JMS/sandbox4

The halotolerant carbonic anhydrase (dCAII) looks like other carbonic anhydrases, 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 full entry on γ-carbonic anhydrase see Gamma Carbonic Anhydrase.

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 (dCA II) 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.

Still, dCAII has a decidedly unusual ability to function in extremely high salt concentrations, but, also, in low salt concentrations.This balance comes from a predominantly negative surface, on the one hand, that is still less negative than that observed in halophiles, on the other hand. Therein lies the difference between Halophile and Halotolerant.

In summary, dCAII has all the properties of CA, and strikes the balance between positive and negative surface residues, which leads to tolerance of extremely salty conditions, and an ability to function in low salt conditions, as well.