Enzyme photoisomerization

Retinol A. can be enzymically formed from retinoic acid. B. is transported from the intestine to the liver in chylomicrons. C. is the light-absorbing portion of rhodopsin. D. is phosphorylated and dephosphorylated during the visual cycle. E. mediates most of the actions of the retinoids. Correct answer = B. Retinyf esters are incorporated into chylomicrons. Retinoic acid cannot be reduced to retinol. Retinal, the aldehyde form of retinol, is the chromophore for rhodopsin. Retinal is photoisomerized during the visual cycle. Retinoic acid, not retinol, is the most important retinoid. 

Luminescence properties of and phenomena in polymer systems continues to be widely researched in connection with mechanisms of polymer degradation and stabilization, molecular dynamics, solubility, blend miscibility, and solar energy harnessing. A number of interesting reviews have appeared. Molecular dynamics of polymers in solution and in the solid state have been covered, as has excimer formation,photoresponsive polymers,behaviour of polymer gels, and photochromic phenomena. Photoisomerization of enzymes and model compounds has also been discussed in depth, with particular emphasis on proteins and synthetic polymers containing azo-compounds or spirobenzopyrans. ... 

Photoswitchable electrical communication between enzymes and electrodes has also been achieved by the application of photoisomerizable electron-transfer mediators [195, 199]. DilTusional electron mediators (viologen or ferrocene derivatives) were functionalized with photoisomerizable spiropyran/merocyanine units. These mediators can be reversibly photoisomerized from the spiropyran state to the merocyanine state (360 < A < 380 nm) and back (A > 475 nm). An enzyme multilayer array composed of glutathione reductase or glucose oxidase was electrically contacted only when the photoactive group linked to the redox relay (viologen or ferrocene derivative, respectively) was in the spiropyran state. 

The photoisomerizable enzyme monolayer electrode also revealed photoswitchable bioelectrocatalytic activity (Figure 7.10). In the presence of ferrocene carboxylic acid (5) as a diffusional electron transfer mediator, the nitrospiropyran-tethered GOx (4a) revealed a high bioelectrocatalytic activity, reflected by a high electrocatalytic anodic current. The protonated nitromerocyanine-GOx (4b) exhibited a two-fold lower activity, as reflected by the decreased bioelectrocatalytic current. By the reversible photoisomerization of the enzyme electrode between the 4a- and 4b-states, the current responses are cycled between high and low values (Figure 7.10, inset). 

Hybrid potentials of the type discussed in this chapter have been in use for twenty five years or so. Some of the earliest examples were those designed to study conjugated organic molecules in which the -electrons of the system were treated with a semiempirical QM method and the cr-bonding framework was described with a MM force field [8, 9], The methods were used to study the structure and spectra of the molecules [10, 11] and photoisomerization processes [12, 13], The first true combined potential in which both a and w electrons were considered in the quantum mechanical region was also developed by Warshel, in collaboration with Levitt, which they used to study the mechanism of the enzyme, lysozyme [14], They combined a semiempirical QM method to describe a portion of the enzyme and the substrate, and a standard MM force field to describe the rest of the atoms. 

The fluorenone-sensitized irradiation of the all rrawj-triene (140) gives a photostationary mixture composed of the trans-cis-trans, cis-trans-trans and cis-cis-trans isomers. The potential energy surfaces of the ground and excited state of the triene (140) have been mapped,and a study of the fluorescence from this triene in lipid bilayers and isotropic solvents has been carried out. The photophysical properties of the dithienylpolyenes (141) and (142) have been measured. The photoisomerism of all-tranj-retinal to the 11-m isomer brought about by honeybee retina enzyme photoisomerase has been described. ... 

The bioelectrocatalyzed oxidation of glucose in this system originates from the primary oxidation of the ferrocene carboxylic acid, (21), to the respective ferrocenylium cation that mediates the oxidation of the enzyme s redox center and its activation towards the oxidation of glucose. Photoisomerization of the enzyme monolayer to the MRH-GO state switched-off the bioelectrocatalytic functions of the protein monolayer, and only the electrical response of the diffusional electron mediator was observed, Fig. 3-31, curves (b) and (d). By the cyclic photoisomerization of the enzyme-monolayer electrode between the SP-GOx and MRlT-GOx states, the reversible photoswitching of the enzyme activity between ON and OFF states was demonstrated, Fig. 3-31 (inset). 

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