光裂合酶

光裂合酶（Photolyases，EC 4.1.99.3）为一种DNA修复酵素，专门修复遭受紫外线破坏的DNA。此酵素的作用需要可见光协助活化^[1]及DNA修复^[2]。光裂合酶修复DNA的过程称为光学活作用（photoreactivation）。

DNA光裂合酶
A deazaflavin photolyase from Anacystis nidulans, illustrating the two light-harvesting cofactors: FADH− (yellow) and 8-HDF (cyan).
鉴定
标志	FAD_binding_7
Pfam	PF03441（旧版）
InterPro（英语：InterPro）	IPR005101
PROSITE（英语：PROSITE）	PDOC00331
SCOP（英语：Structural Classification of Proteins）	1qnf / SUPFAM
现有可用的蛋白结构： Pfam 结构（旧版） / ECOD PDB RCSB PDB; PDBe; PDBj PDBsum（英语：PDBsum） 结构概要 PDB 1u3cA:214-492 1u3dA:214-492 1iquA:176-418 1iqrA:176-418 1dnpB:202-469 1tezD:207-472 1owmA:207-472 1qnf :207-472 1owpA:207-472 1owoA:207-472 1owlA:207-472 1ownA:207-472 1np7B:213-453

由系统发生学来看，光裂合酶是一个相当古老的酵素，从细菌、真菌^[3]，到动物体内都有^[4]，且在植物体内尤为重要。人类等胎盘哺乳类体内则并无此酵素活性，而是使用另一种较无效率的核苷酸切除修复法来取代光裂合酶^[5]。

DNA中相邻的胸腺嘧啶受到紫外线的照射时，会形成共价键，形成环丁烷桥，称为胸腺嘧啶二聚体。形成胸腺嘧啶二聚体的基因会导致基因在复制和转录时出现错误^[6]。光裂合酶会接在DNA上，切除胸腺嘧啶二聚体，并修复回原本的样子。

A UV radiation induced thymine-thymine cyclobutane dimer (right) is the type of DNA damage（英语：Direct DNA damage） which is repaired by DNA photolyase. Note: The above diagram is incorrectly labelled as thymine as the structures lack 5-methyl groups.

光裂合酶为一种黄素蛋白，内含两个捕光性辅因子。所有的光裂合酶都含有FADH⁻，并可根据他的第二辅因子分为两类。第一类为叶酸性光裂合酶，其第二辅因子的成分为次甲基四氢叶酸蝶呤（Methenyltetrahydrofolate，MTHF），称。第二类则为脱氮黄素（英语：deazaflavin）性光裂合酶，其第二辅因子为8-羟-7,8-二甲基-5-脱氮核黄素（8-hydroxy-7,8-didemethyl-5-deazariboflavin，8-HDF）。FAD本身即有酵素活性，第二辅因子则可在低光状态下加速反应进行。被光活化的光裂合酶会将FADH⁻的电子，转移给胸腺嘧啶二聚体来裂解二聚体^[7]。

除了第二辅因子之外，光裂合酶也可根据序列相似性分为两种，第一种主要在革兰氏阴性菌、阳性菌、嗜盐古菌、真菌，和植物体内。Proteins containing this domain also include Arabidopsis thaliana cryptochrome（英语：cryptochrome）s 1 and 2, which are blue light photoreceptors that mediate blue light-induced gene expression and modulation of circadian rhythms.

、可以将光裂合酶分为两类。The first class contains enzymes from Gram-negative and Gram-positive bacteria, the halophilic archaebacteria Halobacterium halobium,以及真菌与植物。阿拉伯芥的Cryptochrome（英语：Cryptochrome） 1与2亦含有此种结构域，作为促进蓝光基因表现与调整昼夜节律的光受体。

The second class are named cryptochrome (Cry), found in species as diverse as Drosophila, Arabidopsis, Synechocystis, and Human (Cry-DASH). These were previously assumed to have no DNA repair activity because of negligible activity on double-stranded DNA. A study^[4] 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 single-stranded DNA. Their study showed that VcCry1 from Vibrio cholerae, X1Cry from Xenopus laevis, and AtCry3 from Arabidopsis thaliana all had photolyase activity on UV irradiated ssDNA in vitro.

有些防晒油含有此酵素，宣称可以治愈紫外线晒伤^[8]。

人类体内含光裂合酶之蛋白

• CRY1
• CRY2（英语：CRY2）