The halotolerant carbonic anhydrase (dCAII) looks like other carbonic anhydrases (CA), showing high structural similarity. But looks aside, dCAII has an unusual ability to weather extreme environments.[1]
Carbonic anhydrases are enzymes which catalyze the interconversion of CO2 and water to bicarbonate and proton. For a complementary entry on carbonic anhydrases, and specifically γ-carbonic anhydrase, the class which included dCAII, see Gamma Carbonic Anhydrase.
dCAII has a decidedly unusual ability to function both in extremely high salt concentrations, and, also, in low salt concentrations.This dual nature results from dCAII well-calibrated amount of negative surface. The prevents misfolding in highly salty environments. But the surface is not so negative that dCAII requires lots of salt to function. Meaning, dCAII is rather negative, but not as negative as halophiles. Therein lies the difference between Halophile and Halotolerant.
Despite dCAII's unusual chemical properties, dCAII has the , such as:
two prominent alpha helixes (here, the two longest helixes)
10 beta sheets
catalytic zinc
dCAII has, too, the CA's architectural elements.
Catalytic zinc coordinating residues
substrate binding
hydrogen binding
lacetate
In the following of the carbonic anhydrase from Dunaliella salina (dCAII) structure. The regions corresponding to conserved regions (CRs, blue), variable regions (VRs, lime), and variable conserved regions (VCRs, red), are positioned on the dCA II structure. The catalytic Zn2+, insertions and deletions in VCRs including L1 (the Zn binding loop), L4 (the Na-binding loop), L5, and two extended α-helices (E and G) are marked. N and C termini are labeled.
In summary, dCAII has shares structural properties of CA, yet still manages to strike the balance between positive and negative surface residues, which leads to tolerance of extremely salty conditions.
