Intraprotein radical transfer during photoactivation of DNA photolyase

Amino-acid radicals play key roles in many enzymatic reactions(1). Catalysi s often involves transfer of a radical character within the protein, as in class I ribonucleotide reductase where radical transfer occurs over 35 Angs trom, from a tyrosyl radical to a cysteine(1-3). It is currently debated wh ether this kind of long-range transfer occurs by electron transfer, followe d by proton release to create a neutral radical, or by H-atom transfer, tha t is, simultaneous transfer of electrons and protons(4-7), The latter mecha nism avoids the energetic cost of charge formation in the low dielectric pr otein(4,5), but it is less robust to structural changes than is electron tr ansfer(7). Available experimental data do not clearly discriminate between these proposals. We have studied the mechanism of photoactivation (light-in duced reduction of the flavin adenine dinucleotide cofactor) of Escherichia coli DNA photolyase(8-10) using time-resolved absorption spectroscopy. Her e we show that the excited flavin adenine dinucleotide radical abstracts an electron from a nearby tryptophan in 30 ps. After subsequent electron tran sfer along a chain of three tryptophans, the most remote tryptophan (as a c ation radical) releases a proton to the solvent in about 300 ns, showing th at electron transfer occurs before proton dissociation. A similar process m ay take place in photolyase-like blue-light receptors.