Protective role of salt in catalysis and maintaining structure of halophilic proteins against denaturation

Abstract

Search for new industrial enzymes having novel properties continues to be a desirable pursuit in enzyme research. The halophilic organisms inhabiting under saline/ hypersaline conditions are considered as promising source of useful enzymes. Their enzymes are structurally adapted to perform efficient catalysis under saline environment wherein n0n-halophilic enzymes often lose their structure and activity. Haloenzymes have been documented to be polyextremophilic and withstand high temperature, pH, organic solvents, and chaotropic agents. However, this stability is modulated by salt. Although vast amount of information have been generated on salt mediated protection and structure function relationship in halophilic proteins, their clear understanding and correct perspective still remain incoherent. Furthermore, understanding their protein architecture may give better clue for engineering stable enzymes which can withstand harsh industrial conditions. The article encompasses the current level of understanding about haloadaptations and analyzes structural basis of their enzyme stability against classical denaturants.

Immobilization of halophilic Bacillus sp. EMB9 protease on functionalized silica nanoparticles and application in whey protein hydrolysis

“…Halophilic proteases, apart from being highly salt stable, have also been recognized for their polyextremophilicity, as evident from their increased resistance to denaturation by higher temperatures, chemical reagents, detergents, chaotropic agents, organic solvents and extreme pH values [22,23]. Immobilization of these enzymes would therefore add to enhance such properties and provide ample scope for expanding their versatility.…”

A polyextremophilic alcohol dehydrogenase from the Atlantis II Deep Red Sea brine pool

“…However, low‐salt conditions destabilized ADH/A1a, possibly because of the increased structural flexibility and partial unfolding promoted by the decline in external ionic forces . Notably, the presence of salt is expected to play an important role in the structural protection of halophilic proteins . Furthermore, ADH/A1a was able to withstand exposure to organic solvents.…”

Systematics of haloarchaea and biotechnological potential of their hydrolytic enzymes

“…A high concentration of ions in salt solutions reduces both water activity and available water molecules, which are required for making a water layer around proteins [193]. The special amino acid compositional features of halophilic proteins are the key reasons for their high solubility in aqueous solutions containing high salt concentrations and their resistance to organic solvents [194].…”

“…Acidic residues are more abundant on the surface of halophilic proteins in comparison with their non-halophilic counterparts. These negatively charged residues compete with salt for hydration [195] and accumulation of water molecules around the halophilic proteins prevents their aggregation, even at high salt concentrations [193]. It has been shown that there are more acidic residues (especially aspartic acid) than basic ones on a halophilic protein surface, which explains the acidic nature of halophilic proteins.…”