title: Electronic structural flexibility of heterobimetallic Mn/Fe cofactors: R2lox and R2c proteins.
abstract: The electronic structure of the Mn/Fe cofactor identified in a new class of oxidases (R2lox) described by Andersson and Högbom [Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 5633] is reported. The R2lox protein is homologous to the small subunit of class Ic ribonucleotide reductase (R2c) but has a completely different in vivo function. Using multifrequency EPR and related pulse techniques, it is shown that the cofactor of R2lox represents an antiferromagnetically coupled Mn(III)/Fe(III) dimer linked by a μ-hydroxo/bis-μ-carboxylato bridging network. The Mn(III) ion is coordinated by a single water ligand. The R2lox cofactor is photoactive, converting into a second form (R2loxPhoto) upon visible illumination at cryogenic temperatures (77 K) that completely decays upon warming. This second, unstable form of the cofactor more closely resembles the Mn(III)/Fe(III) cofactor seen in R2c. It is shown that the two forms of the R2lox cofactor differ primarily in terms of the local site geometry and electronic state of the Mn(III) ion, as best evidenced by a reorientation of its unique (55)Mn hyperfine axis. Analysis of the metal hyperfine tensors in combination with density functional theory (DFT) calculations suggests that this change is triggered by deprotonation of the μ-hydroxo bridge. These results have important consequences for the mixed-metal R2c cofactor and the divergent chemistry R2lox and R2c perform.
authors.affiliation: Max-Planck-Institut für Chemische Energiekonversion , Stiftstrasse 34-36, Mülheim an der Ruhr D-45470, Germany.
authors.forename: Hannah S
authors.initials: HS
authors.lastname: Shafaat
authors.forename: Julia J
authors.initials: JJ
authors.lastname: Griese
authors.forename: Dimitrios A
authors.initials: DA
authors.lastname: Pantazis
authors.forename: Katarina
authors.initials: K
authors.lastname: Roos
authors.forename: Charlotta S
authors.initials: CS
authors.lastname: Andersson
authors.forename: Ana
authors.initials: A
authors.lastname: Popović-Bijelić
authors.forename: Astrid
authors.initials: A
authors.lastname: Gräslund
authors.forename: Per E M
authors.initials: PE
authors.lastname: Siegbahn
authors.forename: Frank
authors.initials: F
authors.lastname: Neese
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Lubitz
authors.forename: Martin
authors.initials: M
authors.lastname: Högbom
authors.forename: Nicholas
authors.initials: N
authors.lastname: Cox
mesh_terms: D002692:Chlamydia trachomatis / Q000737:chemistry / Q000201:enzymology*; D004578:Electron Spin Resonance Spectroscopy; D056506:Geobacillus / Q000737:chemistry / Q000201:enzymology*; D007501:Iron / Q000737:chemistry; D008345:Manganese / Q000737:chemistry; D008958:Models, Molecular; D009169:Mycobacterium tuberculosis / Q000737:chemistry / Q000201:enzymology*; D010088:Oxidoreductases / Q000737:chemistry*; D055668:Photochemical Processes; D011789:Quantum Theory; D012264:Ribonucleotide Reductases / Q000737:chemistry*
chemical_list: D008345:Manganese; D007501:Iron; D010088:Oxidoreductases; D012264:Ribonucleotide Reductases