Possible role of essential tyrosine on the protonation of isoalloxazine ring in the excited singlet state and the triplet state in photolyase from Escherichia coli. Molecular orbital study

Photolyases from Escherichia coli. (PLEC) function to repair UV-damaged DNA. Photolyases and cryptochromes (altogether 42 proteins) are found to contain at least one tyrosine (Tyr) residue near the isoalloxazine ring (Iso) of flavin adenine dinucleotide (FAD) within 1.2 nm, Tyr281 in PLEC. Iso in PLEC is surrounded by four Trp residues and one Tyr within 1.2 nm from Iso. Photoinduced charge transfer interactions (PCT) among these chromophores in the Iso-Tyr281-Trp277-Trp338-Trp382-Trp384 system were studied by means of a semiempirical molecular orbital (MO) method. Structure of the system was extracted from crystal structure of PLEC. In the ground state the PCT interactions between any pairs were negligible. Upon photo-excitation almost one electron transferred from Trp382 to the excited Iso, to which an electron transport chain from Tyr residues at the protein - water interface to Iso is connected. Protonation at N1 or N5 of Iso from Tyr281 markedly stabilized the transition energy in the excited singlet state and triplet state, depending on the polarity of the medium, but not from a source other than Tyr281. In these states, nearly one electron migrates from Tyr281 to Trp382. Effects of the protonation from Tyr281 to the fully reduced Iso were also examined, where total charge of the system was -2. In the ground state the energy of the system was stabilized by the protonation, when the proton was bound to N5 of Iso, but not to N1. In the excited singlet state and triplet state of the fully reduced Iso, the transition energies of the system were also stabilized upon the protonation to N1 or N5 of Iso from Tyr281, but not from a source other than Tyr281. In the ground state almost two negative charges were located at Iso. In the excited singlet state one of two negative charges was mostly localized to Iso and another to Trp277 at the dielectric constant, 3, and to Trp384 at the dielectric constant, 10. In the triplet state without the protonation, one of two negative charges was localized to Iso and another to Trp384 at any dielectric constant. Upon protonation to N1 or N5 of Iso in the triplet state, one of the negative charges was localized to Iso and another to Tyr281 at any value of dielectric constant. Thus Tyr281 closely interacted with Iso in the excited singlet state and triplet state, and other Trp residues in the system through the proton transfer.