title: Solid-state NMR spectroscopy to probe photoactivation in canonical phytochromes.
abstract: The photoreceptor phytochrome switches photochromically between two thermally stable states called Pr and Pfr. Here, we summarize recent solid-state magic-angle spinning (MAS) NMR work on this conversion process and interpret the functional mechanism in terms of a nano-machine. The process is initiated by a double-bond photoisomerization of the open-chain tetrapyrrole chromophore at the methine bridge connecting pyrrole rings C and D. The Pr-state chromophore and its surrounding pocket in canonical cyanobacterial and plant phytochromes has significantly less order, tends to form isoforms and is soft. Conversely, Pfr shows significantly harder chromophore-protein interactions, a well-defined protonic and charge distribution with a clear classical counterion for the positively charged tetrapyrrole system. The soft-to-hard/disorder-to-order transition involves the chromophore and its protein surroundings within a sphere of at least 5.5 Å. The relevance of this collective event for signaling is discussed. Measurement of the intermediates during the Pfr → Pr back-reaction provides insight into the well-adjusted mechanics of a two-step transformation. As both Pr → Pfr and Pfr → Pr reaction pathways are different in ground and excited states, a photochemically controlled hyper-landscape is proposed allowing for ratchet-type reaction dynamics regulating signaling activity.
authors.affiliation: Leids Instituut voor Chemisch Onderzoek, Universiteit Leiden, Leiden, The Netherlands.
authors.forename: Chen
authors.initials: C
authors.lastname: Song
authors.forename: Thierry
authors.initials: T
authors.lastname: Rohmer
authors.forename: Markus
authors.initials: M
authors.lastname: Tiersch
authors.forename: Jan
authors.initials: J
authors.lastname: Zaanen
authors.forename: Jon
authors.initials: J
authors.lastname: Hughes
authors.forename: Jörg
authors.initials: J
authors.lastname: Matysik
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D000458:Cyanobacteria / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D010834:Phytochrome / Q000737:chemistry*; D010940:Plant Proteins / Q000737:chemistry*; D010944:Plants / Q000737:chemistry; D011487:Protein Conformation; D020033:Protein Isoforms / Q000737:chemistry; D011522:Protons; D015398:Signal Transduction; D055672:Static Electricity; D045725:Tetrapyrroles / Q000737:chemistry*
chemical_list: D001426:Bacterial Proteins; D010940:Plant Proteins; D020033:Protein Isoforms; D011522:Protons; D045725:Tetrapyrroles; D010834:Phytochrome