(6-4)DNA photolyase

(6-4)DNA photolyase is an enzyme that repairs DNA damage caused by ultraviolet (UV) radiation, specifically cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts. While CPD photolyases are more common and repair CPDs formed between adjacent pyrimidines on the same DNA strand, (6-4)DNA photolyases specifically target (6-4) photoproducts. These (6-4) photoproducts are formed between the carbon 6 of one pyrimidine and carbon 4 of the adjacent pyrimidine on the same DNA strand, forming a covalent bond.

(6-4)DNA photolyases are flavoproteins that contain a flavin adenine dinucleotide (FAD) cofactor as their chromophore. This FAD cofactor absorbs light energy in the UV-A/blue light spectrum. The enzyme binds to the DNA at the site of the (6-4) photoproduct, and the absorbed light energy is used to break the bond linking the two pyrimidines. This restores the original pyrimidines and repairs the DNA damage.

The reaction mechanism involves the photoactivation of the FAD cofactor. Once activated, the FAD cofactor transfers an electron to the (6-4) photoproduct, causing it to break apart into its original pyrimidine bases. The electron is then transferred back to the FAD cofactor, restoring it to its original state.

(6-4)DNA photolyases are found in a wide variety of organisms, including bacteria, archaea, fungi, plants, and animals. They are an important mechanism for repairing DNA damage caused by exposure to sunlight and preventing mutations and cancer. The enzyme's activity is dependent on the presence of light, making it a light-dependent DNA repair system. In many organisms, the expression of (6-4)DNA photolyase is upregulated in response to UV radiation, further enhancing their capacity to repair damaged DNA.