Photolyase quantum yield

These experiments proved that a light-excited, reduced flavin is indeed able to photoreduce cyclobutane pyrimidine dimers and that these dimers undergo a spontaneous cycloreversion. The quantum yield of about 0=5% clarified that the overall dimer splitting process is highly efficient, even in these simple model systems ((]) photolyase 70%). 

The quantum yield of DNA repair by photolyase (the number of cyclobutane pyrimidine dimers split by the enzyme for each photon absorbed by the enzyme in the enzyme-substrate complex) ranges from 0.7 to 1.0. It should be noted, however, that in photolyase FADH is the catalytic cofactor and MTHF (or 8-HDF) is the photoantenna. As a consequence, the quantum yield of photolyase is the product of three reactions (Payne and Sancar, 1990) energy transfer from MTHF (or 8-HDF) to FADH , electron transfer from (FADH ) to the PyrOPyr, and finally splitting of PyrOPyr °. The latter two reactions are very efficient and occur with nearly 100% efficiency, at least in the case of ToT. Therefore, the critical determinant of overall quantum yield of repair is the quantum yield of energy transfer from the photoantenna to the catalytic cofactor (Kim et aL, 1991, 1992). The efficiency of energy transfer by Forster radiationless... 

Clearly, all indications are that (6—4) photolyase binds DNA and repairs its substrate by a mechanism quite similar to that of classical photolyase. However, there appears to be a fundamental difference in the photochemical reaction catalyzed by the two enzymes. The quantum yield of repair by excited singlet-state flavin by classical photolyase is near unity, whereas the quantum yield of repair by excited flavin in (6-4) photolyase is 0.05-0.10. Whether this low quantum yield of repair by (6—4) photolyase is a result of the low efficiency of formation of the oxetane intermediate thermally, low efficiency of electron transfer from the flavin to the photoproduct, or low efficiency splitting of the oxetane anion coupled with high rate of back electron transfer is not known at present. Furthermore, it was found that (6-4) photolyase can photorepair the Dewar valence isomer of the (6-4) photoproduct (Taylor, 1994) that cannot form an oxetane intermediate, casting some doubt about the basic premise of the retro [2+2] reaction. However, the Dewar isomer is repaired with 300-400 lower quantum yield than the (6-4) photoproduct, and it has been proposed (Zhao et ai, 1997) that the Dewar isomer may be repaired by the enzyme through a two-photon reaction in which the first photon converts the Dewar isomer to the Kekule form and a second electron transfer reaction initiated by the second photon promotes the retro [2+2] reaction. 

The lesions caused by irradiation can be repaired through DNA photoreactivation catalyzed by different photolyases, specifically active either on cyclobutane dimers (with a high quantum yield) or on 6-4 adducts (resulting from the ring opening of oxetanes, with a low quantum yield). The mechanism of the first reaction has been investigated in detail, while attention to the second has grown more recently. 

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