Effects of introducing negative charges into the molecular surface of thermolysin by site-directed mutagenesis on its activity and stability

Microbial-derived salt-tolerant proteases and their applications in high-salt traditional soybean fermented foods: a review

Different microorganisms can produce different proteases, which can adapt to different industrial requirements such as pH, temperature, and pressure. Salt-tolerant proteases (STPs) from microorganisms exhibit higher salt tolerance, wider adaptability, and more efficient catalytic ability under extreme conditions compared to conventional proteases. These unique enzymes hold great promise for applications in various industries including food, medicine, environmental protection, agriculture, detergents, dyes, and others. Scientific studies on microbial-derived STPs have been widely reported, but there has been little systematic review of microbial-derived STPs and their application in high-salt conventional soybean fermentable foods. This review presents the STP-producing microbial species and their selection methods, and summarizes and analyzes the salt tolerance mechanisms of the microorganisms. It also outlines various techniques for the isolation and purification of STPs from microorganisms and discusses the salt tolerance mechanisms of STPs. Furthermore, this review demonstrates the contribution of modern biotechnology in the screening of novel microbial-derived STPs and their improvement in salt tolerance. It highlights the potential applications and commercial value of salt-tolerant microorganisms and STPs in high-salt traditional soy fermented foods. The review ends with concluding remarks on the challenges and future directions for microbial-derived STPs. This review provides valuable insights into the separation, purification, performance enhancement, and application of microbial-derived STPs in traditional fermented foods.

Characterization and mutation analysis of a halotolerant serine protease from a new isolate of Bacillus subtilis

OBJECTIVES

A bacterial halotolerant enzyme was characterized to understand the molecular mechanism of salt adaptation and to explore its protein engineering potential.

RESULTS

Halotolerant serine protease (Apr_No16) from a newly isolated Bacillus subtilis strain no. 16 was characterized. Multiple alignments with previously reported non-halotolerant proteases, including subtilisin Carlsberg, indicated that Apr_No16 has eight acidic or polar amino acid residues that are replaced by nonpolar amino acids in non-halotolerant proteases. Those residues were hypothesized to be one of the primary contributors to salt adaptation. An eightfold mutant substituted with Ala residues exhibited 1.2- and 1.8-fold greater halotolerance at 12.5% (w/v) NaCl than Apr_No16 and Carlsberg, respectively. Amino acid substitution notably shifted the theoretical pI of the eightfold mutant, from 6.33 to 9.23, compared with Apr_No16. The resulting protein better tolerated high salt conditions.

CONCLUSIONS

Changing the pI of a bacterial serine protease may be an effective strategy to improve the enzyme's halotolerance.

MDC-Analyzer-facilitated combinatorial strategy for improving the activity and stability of halohydrin dehalogenase from Agrobacterium radiobacter AD1

Halohydrin dehalogenase from Agrobacterium radiobacter AD1 (HheC) displays a broad substrate range with high regio- and enantioselectivity of both ring-closure and ring-opening reactions, making the enzyme a useful catalyst for the production of optically pure epoxides and β-substituted alcohols. In this study, we report a novel method using an MDC-Analyzer-facilitated combinatorial strategy to improve the activity and stability of HheC by simultaneously randomizing multiple contiguous residues. Six contiguous active-site residues, which are the hotspots for improving the activity of HheC, were simultaneously selected and randomized using the MDC-Analyzer-facilitated combinatorial strategy, resulting in a high-quality mutagenesis library. After screening a total of 1152 clones, three positive mutants were obtained, which exhibited approximately 3.5-5.9-fold higher kcat values than the wild-type HheC toward 1,3-dichloro-2-propanol (1,3-DCP). However, the inactivation half-life of the best mutant (DG9) at 55 °C decreased 9-fold compared with that of the wild-type HheC. To improve the stability of mutant DG9, seven contiguous potential surface amino acids were revealed by using the B-FITTER tool. Two charged amino acids, Glu and Lys, which are more abundant in thermophilic proteins than in their mesophilic counterparts, were selected to substitute those seven amino acids and were combined together via an MDC-Analyzer-facilitated combinatorial strategy. Two mutants displaying 1.6- and 2.3-fold higher half-life τ1/2 (55 °C) values than their DG9 template were obtained after screening only 384 clones. The results indicated that an MDC-Analyzer-facilitated combinatorial strategy represents an efficient tool for the directed evolution of functional enzymes with multiple contiguous targeting sites.

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

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.

Effects of site-directed mutagenesis in the N-terminal domain of thermolysin on its stabilization

The thermolysin variant G8C/N60C/S65P in which the triple mutation in the N-terminal domain, Gly8→Cys/Asn60→Cys/Ser65→Pro, is undertaken increases stability [Yasukawa, K. and Inouye, K. (2007) Improving the activity and stability of thermolysin by site-directed mutagenesis. Biochim. Biophys. Acta 1774, 1281-1288] and its mechanism is examined in this study. The apparent denaturing temperatures based on ellipticity at 222 nm of the wild-type thermolysin (WT), G8C/N60C, S65P and G8C/N60C/S65P were 85, >95, 88 and >95°C, respectively. The first-order rate constants, k(obs), of the thermal inactivation of WT and variants at 10 mM CaCl₂ increased with increasing thermal treatment temperatures (70-95°C), and those at 80°C decreased with increasing CaCl₂ concentrations (1-100 mM). The k(obs) values were in the order of WT > S65P > G8C/N60C≒G8C/N60C/S65P at all temperatures and CaCl₂ concentrations. These results indicate that the mutational combination, Gly8→Cys/Asn60→Cys and Ser65→Pro, increases stability only as high as Gly8→Cys/Asn60→Cys does. Assuming that irreversible inactivation of thermolysin occurs only in the absence of calcium ions, the dissociation constants, K(d), to the calcium ions of WT, G8C/N60C, S65P and G8C/N60C/S65P were 47, 8.9, 17 and 7.2 mM, respectively, suggesting that Gly8→Cys/Asn60→Cys and Ser65→Pro stabilize thermolysin by improving its affinity to calcium ions, most probably the one at the Ca²⁺-binding site III in the N-terminal domain.

The zinc-dependent protease activity of the botulinum neurotoxins

The botulinum neurotoxins (BoNT, serotypes A-G) are some of the most toxic proteins known and are the causative agents of botulism. Following exposure, the neurotoxin binds and enters peripheral cholinergic nerve endings and specifically and selectively cleaves one or more SNARE proteins to produce flaccid paralysis. This review centers on the kinetics of the Zn-dependent proteolytic activities of these neurotoxins, and briefly describes inhibitors, activators and factors underlying persistence of toxin action. Some of the structural, enzymatic and inhibitor data that are discussed here are available at the botulinum neurotoxin resource, BotDB (http://botdb.abcc.ncifcrf.gov).