title: Evidence for Tautomerisation of Glutamine in BLUF Blue Light Receptors by Vibrational Spectroscopy and Computational Chemistry.
abstract: BLUF (blue light sensor using flavin) domains regulate the activity of various enzymatic effector domains in bacteria and euglenids. BLUF features a unique photoactivation through restructuring of the hydrogen-bonding network as opposed to a redox reaction or an isomerization of the chromophore. A conserved glutamine residue close to the flavin chromophore plays a central role in the light response, but the underlying modification is still unclear. We labelled this glutamine with (15)N in two representative BLUF domains and performed time-resolved infrared double difference spectroscopy. The assignment of the signals was conducted by extensive quantum chemical calculations on large models with 187 atoms reproducing the UV-vis and infrared signatures of BLUF photoactivation. In the dark state, the comparatively low frequency of 1,667 cm(-1) is assigned to the glutamine C=O accepting a hydrogen bond from tyrosine. In the light state, the signature of a tautomerised glutamine was extracted with the C=N stretch at ~1,691 cm(-1) exhibiting the characteristic strong downshift by (15)N labelling. Moreover, an indirect isotope effect on the flavin C4=O stretch was found. We conclude that photoactivation of the BLUF receptor does not only involve a rearrangement of hydrogen bonds but includes a change in covalent bonds of the protein.
authors.affiliation: Department of Biomolecular Mechanisms, Max Planck Institute for Medical Research, Jahnstraße 29, 69120, Heidelberg, Germany.
authors.forename: Tatiana
authors.initials: T
authors.lastname: Domratcheva
authors.affiliation: Department of Biomolecular Mechanisms, Max Planck Institute for Medical Research, Jahnstraße 29, 69120, Heidelberg, Germany.
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Hartmann
authors.affiliation: Department of Biomolecular Mechanisms, Max Planck Institute for Medical Research, Jahnstraße 29, 69120, Heidelberg, Germany.
authors.forename: Ilme
authors.initials: I
authors.lastname: Schlichting
authors.affiliation: Physical and Biophysical Chemistry, Department of Chemistry, Bielefeld University, Universitäts straße 25, 33615 Bielefeld, Germany.
authors.forename: Tilman
authors.initials: T
authors.lastname: Kottke
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism / Q000528:radiation effects*; D005973:Glutamine / Q000737:chemistry* / Q000378:metabolism; D006860:Hydrogen Bonding; D007536:Isomerism; D008027:Light; D056004:Molecular Dynamics Simulation; D017434:Protein Structure, Tertiary; D015800:Protozoan Proteins / Q000737:chemistry* / Q000378:metabolism / Q000528:radiation effects*; D013057:Spectrum Analysis
chemical_list: D001426:Bacterial Proteins; D015800:Protozoan Proteins; D005973:Glutamine