Structural and mechanistic studies of a stabilized subunit dimer variant of Escherichia coli bacterioferritin identify residues required for core formation.
Bacterioferritin (BFR) is a bacterial member of the ferritin family that functions in iron metabolism and protects against oxidative stress. BFR differs from the mammalian protein in that it is comprised of 24 identical subunits and is able to bind 12 equivalents of heme at sites located between adjacent pairs of subunits. The mechanism by which iron enters the protein to form the dinuclear (ferroxidase) catalytic site present in every subunit and the mineralized iron core housed within the 24-mer is not well understood. To address this issue, the properties of a catalytically functional assembly variant (E128R/E135R) of Escherichia coli BFR are characterized by a combination of crystallography, site-directed mutagenesis, and kinetics. The three-dimensional structure of the protein (1.8 A resolution) includes two ethylene glycol molecules located on either side of the dinuclear iron site. One of these ethylene glycol molecules is integrated into the surface of the protein that would normally be exposed to solvent, and the other is integrated into the surface of the protein that would normally face the iron core where it is surrounded by the anionic residues Glu(47), Asp(50), and Asp(126). We propose that the sites occupied by these ethylene glycol molecules define regions where iron interacts with the protein, and, in keeping with this proposal, ferroxidase activity decreases significantly when they are replaced with the corresponding amides.
Departments of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada.
Steve G
SG
Wong
Stacey A L
SA
Tom-Yew
Allison
A
Lewin
Nick E
NE
Le Brun
Geoffrey R
GR
Moore
Michael E P
ME
Murphy
A Grant
AG
Mauk
D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry*; D001665:Binding Sites; D002384:Catalysis; D003573:Cytochrome b Group / Q000737:chemistry*; D019281:Dimerization; D004926:Escherichia coli / Q000378:metabolism*; D019855:Ethylene Glycol / Q000737:chemistry; D005293:Ferritins / Q000737:chemistry*; D006418:Heme / Q000737:chemistry; D007501:Iron / Q000737:chemistry; D007700:Kinetics; D008968:Molecular Conformation; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011994:Recombinant Proteins / Q000737:chemistry; D017386:Sequence Homology, Amino Acid
D001426:Bacterial Proteins; D003573:Cytochrome b Group; D011994:Recombinant Proteins; D006418:Heme; D005293:Ferritins; C018032:bacterioferritin; D007501:Iron; D019855:Ethylene Glycol