Practice question

DNA photolyase contains light-harvesting cofactors such as methenyltetrahydrofolate and flavin adenine dinucleotide in reduced FADH- form. Enzyme flips out thymine dimer into active pocket where FADH- absorbs photons in blue region 350-450 nanometer. Upon photon absorption, excited FADH- donates electron to dimer splitting cyclobutane ring via radical mechanism returning dimers to monomeric thymines and restoring FAD radical which recaptures electron completing catalytic cycle. Absorption maximum around 384 nanometer and blue light requirement distinguishes photolyase from UV-C which itself creates dimers, explaining why sunlight can both damage and provide substrate for repair.