Efficient light-dependent DNA repair requires a large cofactor separation

DNA photolyases are repair enzymes which split (repair) UV-induced cyclobut ane DNA lesions. Critical steps in the light-driven repair reaction are the absorption of light by a deazaflavin or methenyl tetrahydrofolate cofactor and the transfer of the excitation energy to a reduced and deprotonated FA DH(-) cofactor, which initiates an electron transfer to the dimer lesion. A lthough most efficient energy transfer requires a close cofactor arrangemen t, there is a separation of > 17 Angstrom between the cofactors in photolya ses. To determine the effect of the large cofactor distance on the repair e fficiency, a systematic study with model compounds was performed. A series of compounds were synthesized which contain a model DNA lesion covalently c onnected to a flavin and a deazaflavin. While the flavin-dimer lesion dista nce was kept constant in all model compounds, the flavin-deazaflavin distan ce was incrementally increased. Investigation of the dimer cleavage efficie ncy shows that compounds with a large cofactor separation possess a low ene rgy-transfer efficiency but split the dimer most efficiently within a few m inutes. Model compounds with a close cofactor orientation feature a highly efficient energy transfer from the deazaflavin to the flavin. They are, how ever, unable to perform the repair of the dimer lesion. At very short cofac tor distances, the light-driven repair process is fully inhibited. This is explained by a competitive electron transfer between both cofactors, which hinders the electron transfer to the dimer lesion and hence the dimer split ting. The presented data suggest that the large cofactor separation (17 Ang strom) found in photolyases is a critical parameter that determines the DNA repair efficiency by photolyases.