PEDS Advance Access published online on February 13, 2004
Protein Engineering Design and Selection, doi:10.1093/protein/gzh022
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1 Department of Biological Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel, Department of Medical Biochemistry and Genetics, Reynolds Building, Texas A and M University, College Station, TX 77843, USA
* To whom correspondence should be addressed. E-mail: Joel.Sussman{at}weizmann.ac.il.
Dunaliella salina is a unicellular green alga thriving in environments ranging from fresh water to hyper-saline lakes, such as the Dead Sea. An unusual, internally-duplicated, 60 kDa Keywords:
Carbonic anhydrase activity/Dunaliella salina/halophilic proteins/protein molecular adaptation/salt-tolerant protein
Accepted January 27, 2004
Oxford University Press
Article
Natural protein engineering: a uniquely salt-tolerant, but not halophilic,
-type carbonic anhydrase from algae proliferating in low- to hyper-saline environments
2 Departments of Biological Chemistry and Structural Biology, Weizmann Institute of Science, Rehovot 76100, Israel
3 Department of Biological Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel
4 Department Structural Biolog, Weizmann Institute of Science, Rehovot 76100, Israel
Abstract 
-type carbonic anhydrase (dCA I), located on the surface of this alga is expected to function over a broad range of salinities. It thus would differ from other carbonic anhydrases that lose activity already at low salinities, as well as from halophilic proteins that require high salinities for conformational stability. Enzymatic analyses indeed indicated that dCA I retained activity at salt concentrations ranging from low salt to at least 1.5 M NaCl or KCl for CO2 hydration, 2.0 M NaCl for esterase activity and 0.5 M for bicarbonate dehydration. Although measurements at higher salinities were constrained by the interference of salt in the respective assayed reactions, activity was noticeable even at 4.0 M NaCl. Comparisons of the internally-duplicated dCA I to single domain derivatives indicated that inter-domain interactions played a decisive role in the stability, activity, salt-tolerance and pH-responses of dCA I. Thus, dCA I is a uniquely salt-tolerant protein, retaining an active conformation over a large range of salinities and as a Zn-metalloenzyme, largely immune to the specific inhibitory effects of anions. Its unique features make dCA I a useful model to understand the physico-chemical basis of halotolerance and protein-salt interactions in general.
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