Hydrogen bond switching among flavin and amino acid side chains in the BLUF photoreceptor observed by ultrafast infrared spectroscopy.
BLUF domains constitute a recently discovered class of photoreceptor proteins found in bacteria and eukaryotic algae. BLUF domains are blue-light sensitive through a FAD cofactor that is involved in an extensive hydrogen-bond network with nearby amino acid side chains, including a highly conserved tyrosine and glutamine. The participation of particular amino acid side chains in the ultrafast hydrogen-bond switching reaction with FAD that underlies photoactivation of BLUF domains is assessed by means of ultrafast infrared spectroscopy. Blue-light absorption by FAD results in formation of FAD(*-) and a bleach of the tyrosine ring vibrational mode on a picosecond timescale, showing that electron transfer from tyrosine to FAD constitutes the primary photochemistry. This interpretation is supported by the absence of a kinetic isotope effect on the fluorescence decay on H/D exchange. Subsequent protonation of FAD(*-) to result in FADH(*) on a picosecond timescale is evidenced by the appearance of a N-H bending mode at the FAD N5 protonation site and of a FADH(*) C=N stretch marker mode, with tyrosine as the likely proton donor. FADH(*) is reoxidized in 67 ps (180 ps in D(2)O) to result in a long-lived hydrogen-bond switched network around FAD. This hydrogen-bond switch shows infrared signatures from the C-OH stretch of tyrosine and the FAD C4=O and C=N stretches, which indicate increased hydrogen-bond strength at all these sites. The results support a previously hypothesized rotation of glutamine by approximately 180 degrees through a light-driven radical-pair mechanism as the determinant of the hydrogen-bond switch.
Bonetti
Cosimo
C
Biophysics Group, Department of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit, Amsterdam, The Netherlands.
Mathes
Tilo
T
van Stokkum
Ivo H M
IH
Mullen
Katharine M
KM
Groot
Marie-Louise
ML
van Grondelle
Rienk
R
Hegemann
Peter
P
Kennis
John T M
JT
D000596:Amino Acids / Q000737:chemistry* / Q000528:radiation effects; D001665:Binding Sites; D003198:Computer Simulation; D005415:Flavins / Q000737:chemistry* / Q000528:radiation effects; D006860:Hydrogen Bonding / Q000528:radiation effects; D008027:Light; D055537:Light Signal Transduction; D008956:Models, Chemical; D020283:Photoreceptors, Microbial / Q000737:chemistry* / Q000528:radiation effects; D011485:Protein Binding; D017434:Protein Structure, Tertiary / Q000528:radiation effects; D013055:Spectrophotometry, Infrared / Q000379:methods
D000596:Amino Acids; D005415:Flavins; D020283:Photoreceptors, Microbial