Multiple Retinal Isomerizations during the Early Phase of the Bestrhodopsin Photoreaction
Spyridon Kaziannis 1,2, Matthias Broser3, Ivo H.M. van Stokkum4, Jakub Dostal1, Wayne Busse3, Arno Munhoven3, Cesar Bernardo1, Miroslav Kloz1, Peter Hegemann3*, John T.M. Kennis4*
1ELI-Beamlines, Institute of Physics, Na Slovance 2, 182 21 Praha 8,
Czech Republic
2Department of Physics, University of Ioannina, Ioannina, Gr-45110
2Institut für Biologie, Experimentelle Biophysik, Humboldt Universität zu Berlin, Invalidenstrasse 42, D-10115 Berlin, Germany
3Department of Physics and Astronomy, Vrije Universiteit Amsterdam, Amsterdam 1081 HV, De Boelelaan 1081, The Netherlands
*corresponding authors, j.t.m.kennis@vu.nl, hegemann@rz.hu-berlin.de
Bestrhodopsins constitute a class of light-regulated pentameric ion channels that consist of one or two rhodopsins in tandem fused with bestrophin ion channel domains. Here, we report on the isomerization dynamics in the rhodopsin tandem domains of Phaeocystis antarctica bestrhodopsin, which binds all-trans retinal via a Schiff-base (RSB) absorbing at 661 nm and, upon illumination, converts to the metastable P540 state with an unusual 11-cis RSB isomer. The primary photoproduct P682 corresponds to a mixture of highly distorted 11-cis and 13-cis RSB isomers directly formed from the excited state in 1.4 ps. P673 evolves from P682 in 500 ps and contains exclusively highly distorted 13-cis retinal, indicating that the 11-cis fraction in P682 has converted to 13-cis RSB. Next, P673 establishes an equilibrium with P595 in 1.2 ms, during which RSB converts to 11-cis in the latter species and then further proceeds to P560 in 48 ms and P540 in 0.8 ms while remaining 11-cis. Hence, extensive isomeric switching occurs on the early ground state potential energy surface (PES) on the hundreds of ps to ms timescale before finally settling on a metastable 11-cis photoproduct. We propose that the early photoproducts P682 and P673 are trapped high up on the ground-state PES after passing through either of two closely located conical intersections that result in 11-cis and 13-cis RSB. Co-rotation of C11=C12 and C13=C14 bonds results in a constricted conformational landscape that allows thermal switching between 11-cis and 13-cis species of highly strained RSB chromophores. Protein relaxation may release RSB strain, allowing it to evolve to a stable 11-cis isomeric configuration within microseconds.
