Abstract
Halotolerant lipolytic enzymes have gained growing interest, due to potential applications under harsh conditions, such as hypersalinity and presence of organic solvents. In this study, a lipolytic gene, est56, encoding 287 amino acids was identified by functional screening of a compost metagenome. Subsequently, the gene was heterologously expressed, and the recombinant protein (Est56) was purified and characterized. Est56 is a mesophilic (Topt 50 °C) and moderate alkaliphilic (pHopt 8) enzyme, showing high thermostability at 30 and 40 °C. Strikingly, Est56 is halotolerant as it exhibited high activity and stability in the presence of up to 4 M NaCl or KCl. Est56 also displayed enhanced stability against high temperatures (50 and 60 °C) and urea (2, 4, and 6 M) in the presence of NaCl. In addition, the recently reported halotolerant lipolytic enzymes were summarized. Phylogenetic analysis grouped these enzymes into 13 lipolytic protein families. The majority (45%) including Est56 belonged to family IV. To explore the haloadaptation of halotolerant enzymes, the amino acid composition between halotolerant and halophilic enzymes was statistically compared. The most distinctive feature of halophilic from non-halophilic enzymes are the higher content of acidic residues (Asp and Glu), and a lower content of lysine, aliphatic hydrophobic (Leu, Met and Ile) and polar (Asn) residues. The amino acid composition and 3-D structure analysis suggested that the high content of acidic residues (Asp and Glu, 12.2%) and low content of lysine residues (0.7%), as well as the excess of surface-exposed acidic residues might be responsible for the haloadaptation of Est56.
Highlights
Halophilic and halotolerant enzymes, which show resistance to salinity, are one of the major groups of extremozymes with industrial relevance
We identified a halotolerant esterase, Est56, from a compost metagenome and compared it to recently reported halotolerant enzymes (Table S1)
The remaining 17 halotolerant enzymes were derived from individual microorganisms of which most thrive in saline environments, i.e., Zunongwangia profunda was derived from a surface seawater [24], Erythrobacter seohaensis from a tidal flat [76], Psychrobacter celer from a deep-sea sediment [77], and Alkalibacterium sp
Summary
Halophilic and halotolerant enzymes, which show resistance to salinity, are one of the major groups of extremozymes with industrial relevance. Microorganisms growing optimally at high salt concentrations are reservoirs for halophilic and halotolerant enzymes They are generally divided into halophiles and halotolerant organisms [1,2]. Halophilic archaea of the Halobacteriales order primarily adopt the “salt-in” strategy by accumulating equimolar concentrations of inorganic ions such as potassium and chloride ions [4,5]. This mechanism of osmoregulation results in intracellular enzymes, which evolve to halophilic types [6,7], which intrinsically show high activity and stability towards increasing salinity. Unlike most halophilic enzymes, which are inactive under low salt concentrations, halotolerant enzymes are still active in the absence of NaCl [14]
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