BB47CH14_Hoff ARI 7 March 2018 13:38 Annual Review of Biophysics Photoreceptors Take Charge: Emerging Principles for Light Sensing Tilman Kottke,1 Aihua Xie,2 Delmar S. Larsen,3 and Wouter D. Hoff 4,5 1Department of Chemistry, Bielefeld University, 33615 Bielefeld, Germany; email:
[email protected] 2Department of Physics, Oklahoma State University, Stillwater, Oklahoma 74078, USA; email:
[email protected] 3Department of Chemistry, University of California, Davis, California 95616, USA; email:
[email protected] 4Department of Microbiology and Molecular Genetics, Oklahoma State University, Stillwater, Oklahoma 74078, USA; email:
[email protected] 5Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078, USA Annu. Rev. Biophys. 2018. 47:14.1–14.23 Keywords The Annual Review of Biophysics is online at chromophore, electrostatic epicenter, isomerization, proton-coupled biophys.annualreviews.org electron transfer, proton transfer, signaling state https://doi.org/10.1146/annurev-biophys- 070317-033047 Abstract Copyright c 2018 by Annual Reviews. The first stage in biological signaling is based on changes in the functional All rights reserved state of a receptor protein triggered by interaction of the receptor with its ligand(s). The light-triggered nature of photoreceptors allows studies on the mechanism of such changes in receptor proteins using a wide range of biophysical methods and with superb time resolution. Here, we critically evaluate current understanding of proton and electron transfer in photosen- sory proteins and their involvement both in primary photochemistry and subsequent processes that lead to the formation of the signaling state. An insight emerging from multiple families of photoreceptors is that ultrafast primary photochemistry is followed by slower proton transfer steps that con- tribute to triggering large protein conformational changes during signaling state formation.