   4.1.99.3 | cyclobutadipyrimidine (in DNA) = 2 pyrimidine residues (in DNA) |
upon absorption of a visible photon, the photoexcited flavin radical FADH abstracts an electron from nearby W382. The tryptophanyl radical W382 thus formed abstracts an electron from the nearby middle tryptophan, W359. W359 radical abstracts an electron from W306. As the second and third steps are faster than the first one, the intermediate states are not populated in wild-type photolyase to any significant extent and escape spectroscopic detection. The described forward electron transfer steps between the tryptophans are in competition with back electron transfer from FADH- to the respective tryptophanyl cation radical, so that the quantum yield of formation of the terminal W306 radical is only 20%. W306 radical releases a proton to the aqueous phase in 200 ns. The resulting neutral radical W306 radical can be reduced by extrinsic reductants, leaving a photolyase that contains FADH as required for photorepair of DNA |
713768 |
