6-4 Photolyase catalysis

To elucidate further the possible role of flavin as a sensitizer in photolyase catalysis, Joms conducted a study on the light-catalyzed monomerization of thymine dimers by reduced flavins and flavin analogs as models for the DNA-PL reaction of E. coli and yeast (162). The reduced forms of 1-deazariboflavin, A(3)-methyllumiflavin, 7,8-dimethyl-1,10-ethyleneisoalloxazinium perchlorate, and 5-deazariboflavin were generated anaerobically with excess dithionite. Of these four flavins, cleavage of cij-syn-[me//iy/- H]thymine dimer to [methyl- H]-thymine was observed with reduced 1-deazariboflavin, and to a lesser extent with... 

Flavins — Riboflavin is first of all essential as a vitamin for humans and animals. FAD and FMN are coenzymes for more than 150 enzymes. Most of them catalyze redox processes involving transfers of one or two electrons. In addition to these well known and documented functions, FAD is a co-factor of photolyases, enzymes that repair UV-induced lesions of DNA, acting as photoreactivating enzymes that use the blue light as an energy source to initiate the reaction. The active form of FAD in photolyases is their two-electron reduced form, and it is essential for binding to DNA and for catalysis. Photolyases contain a second co-factor, either 8-hydroxy-7,8-didemethyl-5-deazariboflavin or methenyltetrahydrofolate. ... 

Weber S. Light-driven enzymatic catalysis of DNA repair a review of recent biophysical studies on photolyase. Biochim Biophys Acta 2005 1707 1-23. 

Figure 1 Reaction mechanism of DNA photolyases (A) mechanism of cyclobutane photolyase and (B) mechanism of (6-4) photolyase. Both photolyases harness blue light energy to remove UV-induced damage and contain two noncovalently bound chromophores. They bind UV-damaged DNA in a reaction that is light independent and carry out catalysis in a light-initiated cyclic electron transfer. In (6-4) photolyase, the (6-4) photoproduct is converted to a four-membered oxetane ring thermally (kT) before the photochemical reaction.

Catalysis by (6—4) photolyase must accomplish two chemical tasks cleavage of the C6—C4 sig a bond, and transfer of the OH (or —NH2) group from the C5 of the 5 base to the G4 of the 3 base. Because formation of the (6—4) photoproduct is presumed to proceed through a four-mem-bered oxetane or azetidine intermediate, it has been proposed that (6—4) photolyase first converts the open form of the (6—4) photoproduct to the four-membered ring by a thermal reaction, and then the four-mem-bered ring is cleaved by retro [2+2] reaction photochemically (Kim et al, 1994 Zhao et aL, 1997). A site-directed mutagenesis study has identified two histidine residues in the active site that may participate in conversion of the (6-4) photoproduct to the oxetane intermediate by general acid-base catalysis (Hitomi et al, 2001). A current model for catalysis by (6-4) photolyase is as follows (Fig. 8) The enzyme binds DNA and flips out the... 

Fig. 8. Reaction mechanism of (6-4) photolyase. The enzyme binds to DNA containing a (6-4) photoproduct and flips out the dinucleotide adduct into the active site cavity, where the open form of the photoproduct is converted to the oxetane intermediate by a light-independent general acid-base mechanism. Catalysis is initiated by light MTHF absorbs a photon and transfers energy to FADH , which then transfers an electron to the oxetane intermediate bond rearrangement in the oxetane radical regenerates two canonical pyrimidines, and back-electron transfer restores the flavin radical to catalytically competent FADH form. The repaired dipyrimidine flips back into the DNA duplex, and the enzyme is dissociated from the substrate.

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