Fe2+ substrate transport through ferritin protein cage ion channels influences enzyme activity and biomineralization
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Ferritins, complex protein nanocages, form internal iron-oxy minerals (Fe2O3·H2O), by moving cytoplasmic Fe2+ through intracage ion channels to cage-embedded enzyme (2Fe2+/O2 oxidoreductase) sites where ferritin biomineralization is initiated. The products of ferritin enzyme activity are diferric oxy complexes that are mineral precursors. Conserved, carboxylate amino acid side chains of D127 from each of three cage subunits project into ferritin ion channels near the interior ion channel exits and, thus, could direct Fe2+ movement to the internal enzyme sites. Ferritin D127E was designed and analyzed to probe properties of ion channel size and carboxylate crowding near the internal ion channel opening. Glu side chains are chemically equivalent to, but longer by one –CH2 than Asp, side chains. Ferritin D127E assembled into normal protein cages, but diferric peroxo formation (enzyme activity) was not observed, when measured at 650 nm (DFP λ max). The caged biomineral formation, measured at 350 nm in the middle of the broad, nonspecific Fe3+–O absorption band, was slower. Structural differences (protein X-ray crystallography), between ion channels in wild type and ferritin D127E, which correlate with the inhibition of ferritin D127E enzyme activity include: (1) narrower interior ion channel openings/pores; (2) increased numbers of ion channel protein–metal binding sites, and (3) a change in ion channel electrostatics due to carboxylate crowding. The contributions of ion channel size and structure to ferritin activity reflect metal ion transport in ion channels are precisely regulated both in ferritin protein nanocages and membranes of living cells.
KeywordsFerritin Iron oxidation Ion channels Crystal structure Electrostatics Di-iron center Diferric peroxo Caged iron-oxy biomineral Oxidoreductase enzyme reactivity
- di-Fe2+/O2 center
Oxidoreductase or ferroxidase center (Fox Center)
We thank Dr. Francesco Oteri, Dr. Ho Leung Ng and Dr. Anshuman Dixit for helpful discussions. We also thank the Advanced Light Source (ALS) at Berkeley National Laboratories and Stanford Synchrotron Radiation Light source (SSRL) for their invaluable help in data collection. This work was supported by the CHORI Partners and National Institutes of Health Grants DK20251 (to E. C. T), AI081161 (to C. W. G) and DST-INSPIRE Faculty Award (to R. K. B).
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