PHOTOCHEMISTRY, PHOTOPHYSICS, AND MECHANISM OF PYRIMIDINE DIMER REPAIR BY DNA PHOTOLYASE

DNA photolyases photorepair pyrimidine dimers (Pyr<>Pyr) 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.