Photoenzymes

Trying to determine the structure of a protein by UV spectroscopy was like trying to determine the structure of a piano by listening to the sound it made while being dropped down a flight of stairs. 

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Several natural photoenzymes with activity controlled by light have been reported to date. Among them are DNA and (6-4) photoproduct photolyases, which are highly efficient light-driven DNA-repair enzymes [14, 15], protochlorophyllide reductase, which is an important enzyme in the chlorophyll biosynthesis pathway [16, 17], nitrile hydratase, which hydirates aliphatic and aromatic nitriles to the... 

Figure 12.2 Cyclic photosynthetic system composed of a reductive photoenzyme Eo-Mb-Co11...

The model systems for enzymes activated by light can be called artificial photoenzymes . This expression comprises two important requirements, which must be fulfilled by system its activity should be triggered by light and it should behave like an artificial enzyme . The term artificial enzyme is very often used inappropriately and applied to any catalysts. The detailed explanation of this term, together with critical analysis of enzyme model systems, can be found in an excellent review by Kirby [35], In general, enzymes are more than just highly developed catalysts ... 

Usually mimetics of enzymes are not as sophisticated and their catalytic efficiency is taken into account as a main objective. The important feature of enzyme mimetics is an initial binding interaction between substrate and the catalyst, thus giving rise to Michaelis-Menten kinetics [35], Therefore for a model system to be called an artificial enzyme it should combine more than one of the key features identified for enzymes [35], For some systems it is difficult to decide if they can be considered as artificial photoenzymes or if they are just photocatalysts (described in Chapters 6, 7,10, and 21). It is not the purpose of this book to judge whether model systems can be called artificial photoenzymes or should be considered as just chemical photocatalysts. To get some feeling which type of model systems have been investigated we describe a few model systems that have claimed to have enzyme-like activity. 

Zahavy E, Willner I. Photoinduced electron transfer in eosin-modified Co(II)-protoporphyrin IX reconstituted myoglobin and a- or /3-hemoglobin subunits photocata-lytic transformations by the reconstituted photoenzymes. / Am Chem Soc 1996 118 12499-514. 

Figure 42. The preparation of a photoenzyme by the reconstitution of a heme protein with Co(II)-protoporphyrin IX and the chemical modification of the protein backbone with a tethered chro-mophore. Hydrogen evolution and hydrogenation of acetylene derivatives photobiocatalyzed by the assembly.

An alternative approach to the preparation of photoenzymes by the reconstitution of apo-proteins includes the application of metalloprotoporphyrins such as Zn(II)-protoporphyrin IX (Zn-P), which exhibits photophysical electron-transfer properties for the reconstitution of apo-proteins derived from hemoproteins. Func-... 

Photoactivated redox proteins will play important roles in the future development of artificial photosynthetic systems. The unique participation of the protein matrices in photoinduced charge separation and the possibility of implanting catalytic sites in the protein indicate that tailored photoenzymes will be important components for the light-driven synthesis of fuels and valuable chemicals, e.g., CO2 and N2 fixation and amino acid synthesis. The areas of bioelectronics and opto-bioelectronics represent exciting interdisciplinary ventures for chemists, biologists, physicists and materials scientists. The advances in these fields oflFer compelling opportunities for the development of electronic biomaterial-based devices. 

Willner, I, Zahavy, E., Heleg-Shabtai, V. (1995). Eosin-modified reconstituted Co(II) protoporphyrin IX myoglobin A semisynthetic photoenzyme for evolution and hydrogenation, J. Am. Chem. Soc., 117 542. 

Bioluminescence imaging of luciferase reporter genes is much more sensitive for in vivo imaging applications due to the lack of background bioluminescence signal [38]. Luciferases are photoenzymes isolated from a wide variety of insects. 

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