Halophilic Enzymes: Characteristics, Structural Adaptation and Potential Applications for Biocatalysis

Nayla      Munawar; Paul      C. Engel

doi:10.2174/18722083113076660033

Abstract

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.