Photolyase

Photolyase is an enzyme that binds complementary DNA strands and breaks pyrimidine dimers that are typically caused by exposure to ultraviolet light. Pyrimidine dimers occur when a pair of thymine bases or cytosine bases on the same strand of DNA bind together, resulting in a 'bulge' of the DNA structure referred to as a lesion. Photolyase has a high affinity for these lesions in the DNA and will reversibly bind and split the dimer using light-energy. This enzyme only functions as a DNA repair mechanism when visible light is available (preferentially from the violet/blue end of the spectrum). This process is also known as photoreactivation.

Photolyases are flavoproteins and contain two light-harvesting cofactors. All photolyases contain the two-electron-reduced FADH2; they are divided into two main classes based on the second cofactor, which may be either the pterin methenyltetrahydrofolate (MTHF) in folate photolyases or the deazaflavin 8-hydroxy-7,8-didemethyl-5-deazariboflavin (8-HDF) in deazaflavin photolyases. Although only FAD is required for catalytic activity, the second cofactor significantly accelerates reaction rate in low-light conditions. The enzyme acts by electron transfer in which the reduced flavin FADH2 acts as an electron donor to break the pyrimidine dimer.

Photolyase is present and functional in prokaryotes, is present in lower eukaryotes (as yeast) where it is thought to have a minor role, and it has not been found in human cells. However, many higher eukaryotes, including humans, possess a homologous protein called cryptochrome that is involved in light-sensitive regulatory activities such as modulating circadian rhythms.