Pereira, AS, C. G. Timoteo, M. Guilherme, F. Folgosa, S. G. Naik, A. G. Duarte, BH HUYNH, and P. Tavares. "
Spectroscopic Evidence for and Characterization of a Trinuclear Ferroxidase Center in Bacterial Ferritin from Desulfovibrio vulgaris Hildenborough."
Journal of the American Chemical Society. 134 (2012): 10822-10832.
AbstractFerritins are ubiquitous and can be found in practically all organisms that utilize Fe. They are composed of 24 subunits forming a hollow sphere with an inner cavity of similar to 80 angstrom in diameter. The main function of ferritin is to oxidize the cytotoxic Fe2+ ions and store the oxidized Fe in the inner cavity. It has been established that the initial step of rapid oxidation of Fe2+ (ferroxidation) by H-type ferritins, found in vertebrates, occurs at a diiron binding center, termed the ferroxidase center. In bacterial ferritins, however, X-ray crystallographic evidence and amino acid sequence analysis revealed a trinuclear Fe binding center comprising a binuclear Fe binding center (sites A and B), homologous to the ferroxidase center of H-type ferritin, and an adjacent mononuclear Fe binding site (site C). In an effort to obtain further evidence supporting the presence of a trinuclear Fe binding center in bacterial ferritins and to gain information on the states of the iron bound to the trinuclear center, bacterial ferritin from Desulfovibrio vulgaris (DvFtn) and its E130A variant was loaded with substoichiometric amounts of Fe2+, and the products were characterized by Mossbauer and EPR spectroscopy. Four distinct Fe species were identified: a paramagnetic diferrous species, a diamagnetic diferrous species, a mixed valence Fe2+Fe3+ species, and a mononuclear Fe2+ species. The latter three species were detected in the wild-type DvFtn, while the paramagnetic diferrous species was detected in the E130A variant. These observations can be rationally explained by the presence of a trinuclear Fe binding center, and the four Fe species can be properly assigned to the three Fe binding sites. Further, our spectroscopic data suggest that (1) the fully occupied trinuclear center supports an all ferrous state, (2) sites B and C are bridged by a mu-OH group forming a diiron subcenter within the trinuclear center, and (3) this subcenter can afford both a mixed valence Fe2+Fe3+ state and a diferrous state. Mechanistic insights provided by these new findings are discussed and a minimal mechanistic scheme involving O-O bond cleavage is proposed.
Jameson, G. N. L., W. Jin, C. Krebs, A. S. Perreira, P. Tavares, X. F. Liu, E. C. Theil, and BH HUYNH. "
Stoichiometric production of hydrogen peroxide and parallel formation of ferric multimers through decay of the diferric-peroxo complex, the first detectable intermediate in ferritin mineralization."
Biochemistry. 41 (2002): 13435-13443.
AbstractThe catalytic step that initiates formation of the ferric oxy-hydroxide mineral core in the central cavity of H-type ferritin involves rapid oxidation of ferrous ion by molecular oxygen (ferroxidase reaction) at a binuclear site (ferroxidase site) found in each of the 24 subunits. Previous investigators have shown that the first detectable reaction intermediate of the ferroxidase reaction is a diferric-peroxo intermediate, F-peroxo, formed within 25 ms, which then leads to the release of H2O2 and formation of ferric mineral precursors. The stoichiometric relationship between F-peroxo, H2O2, and ferric mineral precursors, crucial to defining the reaction pathway and mechanism, has now been determined. To this end, a horseradish peroxidase-catalyzed spectrophotometric method was used as an assay for H2O2. By rapidly mixing apo M ferritin from frog, Fe2+, and O-2 and allowing the reaction to proceed for 70 ms when F-peroxo has reached its maximum accumulation, followed by spraying the reaction mixture into the H2O2 assay solution, we were able to quantitatively determine the amount of H2O2 produced during the decay of F-peroxo. The correlation between the amount of H2O2 released with the amount of F-peroxo accumulated at 70 ms determined by Mossbauer spectroscopy showed that F-peroxo decays into H2O2 with a stoichiometry of 1 F-peroxo:H2O2. When the decay of F-peroxo was monitored by rapid freeze-quench Mossbauer spectroscopy, multiple diferric mu-oxo/mu-hydroxo complexes and small polynuclear ferric clusters were found to form at rate constants identical to the decay rate of F-peroxo. This observed parallel formation of multiple products (H2O2, diferric complexes, and small polynuclear clusters) from the decay of a single precursor (F-peroxo) provides useful mechanistic insights into ferritin mineralization and demonstrates a flexible ferroxidase site.