Photolyase

Photolyase is an enzyme that binds complementary DNA strands and breaks certain types of pyrimidine dimers that are caused by exposure to ultraviolet light. Two kinds of pyrimidine dimers arise when a pair of thymine bases or cytosine bases on the same strand of DNA become covalently linked. These dimer result in a 'bulge' of the DNA structure, referred to as a lesion. The more common covalent linkage involves the formation of a cyclobutane bridge. Photolyase has a high affinity for these lesions and reversibly binds and converts them back to the original bases. This DNA repair enzyme mechanism requires visible light, preferentially from the violet/blue end of the spectrum. This repair process is 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. [Sancar A. (2003). Structure and function of DNA photolyase and cryptochrome blue-light photoreceptors. "Chem Rev" 103(6):2203-37. PMID 12797829 ]

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.

Some sunscreens include photolyase in their ingredients, claiming a reparative action on UV-damaged skin. [Dagmar Kulms, Birgit Pöppelmann, Daniel Yarosh, Thomas A. Luger, Jean Krutmann and Thomas Schwarz (1999). Nuclear and cell membrane effects contribute independently to the induction of apoptosis in human cells exposed to UVB radiation "PNAS" 96(14):7974-7979]

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