1-Deazariboflavin, reduced forms

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. ... 

The central ring of 1-deazaflavins remains a pyrazine in X, a di-hydropyrazine in the two-electron-reduced form, XI, and continues to dominate the chemistry with oxygen. Like the parent riboflavins, and unlike the 5-deazaflavins, the dihydro- 1-deaza system, XI, is reoxidized by 02 in a fraction of a second in air-saturated solutions (Table II) the semiquinone is accessible and 1-deazaFAD enzymes show full catalytic competence with flavoprotein dehydrogenases and oxidases (24). Turnover numbers vary from about 1% to 100% that of cognate FAD-enzymes but this variation reflects the -280 mV vs. —200 mV E° values, respectively, for 1-deazariboflavin vs. riboflavin. The redox steps may or may not limit Vmax with a given enzyme (15, 24). 

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... 

A flash photolysis method has been developed that prepares the MoVI-Fe11 state and thus allows the rate constants k3 and k 3 to be measured. Solutions containing 5-deazariboflavin, semicarbazide, and sulfite oxidase are subjected to 555 nm flash photolysis. The deazariboflavin is excited to a triplet state, which is then reduced by semicarbazide to form the 5-deazariboflavin semiquinone radical. This radical is then rapidly oxidized back to its parent species through the one-electron reduction of sulfite oxidase. 

Based on the known chemistry of flavin photolysis reactions, it appears unlikely that thymine dimer cleavage occurs via a direct energy transfer mechanism (160). One proposal suggests that in the model reaction with 1-deazariboflavin, the thymine dimer radical anion is formed via electron donation from the excited sensitizer (164). Alternatively, electron abstraction by the excited flavin could occur, resulting in the thymine dimer radical cation (159, 160), although it is unlikely that reduced flavin would act as an electron acceptor. A schematic for this mechanism is illustrated in Scheme 33, where the initial formation of a sensitizer-dimer complex is consistent with the observed saturation kinetics. The complex is activated by excitation of the ionized sensitizer (pH > 7), and electron donation to the dimer forms the dimer radical anion and the zwitterionic, neutral 1-deazariboflavin radical (162). Thymine dimer radical would spontane-... 

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