segment disc is formed and shed in -2 weeks. The phagocytosis of discs occurs in circadian bursts (rods at the end of night, cones at the end of day). 

Briefly summai ize the major steps in photo transduction in rods. 

Suimnarize the enzymatic mechanisms by which photo transduction is terminated after photostimulation. 

Cideciyan AV, Jacobson SG. An alternative photo transduction model for human rod and cone ERG a-waves normal parameters and variation with age. Vision Res 1996 36 2609-2621. 

Vision researcher have been concerned with post-translational modifications and differential expression methods years before the term proteomics existed. Thanks to their dedication we now understand the basic mechanisms of photo-transduction. Already in the eighties, investigators performed differential studies using ID or 2D electrophoresis, localised differentially expressed proteins on their gels and were left wondering how to find out their identity. We will mention several of them, who managed, through their remarkable persistence, to identify their protein of interest by a variety of methods. 

Shinozawa, X, Hashimoto, H., Fujita, J., and Nakaoka, Y., Participation of GTP-binding protein in the photo-transduction of Paramecium bursaria. Cell Struct. Funct., 21,469, 1996. 

Fig. 12. The different steps of energy transfer in photo-transduction, with the efficiencies calculated from our models.

Halavaty, A. S. and K. Moffat (2007). N- and C-terminal flanking regions modulate light-induced signal transduction in the LOV2 domain of the blue-light sensor photo tropin 1 from Avena sativa. Biochemistry 46 14001-14009. 

Scheme 3. Flavins are capable to undergo both 1 e and 2 e input/output redoxreactions as indicated. Since cytochrome b is a typical 1 e redox reagent, Hemmerich and Schmidt86) suggest a radical mechanism of the sensory transduction (assuming that the cyt b photoreduction is photo-tropically relevant). The nature and fate of the flavin-photosubstrate XH remains obsure. For the case X = cyt b the scheme represents a reversed respiration electron pathway...

Besides these shortcomings the bluelight receptor and sensory transduction problem is recently being attacked on the basis of completely artificial flavin/membrane systems. These appear to provide well-defined model systems to study anisotropic flavin (photo-) chemistry. This, in turn, is an essential prerequisite which allows the primary photo-events of physiological bluelight reception to be imitated and elucidated. 

For instance, conductometric transduction has been used for determination of an atrazine herbicide in the concentration range 4.6-231.8 mM [165]. A sintered glass frit was used as the support for the MIP film prepared by thermo-radical polymerization. However, both the response time of 30 min and the chemosensor recovery time of 12 h were long. For better performance, the MIP film was prepared by photo-radical co-polymerization of a chloroform solution, which contained tri (ethylene glycol)dimethacrylate (TEGDMA), MAA, and oligourethane acrylate, sandwiched between two quartz slides (Table 6). 

Herbicides, like demestryn, have been determined using a chemosensor based on the MIP film recognition and capacitive transduction [193, 194]. In this determination, photografting polymerization has been demonstrated as an efficient procedure for fabrication of a capacitive chemosensor. This procedure involved immobilization of an initiator on the electrode. In this case, first, an alkanethiol monolayer was self-assembled on the gold electrode. This monolayer was perfectly dielectric. Then, an MIP film was deposited on top of this monolayer by photo-radical polymerization in the acetone solution with benzophenone, 2-acrylamido-2-methyl-1-propane sulphonic acid, /V./V -iriethylenediacrylamide and demestryn used as the initiator, cross-linker, functional monomer and template, respectively. Subsequently, the template was extracted with methanol. The capacitance decreased by 20% upon binding the demestryn analyte by the MIP film. Similarly, a creatine chemosensor was constructed [194],... 

Scheme 7 Electronic transduction of photo-switchable bioelectrocatalytic functions of proteins, (A) by the tethering of photoisomerizable units to the protein (R is a diffusional electron mediator that electrically contacts the redox...

GOx (22a). (b) protonated nitromerocyanine-tethered GOx, (22b). I nset Reversible photo-switchable amperometric transduction of the bioelectrocatalyzed oxidation of glucose by 22a - ( ) and 22b-(o). 

