PHOTOCHEMISTRY, PHOTOPHYSICS, AND MECHANISM OF PYRIMIDINE DIMER REPAIR BY DNA PHOTOLYASE |
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Authors: | Sang-Tae Kim Aziz Sancar |
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Institution: | Department of Biochemistry and Biophysics, University of North Carolina School of Medicine, Chapel Hill, NC 27599-7260, USA |
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Abstract: | Abstract— DNA photolyases photorepair pyrimidine dimers (PyroPyr) in DNA as well as RNA and thus reverse the harmful effects of UV-A (320–400 nm) and UV-B (280–320 nm) radiations. Photolyases from various organisms have been found to contain two noncovalently bound cofactors; one is a fully reduced flavin adenine dinucleotide (FADH-) and the other, commonly known as second chromophore, is either methenyltetrahydrofolate (MTHF) or 8-hydroxydeazaflavin (8-HDF). The second chromophore in photolyase is a light-harvesting molecule that absorbs mostly in the near-UV and visible wavelengths (300–500 nm) with its high extinction coefficient. The second chromophore then transfers its excitation energy to the FADH-. Subsequently, the photoexcited FADH- transfers an electron to the Pyr<>Pyr generating a dimer radical anion (Pyr<>Pyr-) and a neutral flavin radical (FADH-). The Pyr<>Pyr- is very unstable and undergoes spontaneous splitting followed by a back electron transfer to the FADH-. In addition to the main catalytic cofactor FADH-, a Trp (Trp277 in Escherichia coli ) in apophotolyase, independent of other chromophores, also functions as a sensitizer to repair Pyr <> Pyr by direct electron transfer. |
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