Photolyases DNA repair enzymes

Photolyases  DNA repair enzymes 

That repair damage caused by exposure to ultraviolet light. This enzyme mechanism requires visible light, preferentially from the violet/blue end of the spectrum, and is known as photoreactivation.

Photolyase is a phylogenetically old enzyme which is present and functional in many species, from the bacteria to the fungi to the animals. However it is no longer working in humans and other placental mammals who instead rely on the less efficient nucleotide excision repair mechanism.

Photolyases bind complementary DNA strands and break certain types of pyrimidine dimers that arise when a pair of thymine or cytosine bases on the same strand of DNA become covalently linked. These dimers result in a 'bulge' of the DNA structure, referred to as a lesion. The more common covalent linkage involves the formation of a cyclobutane bridge. Photolyases have a high affinity for these lesions and reversibly bind and convert them back 

A UV radiation induced thymine-thymine cyclobutane dimer (right) is the type of DNA damage which is undone by photolyase.

Photolyases are flavoproteins and contain two light-harvesting cofactors. All photolyases contain the two-electron-reduced FADH^-; they are divided into two main classes based on the second cofactor, which may be either the pterinmethenyltetrahydrofolate (MTHF) in folate photolyases or the deazaflavin 8-hydroxy-7,8-didemethyl-5-deazariboflavin (8-HDF) indeazaflavin photolyases. Although only FAD is required for catalytic activity, the second cofactor significantly accelerates reaction rate in low-light conditions. The enzyme acts by electron transfer in which the reduced flavin FADH^- is activated by light energy and acts as an electron donor to break the pyrimidine dimer

On the basis of sequence similarities DNA photolyases can be grouped into two classes. The first class contains enzymes from Gram-negative and Gram-positive bacteria, the halophilic archaebacteria Halobacterium halobium, fungi and plants. Proteins containing this domain also include Arabidopsis thaliana cryptochromes 1 and 2, which are blue light photoreceptors that mediate blue light-induced gene expression and modulation ofcircadian rhythms.