Photolyasen

Photolyases are DNA repair enzymes that reverse ultraviolet light–induced lesions in DNA through a light-dependent process known as photoreactivation. They are flavoproteins that use flavin adenine dinucleotide (FAD) as the catalytic cofactor, and many also contain an antenna chromophore to harvest light energy and extend absorption into the blue region of the spectrum.

Mechanism and activity: When exposed to blue light, FAD is excited and enters a reduced signaling state. An electron is transferred through a chain of amino acids, typically tryptophan residues, to the DNA lesion. This electron transfer enables repair of cyclobutane pyrimidine dimers (CPDs) and, in some photolyases, (6-4) photoproducts, restoring the original DNA bases. After repair, the electron returns to FAD, and the enzyme returns to its resting state. Photoreactivation is strictly light-dependent and cannot occur in darkness.

Distribution and evolution: Photolyases are widespread in bacteria, archaea, plants, fungi, and many algae and invertebrates. They are absent or nonfunctional in placental mammals, including humans, which rely primarily on nucleotide excision repair pathways to fix UV-induced lesions. Photolyases belong to the cryptochrome/photolyase family and are categorized into CPD photolyases (repair CPDs) and (6-4) photolyases (repair (6-4) photoproducts). Some enzymes can repair both types, with varying efficiency.

Biological significance and applications: Photoreactivation provides a rapid, light-driven means to repair UV damage, complementing other DNA repair systems in organisms exposed to sunlight. Research and some commercial formulations explore photolyase activity for reducing UV-induced DNA damage in skin and other contexts, though effectiveness depends on light exposure and biological conditions.