The researchers introduced site-specific mutations at the core of the ferritin nanocage to increase its uptake of the iridium complex IrCp*. Image: Takafumi Ueno from Tokyo Institute of Technology.Researchers from the Tokyo Institute of Technology in Japan have shown that a novel hybrid ferritin nanocage with histidine residues shows 1.5 times higher metal ion uptake and improved catalytic efficiency for alcohol production. Their findings, reported in a paper in Angewandte Chemie, suggest that hybrid bio-nanocages could effectively catalyze reactions to yield industrially important products.
Biological polymers can spontaneously self-assemble into complex structures that resemble tiny vessels or cages, termed 'nano-cages'. These structures can accommodate a wide range of molecules inside them that behave as 'guests'. One popular example is the 'ferritin nanocage', which is formed from the self-assembly of 24 subunits of the protein ferritin and can enclose metal ions that are important catalysts. Although widely known, the ferritin cage's potential applications in industry have yet to be fully explored.
Thus far, most efforts to increase metal ion uptake in ferritin have resulted in cages with low stability. To make the 'guest' molecule sit well within the cage, effective design is the key. Keeping that in mind, a team of scientists led by Takafumi Ueno at Tokyo Tech introduced site-specific mutations at the core of the ferritin nanocage to increase its uptake of an iridium complex (IrCp*). Iridium is a vital catalyst in the alcohol production pathway, and is used commercially in the pharmaceutical, food and chemical industries.
"Based on previous literature, we knew that the presence of coordination amino acids in the cage improve iridium activity, and that substituting these amino acids with appropriate residues could alleviate the problem," explains Ueno. "Since iridium complex behaves as a catalyst, coordination residues would do the job." The researchers replaced two residues in the regular (wild-type) ferritin cages – arginine and aspartic acid – with histidine to create the mutants R52H and D38H. Remarkably, the assembly structure and cage size were not affected by these changes.
Next, they added IrCp* to the mutants and found that R52H was able to embed 1.5 times more iridium atoms than the wild-type cage. In contrast, the D38H mutant behaved exactly like the wild-type cage. So, why didn't both mutations have the same effect?
"This implies that it is not only the presence of the histidine residue but also its position that is crucial to determine uptake efficiency in the cage," says Ueno.
Using the new catalytic cages, the researchers were able to achieve alcohol production rates as high as 88%. Evidently, the mutations favored a structural re-arrangement of the reaction components, which enhanced the conversion rate.
The researchers used simulations to understand how the substrate behaved inside the cage, which revealed that the substrate molecules could move freely. They observed some interactions between the substrate and histidine in the R52H mutant that were not present in the wild-type cage, i.e., the substrate showed preferential binding within the nanocage.
"These hybrid bio-nanocages were also found to be highly stable, suggesting that they could be used as viable catalysts in industrial applications," concludes Ueno. The same structure-based design of the metal ion binding site could also be utilized to create novel ferritin mutants able to take up other guest molecules for varied catalytic applications in the chemical and pharmaceutical industries.
This story is adapted from material from the Tokyo Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.