- Photolyase
Photolyase is an
enzyme that binds complementary DNA strands and breaks certain types ofpyrimidine dimers that are caused by exposure toultraviolet light. Two kinds ofpyrimidine dimers arise when a pair ofthymine bases orcytosine 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 acyclobutane bridge. Photolyase has a high affinity for these lesions and reversibly binds and converts them back to the original bases. ThisDNA repair enzyme mechanism requiresvisible light , preferentially from the violet/blue end of the spectrum. This repair process is known as "photoreactivation."Photolyases are
flavoprotein s and contain two light-harvestingcofactor s. All photolyases contain the two-electron-reduced FADH2; they are divided into two main classes based on the second cofactor, which may be either thepterin methenyltetrahydrofolate (MTHF) in "folate photolyases" or thedeazaflavin 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 byelectron 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 lowereukaryotes (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 calledcryptochrome that is involved in light-sensitive regulatory activities such as modulatingcircadian rhythm s.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]
References
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