Process of Accumulation of Metal Ions on the Interior Surface of apo-Ferritin: Crystal Structures of a Series of apo-Ferritins Containing Variable Quantities of Pd(II) Ions

Abstract

Accumulation of metal ions on protein surfaces is an important subject in the field of materials science because these processes are applicable to the preparation of bioinspired inorganic materials. While previous studies related to this subject have focused on the preparation of nanomaterials using protein scaffolds, the detailed processes of metal ion deposition and metal core formation on a protein surface require clarification. Elucidation of the coordination structures of multinuclear metal binding sites on proteins at an early stage as well as intermediate and fully occupied stages of the metal ion deposition will help us to understand the reaction mechanisms so that desirable inorganic materials can be prepared using protein scaffolds. In this Article, we report on the detailed processes of accumulation of Pd(II) ions demonstrated by a series of X-ray crystal structural analyses of apo-ferritin (apo-Fr), an iron storage protein, containing different amounts of Pd(II) ions in the protein cage. We have identified the specific binding sites of Pd(II) ions and analyzed the dynamic changes in the coordination structure by a combination of the crystal structures and ICP quantitative analyses of apo-Fr containing low, intermediate, and high content of Pd(II) ions. Our studies on Pd(II).apo-Frs provide intriguing implications for the preparation of many other inorganic materials using protein surfaces.

Use of the confined spaces of apo-ferritin and virus capsids as nanoreactors for catalytic reactions

“…The cage can accommodate a number of other metal ions/ complexes or small organic molecules [19][20][21]. The pHdependent assembly/disassembly properties are advantageous with respect to encapsulation of larger metal complexes, organic molecules or nanoparticles [22,23].…”

Biomimetic Materials Synthesis from Ferritin-Related, Cage-Shaped Proteins

“…It is important to notice that most of the mineralization reactions that have been successfully carried out are not specific to iron, suggesting that the electrostatic character of the interior surface of the protein cage is the major player in the mineralization process. Besides the presence of an electrostatic gradient that favours the concentration of cations inside the cavity, it is of great value to have multiple metal binding/nucleation sites able to incorporate efficiently the required metal, which are likely to act as starting points and metal particle seeds for the mineralization reaction Ueno et al, 2009). Other parameters have to be taken into account for the preparation of protein-enclosed nanomaterials of biotechnological value, such as: the specific ferritin molecule to be used as a template; the oxidant/reducing agent; and the reaction conditions (i.e., ionic strength, pH, temperature, presence of chelating molecules, etc.).…”

Elucidation of Metal‐Ion Accumulation Induced by Hydrogen Bonds on Protein Surfaces by Using Porous Lysozyme Crystals Containing RhIIIIons as the Model Surfaces

“…These Asp and His functional groups can be bound to Pd II ions with cooperative disruption of the hydrogen bonds and alterations of the conformation of coordinated amino acids in a manner similar to that which occurs at Site B of Rh·HEWL (Figure 4 a). [23] Therefore, it is suggested that these metal clusters in native proteins are formed by accumulation of aqua metal complexes. The reactions are coupled with the rearrangement of hydrogen bonds and conformational changes of amino acid residues, as observed in our model systems.…”

“…The results indicate that the number of Pd II ions accumulated by apo-Fr increases when different coordination structures are adopted at the specific binding sites on the interior surface of apo-Fr. [23] However, there is only limited information about the cooperative roles of hydrogen bonds and amino-acid residues in the accumulation reactions, because the crystal structure with a high content of Pd II ions was obtained at low resolution. [23,30] If we can construct appropriate metal ion/protein surface models, we expect to gain an in-depth understanding of the accumulation mechanism.…”

“…This is possible because more flexibility is achievable with respect to reaction conditions for PPCs (including pH, and the identity and quantity of metal ions) relative to the more restrictive conditions required for working with crystals of apo-Fr containing metal ions. [23] We demonstrate a snapshot analysis of protein-mediated metal accumulation processes by using porous hen egg white lysozyme (HEWL) crystals that contain different amounts of Rh III ions to elucidate the metal accumulation mechanism, as a preorganization of metal ions for biomineralization reactions at the atomic level. HEWL, a small hydrolytic enzyme, forms a tetragonal crystal (space group P4 3 2 1 2) with different types of pores, as shown in Fig-A C H T U N G T R E N N U N G ure 1 a.…”