Variable Electron Transfer Pathways in an Amphibian Cryptochrome TRYPTOPHAN VERSUS TYROSINE-BASED RADICAL PAIRS

Till Biskup; Bernd Paulus; Asako Okafuji; Kenichi Hitomi; Elizabeth D. Getzoff; Stefan Weber; Erik Schleicher

doi:10.1074/jbc.M112.417725

Variable Electron Transfer Pathways in an Amphibian Cryptochrome

TRYPTOPHAN VERSUS TYROSINE-BASED RADICAL PAIRS*

From the ^‡Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ, United Kingdom,
the ^§Albert-Ludwigs-Universität Freiburg, Institute of Physical Chemistry, Albertstraße 21, 79104 Freiburg, Germany,
the ^‖Section of Laboratory Equipment, National Institute of Biomedical Innovation, 7-6-8, Saito-Asagi, Ibaraki, Osaka 567-0085, Japan, and
the ^¶Department of Molecular Biology and the Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, California 92037

↵3 To whom correspondence should be addressed. Tel.: 49-761-203-6204; Fax: 49-761-203-6222; E-mail: erik.schleicher{at}physchem.uni-freiburg.de.

↵1 Present address: Albert-Ludwigs-Universität Freiburg, Institute of Physical Chemistry, Albertstraße 21, 79104 Freiburg, Germany.
↵2 Present address: Molecular Informatics Corp., 1-1-14 Izumi-machi, Chuo-ku, Osaka 540-0019, Japan.

Background: Cryptochrome/photolyase proteins maintain a tryptophan electron transfer pathway allowing for efficient light-induced reduction of the FAD cofactor.

Results: When this canonical pathway is blocked, electron transfer in a frog cryptochrome occurs through a tyrosine radical, identified by EPR spectroscopy.

Conclusion: Alternative electron transfer pathways provide robust photochemistry in cryptochromes.

Significance: Proteins can preserve electron transfer functions through diverse compensatory pathways.

Abstract

Electron transfer reactions play vital roles in many biological processes. Very often the transfer of charge(s) proceeds stepwise over large distances involving several amino acid residues. By using time-resolved electron paramagnetic resonance and optical spectroscopy, we have studied the mechanism of light-induced reduction of the FAD cofactor of cryptochrome/photolyase family proteins. In this study, we demonstrate that electron abstraction from a nearby amino acid by the excited FAD triggers further electron transfer steps even if the conserved chain of three tryptophans, known to be an effective electron transfer pathway in these proteins, is blocked. Furthermore, we were able to characterize this secondary electron transfer pathway and identify the amino acid partner of the resulting flavin-amino acid radical pair as a tyrosine located at the protein surface. This alternative electron transfer pathway could explain why interrupting the conserved tryptophan triad does not necessarily alter photoreactions of cryptochromes in vivo. Taken together, our results demonstrate that light-induced electron transfer is a robust property of cryptochromes and more intricate than commonly anticipated.

Footnotes

↵* This work was supported, in whole or in part, by National Institutes of Health Grant GM37684 (to E. D. G.), Deutsche Forschungsgemeinschaft Grants BI 1249/1-1 and BI 1249/1-2 (to T. B.), and a grant from the Skaggs Institute for Chemical Biology (to K. H.).
↵ This article contains supplemental Figs. S1–S4 and Tables S1 and S2.