Photolyase

From Wikipedia, the free encyclopedia

A deazaflavin photolyase from Anacystis nidulans, illustrating the two light-harvesting cofactors: FADH₂ (yellow) and 8-HDF (cyan).

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.