Halophilic Enzymes: Characteristics, Structural Adaptation and Potential Applications for Biocatalysis
Paul C. Engel.
This review discusses initially the growing interest in biocatalysis for ‘green chemistry’ applications and
suggests that for many such applications the enzymes of halophilic organisms may offer ideal candidates. Halophilic
organisms and the nature of their adaptation to the rigours of their environment are discussed. In the context of this article
it is the adaptations of individual proteins at a molecular level that are of particular significance. In general the enzymes of
halophiles are tolerant not only of high-salt conditions but also of high temperature and of organic solvents. We review
the growing body of information from amino acid sequences and from 3-D structures pointing to a general pattern of high
negative charge density on the surface of halophilic proteins and explore various current ideas as to how this structural
pattern may account for the observed functional properties. The biotechnological potential of these enzymes is reviewed
in the light of their versatile and robust properties. A significant barrier to their wider adoption in industry up till now has
been the difficulty of producing halophilic enzymes in bulk with the same ease that characterizes mesophilic systems.
Recent developments in relation to soluble expression in halophilic hosts and also in purification procedures have gone a
long way towards overcoming these important practical problems and the new methods are surveyed. Finally it is
concluded that, whilst the depth of our understanding of structure-function relationships in halophilic enzymes lags behind
the wealth of descriptive information that is accumulating, empirical application of our existing knowledge and
procedures should lead to a widespread adoption of halophilic biocatalysts in the years ahead.
Keywords: Acidic biosensors, chiral synthesis, enzyme isoforms, halophiles, halophilic expression hosts, homology modeling,
hydantoinase, hydrolases, ordered water, oxidoreductases, reverse micelles, salt tolerance, site-directed mutagenesis, solvent
stability, surface residues, thermostability.
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