An extracellular protease has been purified to how to improve conventional enzymes for catalysis from the extreme halophile, Halobacterium halobium.
under harsh conditions as well as to increase dramati-
The irreversible inactivation kinetics of this halophilic procally the repertoire of enzymatic reactions and reaction tease in salt concentrations below 4M consists of autolytic conditions available to the biosynthetic chemist.
and nonautolytic (steady-state denaturation) compo-High salt content is a particularly harsh environment nents. Addition of organic solvents has a dramatic effect for proteins (Eisenberg and Wachtel, 1987). Nevertheon enzyme stability in low salt media. For example, in 0.36M NaCl, the inactivation rate constant for the non-less, enzymes from extreme halophiles flourish in, and
indeed require, nearly saturating salt concentrations.
orders of magnitude lower than in 20% (v/v) tetrahydro-Furthermore, halophilic proteins have proven to be furan. Enzyme stability in different aqueous/organic solunique resources to study the basic tenets of protein vent mixtures correlates strongly to the salting-out capacstructure and stability (Zaccai and Eisenberg, 1990). We ity of the solvent. Solvents that act to increase the have been keenly interested in the mechanism(s) that apparent hydrophobicity of the enzyme's core stabilize the enzyme in much the same way as salting-out salts.
nature uses to enable biocatalysis to take place in nearly
This mechanism is not important for the nonhalophilic saturating concentrations of salt. Such molecular level protease, subtilisin Carlsberg, and demonstrates that adaptation is still not completely understood; however, halophilic enzymes have evolved highly specialized reacseveral things are known. For example, in salt concention medium requirements. Moreover, through the use trations below 0.2M, charge screening of the negative of organic solvents, it is shown that high concentrations amino acid residues on halophilic proteins by the salt of salts are not absolutely necessary for high enzyme stability, and this may have important process consideracan play an important role in stabilizing the protein tions.