Thermostable Carbonic Anhydrases in Biotechnological Applications

Abstract

Carbonic anhydrases are ubiquitous metallo-enzymes which catalyze the reversible hydration of carbon dioxide in bicarbonate ions and protons. Recent years have seen an increasing interest in the utilization of these enzymes in CO2 capture and storage processes. However, since this use is greatly limited by the harsh conditions required in these processes, the employment of thermostable enzymes, both those isolated by thermophilic organisms and those obtained by protein engineering techniques, represents an interesting possibility. In this review we will provide an extensive description of the thermostable carbonic anhydrases so far reported and the main processes in which these enzymes have found an application.

Keywords: CO2 capture process; carbonic anhydrases; protein engineering; thermostable enzyme.

Figure 1

Schematic representation of a chemical absorption process. Red lines indicate the absorber regeneration path.

Figure 2

Schematic representation of CA-based CO₂ capture system and its storage as CaCO₃. CO₂ was sequestered in the scrubber column and the obtained carbonate ions were transformed in CaCO₃ in presence of CaCl₂ in the precipitation column.

Figure 3

(A) SspCA dimer structure (PDB code 4G7A) with one monomer shown in cyan and the other one in magenta; (B) enlarged view of SspCA active site showing the zinc ion coordination.

Figure 4

Superposition of the dimeric structure of SazCA (PDB code 4X5S, green) with those of SspCA (PDB code 4G7A, magenta) [67] and TaCA (PDB code 4C3T, blue) [62]. TaCA dimer was generated using PISA program (http://www.ebi.ac.uk) [71] on the crystallographic coordinates (PDB entry 4C3T).

Figure 5

TaCA tetramer structure (PDB code 4C3T) with one dimer shown in green and the other one in magenta. Intermolecular disulfide bonds are also shown.

Figure 6

Ribbon representation of Cab dimer structure (PDB code 1G5C) [77] with one monomer colored in red and the other one in blue.

Figure 7

Cab active site representation showing the coordination of catalytic zinc ion.

Figure 8

MtCam monomer overall fold (PDB code 1QRG). β-strands are shown in cyan, α-helices in yellow and the 3₁₀-helix in red. Secondary structure assignments were obtained from PROMOTIF [93].

Figure 9

(A) Two different views of the MtCam trimer (PDB code 1QRG). The three monomers are reported in green, red and blue. Zinc ions are shown as spheres and their coordinating residues are shown in ball-and-stick representation. Secondary structure assignments were obtained from PROMOTIF [93]; (B) enlarged view of the active site of MtCam.

Figure 10

(A) Ribbon diagram showing the superposition of MtCam (blue), TeCcmM (green), and PhCamH (red) monomers; (B) structure-based sequence alignment of thermostable γ-CAs with known 3D structure. MtCam secondary structure elements are indicated and named according to PROMOTIF analysis [93]. α-helices, 3₁₀-helices and β-strands for MtCam (PDB code 1QRG), TeCcmM209 (PDB code 3KWC), and PhCamH (PDB code 1V3W) are highlighted in green, yellow and cyan, respectively. Catalytic histidines, Glu84 and Glu62 of MtCam are indicated with asterisks, while the “acidic loop” (Leu83-Asn96) is boxed.