Protein association and dissociation regulated by ferric ion: a novel pathway for oxidative deposition of iron in pea seed ferritin.
Iron stored in phytoferritin plays an important role in the germination and early growth of seedlings. The protein is located in the amyloplast where it stores large amounts of iron as a hydrated ferric oxide mineral core within its shell-like structure. The present work was undertaken to study alternate mechanisms of core formation in pea seed ferritin (PSF). The data reveal a new mechanism for mineral core formation in PSF involving the binding and oxidation of iron at the extension peptide (EP) located on the outer surface of the protein shell. This binding induces aggregation of the protein into large assemblies of approximately 400 monomers. The bound iron is gradually translocated to the mineral core during which time the protein dissociates back into its monomeric state. Either the oxidative addition of Fe(2+) to the apoprotein to form Fe(3+) or the direct addition of Fe(3+) to apoPSF causes protein aggregation once the binding capacity of the 24 ferroxidase centers (48 Fe(3+)/shell) is exceeded. When the EP is enzymatically deleted from PSF, aggregation is not observed, and the rate of iron oxidation is significantly reduced, demonstrating that the EP is a critical structural component for iron binding, oxidation, and protein aggregation. These data point to a functional role for the extension peptide as an iron binding and ferroxidase center that contributes to mineralization of the iron core. As the iron core grows larger, the new pathway becomes less important, and Fe(2+) oxidation and deposition occurs directly on the surface of the iron core.
Li
Chaorui
C
From the College of Food Science and Nutritional Engineering, China Agricultural University, Key Laboratory of Fruits and Vegetables Processing, Ministry of Agriculture, Beijing 100083, China.
Fu
Xiaoping
X
From the College of Food Science and Nutritional Engineering, China Agricultural University, Key Laboratory of Fruits and Vegetables Processing, Ministry of Agriculture, Beijing 100083, China.
Qi
Xin
X
National Institute of Metrology, Beijing 100013, China.
Hu
Xiaosong
X
From the College of Food Science and Nutritional Engineering, China Agricultural University, Key Laboratory of Fruits and Vegetables Processing, Ministry of Agriculture, Beijing 100083, China.
Chasteen
N Dennis
ND
Department of Chemistry, University of New Hampshire, Durham, New Hampshire 03824.
Zhao
Guanghua
G
From the College of Food Science and Nutritional Engineering, China Agricultural University, Key Laboratory of Fruits and Vegetables Processing, Ministry of Agriculture, Beijing 100083, China. Electronic address: gzhao1000@yahoo.com.
D000595:Amino Acid Sequence; D001059:Apoproteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D002412:Cations / Q000737:chemistry / Q000378:metabolism; D005293:Ferritins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D007501:Iron / Q000737:chemistry / Q000378:metabolism*; D007700:Kinetics; D008027:Light; D008954:Models, Biological; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D018532:Pisum sativum / Q000235:genetics / Q000378:metabolism; D010940:Plant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D012542:Scattering, Radiation; D012639:Seeds / Q000378:metabolism
D001059:Apoproteins; D002412:Cations; D010940:Plant Proteins; D005293:Ferritins; D007501:Iron