Research

Chemistry of a huge hollow protein

Functions of proteins are diverse. One of the major protein functions is to catalyze conversion of a substrate to other molecule (Enzyme). Molecular storage and convey are also important roles of proteins. Among them, ferritin that we work with has unique properties. Four a-helices form a subunit, and 24 subunits shows self-assembling in a cage which has a huge spherical cavity of 8nm diameter (Figure 1). The physiological function of ferritin is to form ferric ions by oxidation of ferrous ions that generate harmful hydroxyl radical. About 4500 ferric ions are stored in the cage as an oxidized core species that is an useful form for organisms under depletion conditions of iron ions. The ferrous ions are oxidized at the ferroxidase center which is located at the center of each subunit (Figure 2).

Figure 1. Structure of ferritin (a) Subunit (b) Whole structure (c) Dimension of the cavity

Figure 2. Ferroxidase center

Ferritin has two types of channels. One is 8 hydrophilic channels, so called three holds channel, which is constituted with glutamate and ascorbate residues (Figure 3d). Another is 6 hydrophobic channels, so called four folds channel, which is constituted with leucine residues (Figure 3d). These channels serve as gateways to incorporate small organic species and inorganic ions in the cavity. Creation of a novel bio-catalysts or novel nano-scale molecular carrier based on the two original functions mentioned above is our research targets. The following heads are our research projects with ferritin on going.

Figure 3. (d) Three folds channel (e) Four folds channel

Development of novel oxidative reaction using ferroxidase center
The molecular structure of the ferroxidase center that I showed in figure 2 is reminiscent of the strong oxidizing enzyme called the methane monooxygenase. Therefore, modulation of the coordination sphere of the ferroxidase center could provide an active center for a novel oxidative reaction taken place in the husk of the protein. Our previous researches succeeded to introduce catalytic sites in the cage consisting of metal nano-particle or metal complexes [Ref. 1]. We are planning to construct the multi-stage reaction using the ferroxidase center and a reactive site introduced and/or formed in the cavity, which may enables a novel reaction only possible by multi-functionalized ferrtin.

Attachment of a molecular cover to outer sphere gates of the three folds channels — For the use as the small molecule carrier —
There are some studies with ferritin investigating uptake and release of the small molecule, aiming at a future use of the ferritin as drug carrier [Ref. 2]. Those studies use denaturation and reconstitution process of ferritin by a denaturing agent such as urea to capture the molecule, while the channels which are characteristic of ferritin are not exploited in the uptake processes. Low-yield of the reconstituted ferritin with the target molecule is one of the major problems of this method, especially the case to incorporate the expensive molecules such as carcinostatics. In addition, the reconstituted ferritin may not be able to dose the incorporated molecules (medicine) effectively even after the transportation to an appropriate place as the method has no mechanism to open the cavity depending on some signal. To settle these problems in the previous works, we have commenced a study that uses the ferritin channel to incorporate and release small molecules. As the incorporation and release of the molecule through the channel is equilibrium, a cover molecule that opens and closes the gate of the channels upon some stimulus should be introduced on the ferritin. Our present focus is design of the cover molecule having appropriate structure and function to sense the stimulus.

References）

1. Xing, R.M. et al. J. Inorg. Biochem. 2009, 103, 1039; Yang, Z. et al. Chem. Commmun. 2007, 3453.
2. Abe S. et al., Inorg. Chem. 2010, 49, 6967; Takezawa Y. et al. Dalton Trans 2011, 40, 2190.

Our paper concerned with this study）

Incorporation of organometallic Ru complexes into apo-ferritin cage, Takezawa, Y., Bockmann, P., Sugi, N., Wang, Z. Y., Abe, S., Murakami, T., Hikage, T., Erker, G., Watanabe, Y., Kitagawa, S., Ueno, T. Dalton Transactions 2011, 40, 2190-2195.

Spherical protein cages such as an iron storage protein, ferritin, have great potential as nanometer-scale capsules to assemble and store metal ions and complexes. We report herein the synthesis of a composite of an apo-ferritin cage and Ru(p-cymene) complexes. Ru complexes were efficiently incorporated into the ferritin cavity without degradation of its cage structure. X-Ray crystallography revealed that the Ru complexes were immobilized on the interior surface of the cage mainly by the coordination of histidine residues.