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Excited flavin and pterin molecules are active in intermolecular energy transfer and in photocatalysis of redox reactions resulting in conservation of free energy. Flavin-containing pigments produced in models of the prebiotic environment are capable of converting photon energy into the energy of phosphoanhydride bonds of ATP. However, during evolution photochemical reactions involving excited FMN or FAD molecules failed to become participants of bioenergy transfer systems, but they appear in enzymes responsible for repair of UV-damaged DNA (DNA photolyases) and also in receptors of blue and UV-A light regulating vital functions of organisms. The families of these photoproteins (DNA-photolyases and cryptochromes, LOV-domain- and BLUF-domain-containing proteins) are different in the structure and in mechanisms of the photoprocesses. The excited flavin molecules are involved in photochemical processes in reaction centers of these photoproteins. In DNA photolyases and cryptochromes the excitation energy on the reaction center flavin is supplied from an antenna molecule that is bound with the same polypeptide. The role of antenna is played by MTHF or by 8-HDF in some DNA photolyases, i.e. also by molecules with known coenzyme functions in biocatalysis. Differences in the structure of chromophore-binding domains suggest an independent origin of the photoprotein families. The analysis of structure and properties of coenzyme molecules reveals some specific features that were significant in evolution for their being selected as chromophores in these proteins.

Tetrahydrobiopterin (H4 Bip) is a cofactor for several key enzymes, including NO synthases and aromatic amino acid hydroxylases (AAHs). Normal functioning of the H4 Bip regeneration cycle is extremely important for the work of AAHs. Oxidized pterins may accumulate if the H4 Bip regeneration cycle is disrupted or if H4 Bip autoxidation occurs. These oxidized pterins can photosensitize the production of singlet molecular oxygen (1)O2 and thus cause oxidative stress. In this context, we studied the photooxidation of H4 Bip in phosphate buffer at pH 7.2. We found that UV irradiation of H4 Bip affected its oxidation rate (quantum yield Φ300 = (2.7 ± 0.4) × 10(-3)). The effect of UV irradiation at λ = 350 nm on H4 Bip oxidation was stronger, especially in the presence of biopterin (Bip) (Φ350 = (9.7 ± 1.5) × 10(-3)). We showed that the rate of H4 Bip oxidation linearly depends on Bip concentration. Experiments with KI, a selective quencher of triplet pterins at micromolar concentrations, demonstrated that the oxidation is sensitized by the triplet state biopterin (3) Bip. Apparently, electron transfer sensitization (Type-I mechanism) is dominant. Energy transfer (Type-II mechanism) and singlet oxygen generation play only a secondary role. The mechanisms of H4 Bip photooxidation and their biological meaning are discussed.

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