PEDS Advance Access originally published online on February 13, 2004
Protein Engineering Design and Selection 2004 17(2):191-200; doi:10.1093/protein/gzh022
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© 2004 Oxford University Press
Natural protein engineering: a uniquely salt-tolerant, but not halophilic, 
-type carbonic anhydrase from algae proliferating in low- to hyper-saline environments
Departments of 1Biological Chemistry and 4Structural Biology, Weizmann Institute of Science, Rehovot 76100, Israel and 2Department of Medical Biochemistry and Genetics, Reynolds Building, Texas A & M University, College Station, TX 77843, USA 3These authors contributed equally to this work
5 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 
-type carbonic anhydrase (dCA I), located on the surface of this alga, is expected to function over a broad range of salinities. It would therefore differ from other carbonic anhydrases that already lose activity at low salinities and also 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. Hence 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.
Received December 30, 2003; accepted January 27, 2004 Edited by Dan Tawfik
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