Spectroscopic Evidence for and Characterization of a Trinuclear Ferroxidase Center in Bacterial Ferritin from Desulfovibrio vulgaris Hildenborough

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• 作者：Alice S. Pereira ; Cristina G. Tim贸teo ; M谩rcia Guilherme ; Filipe Folgosa ; Sunil G. Naik ; Am茅rico G. Duarte ; Boi Hanh Huynh ; Pedro Tavares
• 刊名：The Journal of the American Chemical Society
• 出版年：2012
• 出版时间：July 4, 2012
• 年：2012
• 卷：134
• 期：26
• 页码：10822-10832
• 全文大小：562K
• 年卷期：v.134,no.26(July 4, 2012)
• ISSN：1520-5126

文摘

Ferritins 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 80 脜 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 M枚ssbauer 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 渭-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鈥揙 bond cleavage is proposed.