Part:BBa_K4665120

Biology and Usage

Biomineralisation is the process by which living organisms synthesise minerals (Dhami et al., 2013). One of the main metabolic pathways of microbial calcium carbonate production utilizes the zinc metallo-enzyme carbonic anhydrase as the catalyst of the reaction (Chaparro-Acuña et al., 2019). This pathway produces no pollutant byproduct (as is often the case with other biomineralisation pathways) and extracts CO₂ from the atmosphere as a substrate for the reaction catalyzed by the carbonic anhydrase.

SazCA is a thermostable α-carbonic anhydrase derived from the thermophilic bacterium Sulfurihydrogenibium azorense and has been characterised as the fastest known carbonic anhydrase to date, possessing a kcat/KM value of 3.5 × 108 M⁻¹ s^{−1^{. SazCA facilitates the reversible hydration of carbon dioxide to bicarbonate and protons (see fig. 1) (De Simone et al, 2015). This process creates alkaline conditions which increase the solubility of Ca²⁺ ions trapped on the extracellular matrix (EPS) of bacterial cells, which readily bond to HCO^3- , facilitating the formation of calcium carbonate crystals (Anbu, et al. 2016).}}

CO₂ and HCO^3- are small molecules capable of diffusing in and out of cells, yet the intracellular activity of CA is significantly limited by the permeability of the cell membrane, as it restricts both the amount of substrate available for catalysis and the subsequent secretion of the product, reducing overall enzymatic efficiency (Jo, 2013). Therefore, the proposed approach is to express the protein on the surface of the cell, directly exposing it to extracellular concentrations of CO₂, and bypassing cellular secretion of bicarbonate ions, which ultimately enhances the production of calcium carbonate on the surface of limestone cracks.

References

Anbu, P. et al. (March 1, 2016). Formations of calcium carbonate minerals by bacteria and its multiple applications. Springerplus 5(250). https://doi.org/10.1186/s40064-016-1869-2

Chaparro-Acuña, S.P., et al. (June, 2018). Soil bacteria that precipitate calcium carbonate: mechanism and applications of the process. Acta Agronómica 67(2). https://doi.org/10.15446/acag.v67n2.66109

De Luca, V. et al. (March 15, 2013). An α-carbonic anhydrase from the thermophilic bacterium Sulphurihydrogenibium azorense is the fastest enzyme known for the CO2 hydration reaction. Bioorganic & Medicinal Chemistry Letters, 21(6): 1465.1469. https://doi.org/10.1016/j.bmc.2012.09.047

De Simone, G., et al. (May 1, 2015). Crystal structure of the most catalytically effective carbonic anhydrase enzyme known, SazCA from the thermophilic bacterium Sulfurihydrogenibium azorense. Bioorganic & Medicinal Chemistry Letters, 1;25(9): 2002-2006. https://doi.org/10.1016/j.bmcl.2015.02.068

Dhami, N.K., et al. ( May 2013). Biomineralization of calcium carbonate polymorphs by the bacterial strains isolated from calcareous sites. Journal of Microbiology and Biotechnology, 23(5): 707-714. https://doi.org/10.4014/jmb.1212.11087