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Phosphate Capping
Ferritin is an apoenzyme that has variable amounts of a basic ferric phosphate from one organism to the next. For example, Azotobacter vinelandii (AV) has a ferritin with an iron to phosphate ratio of 1:1.7 while the Fe:P ratio in horse spleen is 1:0.125. Ferritin iron cores can be crystalline, but the iron cores with high phosphate content lead to greater disorder in the iron core and also influences the magnetic interactions in the iron core of native and reconstituted ferritinslk. The Fe-Fe bond distance increases from 30.3 nm to 35.0 nm in the presence of the phosphate. It reduces the number of near iron neighbor due to phosphate bridging between adjacent Fe atoms. The iron-oxy polymers (crystallite) can also be terminated by phosphates as they form terminal iron-phosphate complexes that introduce disorder in the iron core.

Removal of the ferritin proteins for the isolation of the iron core, done under 1 M NaOH treatment caused up to 80% of the inorganic phosphate that is associated with the ferritin iron core to be free in solution. In early studies of ferritin, a comparison of the X-ray diffraction pattern of NaOH treated and natural ferritin results showed no difference. This conclusion is also confirmed by virtue that phosphate ferritin can be reconstituted in the absence of phosphate. Although, the formation of apoferritin takes up inorganic phosphate if present during reconstruction. Kinetic studies, conversely have been shown that the rate at which Fe(II) is oxidized, measured by Mossbauer spectroscopy, is significantly increased in the presence of phosphate. The associated anion, phosphate, causes the surface binding and oxidation of Fe2+ and maybe incorporated into the core via this association leading to an accelerated growth of the core.

Reference Work