Moving Iron through ferritin protein nanocages depends on residues throughout each four ?-helix bundle subunit.

ABSTRACT: Eukaryotic H ferritins move iron through protein cages to form biologically required, iron mineral concentrates. The biominerals are synthesized during protein-based Fe²?/O? oxidoreduction and formation of [Fe³?O](n) multimers within the protein cage, en route to the cavity, at sites distributed over ~50 ?. Recent NMR and Co²?-protein x-ray diffraction (XRD) studies identified the entire iron path and new metal-protein interactions: (i) lines of metal ions in 8 Fe²? ion entry channels with three-way metal distribution points at channel exits and (ii) interior Fe³?O nucleation channels. To obtain functional information on the newly identified metal-protein interactions, we analyzed effects of amino acid substitution on formation of the earliest catalytic intermediate (diferric peroxo-A(650 nm)) and on mineral growth (Fe³?O-A(350 nm)), in A26S, V42G, D127A, E130A, and T149C. The results show that all of the residues influenced catalysis significantly (p < 0.01), with effects on four functions: (i) Fe²? access/selectivity to the active sites (Glu¹³?), (ii) distribution of Fe²? to each of the three active sites near each ion channel (Asp¹²?), (iii) product (diferric oxo) release into the Fe³?O nucleation channels (Ala²?), and (iv) [Fe³?O](n) transit through subunits (Val?², Thr¹??). Synthesis of ferritin biominerals depends on residues along the entire length of H subunits from Fe²? substrate entry at 3-fold cage axes at one subunit end through active sites and nucleation channels, at the other subunit end, inside the cage at 4-fold cage axes. Ferritin subunit-subunit geometry contributes to mineral order and explains the physiological impact of ferritin H and L subunits.

SUBMITTER: Haldar S 

PROVIDER: S-EPMC3138320 | biostudies-literature | 2011 Jul

REPOSITORIES: biostudies-literature