Impairment of the retinoid signal transduction pathways occurs as a result of prolonged UV exposure. Down regulation of nuclear receptors for Vitamin A occurs,269 resulting in a functional deficiency of Vitamin A. Application of Vitamin A derivatives would appear to be an obvious treatment modality. Topical application of Vitamin A does increase the HA in the epidermal layer, increasing the thickness of the HA meshwork after prolonged treatment.270 Vitamin A thus enhances repair, as can be demonstrated in photo-aged hairless mouse model.271 The decline in GAG, and in particular HA deposition that occurs with UVB irradiation, can be entirely prevented by retinoic acid treatment. 

Van Brederode, M. E., Van Mourik, F., Van Stokkum, I. H. M., Jones, M. R., and Van Grondelle, R., 1999a, Multiple padiways for ultrafast transduction of light energy in die photos>nthetic reaction center of Rhodobacter sphaeroides. Proc. Natl. Acad. Sci. USA, 96 2054n2059. 

Several arguments support the view that methyl-accepting proteins are involved in the sensory transduction machinery of Halobacterium [46,41] (1) Demethylation is transiently increased by photo- and chemostimuli which are known to control behaviour. 

Katz, E., WiHner, B., and Willner, I. Light-controlled electron transfer reactions at photo isomerizable monolayer electrodes by means of electrostatic interactions Active interfaces for the amperometric transduction of recorded optical signals. Biosens. Bioelectron. 1997, 22, 703-719. 

Katz, E, Lion-Dagan, M., and Willner, I. Control of electrochemical processes by photo-isomerizable spiropyran monolayers immobilized onto Au-electrodes Amperometric transduction of optical signals. J. Electroanal. Chem. 1995, 382,25-31. 

Enzyme biosensors have been described using a range of transduction elements (amper-ometry, potentiometry, optical and photo-thermal). The first biosensor was described in the literature by Clarck and Lyons (1962a) and was based on the use of glucose oxidase combined with electrochemical detection. Since then, this principle has been widely applied in biosensor development, and the enzyme systems used have been mainly oxido-reductases (e.g. tyrosinase, peroxidase and lactase) (Cass etal., 1984 Kulis and Vidziunaite, 2003), and hydrolases (choline esterases) (Andreescu etal., 2002 Nunes etal., 1998). 

Pigmented Bilayer Lipid Membranes, Chlorophyll molecules have been incorporated into two types of artificial bllayer lipid membrane systems for the study of photoenergy transduction. The first consists of a planar bllayer lipid membrane (BLM) separating two aqueous solutions where photovoltaic effects can be Induced. The second system comprises liposomes which are Ideally suited for studies of photo-induced permeability, spectroscopy and chemical reactions. For more complete technical details, two pertinent publications are available (42,43). 

Photo-responsive Synthetic Membranes. Although the visual Information transduction Is too complexed to be realized in vitro, the photoeffects of retinal-containing synthetic membranes have been investigated. Alzawa et al. (74) prepared a photo-responsive membrane from a solution of 11-cis retinal, phosphatidyl choline and trlacetyl cellulose. The retinal was assumed to be incorporated into the molecular assemblies of phosphatidyl choline, which were dispersed In the trlacetyl cellulose membrane matrix. The membrane responded to visible light by showing a transmembrane potential in association with the photoisomerization of membrane-bound 11-cis retinal. On the other hand, a membrane Incorporating 11-cls retinal without phosphatidyl choline exhibited little light-induced transmembrane potential (75). 

It has been proposed that the structural changes associated with the protonation of the chromophoric phenol are responsible for the signal transduction by the pB state. The pB—>pR conversion is photo reversible and its kinetics have been examined [11], Calculations [12] have shown that proton transfer is much more likely in the cis pR state than in the initial dark trans pG state. The PYP photocycle is completed by the pB state relaxing back to the pG dark state. This deprotonation and reisomerization is pH-dependent (r = 140 ms). It has been shown that isomerization of the deprotonated chromophore is faster than for the protonated form, therefore it has been suggested that protonation precedes isomerization. 

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