光裂合酶 光裂合酶PhotolyasesEC 為一種DNA修復酵素專門修復遭受紫外線破壞的DNA此酵素的作用需要可見光協助活化及DNA修復光裂合酶修復DNA的過程稱為光學活作用photoreactivation 由系統發生學來看光裂合酶是一個相當古老的酵素從細菌真菌到動物體內都有且在植物體內尤為重要人類等胎盤哺乳類體內則並無此酵素活性而是使用另一種較無效率的核苷酸切除修復法來取代光裂合酶 DNA中相鄰的胸腺嘧啶受到紫外線的照射時會形成共價鍵形成環丁烷橋稱為胸腺嘧啶二聚體形成胸腺嘧啶二聚體的基因會導致基因在複製和轉錄時出現錯誤光裂合酶會接在DNA上切除胸腺嘧啶二聚體並修復回原本的樣子 光裂合酶為一種黄素蛋白內含兩個捕光性辅因子所有的光裂合酶都含有FADH並可根據他的第二輔因子分為兩類第一類為葉酸性光裂合酶其第二輔因子的成分為次甲基四氫葉酸蝶呤MethenyltetrahydrofolateMTHF稱第二類則為脫氮黃素英语deazaflavin性光裂合酶其第二輔因子為羥二甲基脫氮核黃素hydroxydidemethyldeazariboflavinHDFFAD本身即有酵素活性第二輔因子則可在低光狀態下加速反應進行被光活化的光裂合酶會將FADH的電子轉移給胸腺嘧啶二聚體來裂解二聚體 除了第二輔因子之外光裂合酶也可根據序列相似性分為兩種第一種主要在革蘭氏陰性菌陽性菌嗜鹽古菌真菌和植物體內Proteins containing this domain also include Arabidopsis thaliana cryptochrome英语cryptochromes  and  which are blue light photoreceptors that mediate blue lightinduced gene expression and modulation of circadian rhythms 可以將光裂合酶分為兩類The first class contains enzymes from Gramnegative and Grampositive bacteria the halophilic archaebacteria Halobacterium halobium以及真菌與植物阿拉伯芥的Cryptochrome英语Cryptochrome 與亦含有此種結構域作為促進藍光基因表現與調整昼夜节律的光受體 The second class are named cryptochrome Cry found in species as diverse as Drosophila Arabidopsis Synechocystis and Human CryDASH These were previously assumed to have no DNA repair activity because of negligible activity on doublestranded DNA A study by A Sancar and P Selby provided evidence to suggest this branch of cryptochromes have photolyase activity with a high degree of specificity for cyclobutane pyrimidine dimers in singlestranded DNA Their study showed that VcCry from Vibrio cholerae XCry from Xenopus laevis and AtCry from Arabidopsis thaliana all had photolyase activity on UV irradiated ssDNA in vitro 有些防曬油含有此酵素宣稱可以治癒紫外線曬傷