Ferritin Ion Channel Disorder Inhibits Fe(II)/O2 Reactivity at Distant Sites

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• 作者：Takehiko Tosha ; Rabindra K. Behera ; Elizabeth C. Theil
• 刊名：Inorganic Chemistry
• 出版年：2012
• 出版时间：November 5, 2012
• 年：2012
• 卷：51
• 期：21
• 页码：11406-11411
• 全文大小：410K
• 年卷期：v.51,no.21(November 5, 2012)
• ISSN：1520-510X

文摘

Ferritins, a complex, mineralized, protein nanocage family essential for life, provide iron concentrates and oxidant protection. Protein-based ion channels and Fe(II)/O₂ catalysis initiate conversion of thousands of Fe atoms to caged, ferritin Fe₂O₃路H₂O minerals. The ion channels consist of six helical segments, contributed by 3 of 12 or 24 polypeptide subunits, around the 3-fold cage axes. The channel structure guides entering Fe(II) ions toward multiple, catalytic, diiron sites buried inside ferritin protein helices, 20 脜 away from channel internal exits. The catalytic product, Fe(III)鈥揙(H)鈥揊e(III), is a mineral precursor; mineral nucleation begins inside the protein cage with mineral growth in the central protein cavity (5鈥? nm diameter). Amino acid substitutions that changed ionic or hydrophobic channel interactions R72D, D122R, and L134P increased ion channel structural disorder (protein crystallographic analyses) and increased Fe(II) exit [chelated Fe(II) after ferric mineral reduction/dissolution]. Since substitutions of some channel carboxylate residues diminished ferritin catalysis with no effect on Fe(II) exit, such as E130A and D127A, we investigated catalysis in ferritins with altered Fe(II) exit, R72D, D122R and L134P. The results indicate that simply changing the ionic properties of the channels, as in the R72D variant, need not change the forward catalytic rate. However, both D122R and L134P, which had dramatic effects on ferritin catalysis, also caused larger effects on channel structure and order, contrasting with R72D. All three amino acid substitutions, however, decreased the stability of the catalytic intermediate, diferric peroxo, even though overall ferritin cage structure is very stable, resisting 80 掳C and 6 M urea. The localized structural changes in ferritin subdomains that affect ferritin function over long distances illustrate new properties of the protein cage in natural ferritin function and for applied ferritin uses.