Stability and iron oxidation properties of a novel homopolymeric plant ferritin from adzuki bean seeds: a comparative analysis with recombinant soybean seed H-1 chain ferritin.

BACKGROUND

All reported plant ferritins are heteropolymers comprising two different H-type subunits. Whether or not homopolymeric plant ferritin occurs in nature is an open question.

METHODS

A homopolymeric phytoferritin from adzuki bean seeds (ASF) was obtained by various protein purification techniques for the first time, which shares the highest identity (89.6%) with soybean seed H-1 ferritin (rH-1). Therefore, we compared iron oxidation activity and protein stability of ASF with those of rH-1 by stopped-flow combined with light scattering or UV/Vis spectrophotography, SDS- and native- PAGE analyses. Additionally, a new rH-1 variant (S68E) was prepared by site-directed mutagenesis approach to elucidate their difference in protein stability.

RESULTS

At high iron loading of protein, the extension peptide (EP) of plant ferritin was involved in iron oxidation, and the EP of ASF exhibited a much stronger iron oxidative activity than that of rH-1. Besides, ASF is more stable than rH-1 during storage, which is ascribed to one amino acid residue, Ser68.

CONCLUSIONS

ASF exhibits a different mechanism in iron oxidation from rH-1 at high iron loading of protein, and a higher stability than rH-1. These differences are mainly stemmed from their different EP sequences.

GENERAL SIGNIFICANCE

This work demonstrates that plant cells have evolved the EP of phytoferritin to control iron chemistry and protein stability by exerting a fine tuning of its amino acid sequence.