Encapsulation of carbonic anhydrase in metal-organic frameworks to facilitate CO2 capture

Abstract

Reducing carbon emissions is crucial to stop global warming. The current methods to capture and store carbon dioxide are expensive and energy demanding. Therefore, a new technique using the enzyme carbonic anhydrase was evaluated in this project. Five protocols were tested to encapsulate SazCA, which is a carbonic anhydrase from the thermophilic bacterium Sulfurihydrogenibium azorense, in metal-organic frameworks (MOFs) called zeolitic imidazolate framework-8 (ZIF-8). Encapsulating enzymes in metal-organic frameworks has previously been shown to increase their reusability and thermostability. The enzyme was first produced through heterologous expression from Escherichia coli and purified with affinity chromatography. The MOF was then synthesized together with SazCA. It was confirmed through X-ray diffraction that two of the protocols were successful in producing pure SazCA@ZIF-8. The structure was then identified with scanning electron microscopy, which showed that the protocol with a ratio of 70:1 of the precursors 2-methylimidazole and zinc nitrate hexahydrate had particles with the most desirable cubical shape and were also mostly uniform in size. However, evaluation of the enzymatic activity with Wilbur-Anderson units assay after the encapsulation showed that it only reached about 2-5 % of the activity in free SazCA. This was shown to most likely partly be because the free zinc ions were inhibiting the enzyme. However, it was also shown that the activity improved with 122.41 % by introducing sonication, which increases the diffusion rate. Finally the thermostability was tested and compared to that of free SazCA at the temperatures 75, 85 and 95 ◦C for 1 and 4 hours. At 85 ◦C it was observed that the thermostability in the encapsulated SazCA was higher than in the free SazCA. After 1 h at this temperature the relative activity compared to the free SazCA was 11.85 ± 0.72 %, compared to 1.97 ± 0.30 %, which it was initially. In conclusion, even though the SazCA encapsulated in ZIF-8 lost most of its original activity, important design principles were identified, which could benefit future research within the field.