Photolyasecryptochromes

Photolyase and cryptochromes are a fascinating class of flavoproteins found in a wide range of organisms, from bacteria to plants and animals. They share a common evolutionary origin and are characterized by the presence of a flavin adenine dinucleotide (FAD) cofactor, and in many cases, a light-harvesting cofactor like 5,10-methenyltetrahydrofolate or 8-hydroxy-7,8-didemethyl-5-deazaflavin.

Photolyases are primarily known for their role in DNA repair. Specifically, they catalyze the direct photoreversal of UV-induced DNA lesions, such as cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts. Upon absorbing light energy, the photolyase molecule transfers an electron to the damaged DNA site, effectively breaking the covalent bonds that form the lesion and restoring the DNA to its original undamaged state. This process is crucial for preventing mutations and cell death caused by UV radiation.

Cryptochromes, on the other hand, are involved in a broader range of light-dependent physiological processes. While they also absorb blue and UV-A light, their functions are more diverse. In plants, cryptochromes are key regulators of photomorphogenesis, influencing processes like seed germination, stem elongation, and flowering time. In animals, including humans, cryptochromes play a critical role in the circadian clock, the internal biological clock that regulates daily rhythms of sleep, wakefulness, and other physiological functions. They are believed to be light sensors that entrain the circadian system to the environmental light-dark cycle. The precise mechanisms by which cryptochromes mediate these diverse signaling pathways are still areas of active research, but they are known to interact with other proteins to transmit light signals throughout the cell.