Homologous expression and biochemical characterization of the arylsulfatase from Kluyveromyces lactis and its relevance in milk processing

Detection, production, modification, and application of arylsulfatases

Arylsulfatase is a subset of sulfatase which catalyzes the hydrolysis of aryl sulfate ester. Arylsulfatase is widely distributed among microorganisms, mammals and green algae, but the arylsulfatase-encoding gene has not yet been found in the genomes of higher plants so far. Arylsulfatase plays an important role in the sulfur flows between nature and organisms. In this review, we present the maturation and catalytic mechanism of arylsulfatase, and the recent literature on the expression and production of arylsulfatase in wild-type and engineered microorganisms, as well as the modification of arylsulfatase by genetic engineering are summarized. We focus on arylsulfatases from microbial origin and give an overview of different assays and substrates used to determine the arylsulfatase activity. Furthermore, the researches about arylsulfatase application on the field of agar desulfation, soil sulfur cycle and soil evaluation are also discussed. Finally, the perspectives concerning the future research on arylsulfatase are prospected.

Characterization of putative mannoprotein in Kluyveromyces lactis for lactase production

Lactase is a member of the β-galactosidase family of enzymes that can hydrolyze lactose into galactose and glucose. However, extracellular lactase production was still restricted to the process of cell lysis. In this study, lactase-producing Kluyveromyces lactis JNXR-2101 was obtained using a rapid and sensitive method based on the fluorescent substrate 4-methylumbelliferyl-β-d-galactopyranoside. The purified enzyme was identified as a neutral lactase with an optimum pH of 9. To facilitate extracellular production of lactase, a putative mannoprotein KLLA0_E01057g of K. lactis was knocked out. It could effectively promote cell wall degradation and lactase production after lyticase treatment, which showed potential on other extracellular enzyme preparation. After optimizing the fermentation conditions, the lactase yield from mannoprotein-deficient K. lactis JNXR-2101ΔE01057g reached 159.62 U/mL in a 5-L fed-batch bioreactor.

Engineering the optimum pH of β-galactosidase from Aspergillus oryzae for efficient hydrolysis of lactose

β-Galactosidase (lacA) from Aspergillus oryzae is widely used in the dairy industry. Its acidic pH optimum and severe product inhibition limit its application for lactose hydrolysis in milk. In the present study, structure-based sequence alignment was conducted to determine the candidate mutations to shift the pH optimum of lacA to the neutral range. The Y138F and Y364F mutants shifted the pH optimum of lacA from 4.5 to 5.5 and 6.0, respectively. The acid dissociation constant (pKa) values of catalytic acid/base residues with upwards shift were consistent with the increased pH optimum. All variants in the present study also alleviated galactose inhibition to various extents. Molecular dynamics demonstrated that the less rigid tertiary structures and lower galactose-binding free energy of Y138F and Y364F might facilitate the release of the end product. Both Y138F and Y364F mutants exhibited better hydrolytic ability than lacA in milk lactose hydrolysis. The higher pH optimum and lower galactose inhibition of Y138F and Y364F may explain their superiority over lacA. The Y138F and Y364F mutants in the present study showed potential in producing low-lactose milk, and our studies provide a novel strategy for engineering the pH optimum of glycoside hydrolase.

Characterization and substrate-accelerated thermal inactivation kinetics of a new serine-type arylsulfatase

Arylsulfatase is useful in industrial agar processing by removing sulfate groups. A full-length arylsulfatase gene, designated ArySMA1, was obtained from marine bacteria Serratia sp. SM1. The ArySMA1 gene encoded a novel serine-type arylsulfatase and the enzymatic properties were characterized. The enzyme presented notable capacity of removing sulfate groups from natural algae substrates. Kinetic study suggested that the microscopic thermal inactivation rate of ArySMA1 in free form was smaller than that of the enzyme-substrate complex. The presence of substrate could unexpectedly accelerate ArySMA1 to deactivate at high temperature. Such phenomenon was opposite to many findings that substrate could stabilize enzymes against heat. Molecular dynamics simulation and ANS fluorescent assay indicated the substrate led the hydrophobic regions of the active site more flexible and the sulfate group of the substrate could retard the processivity of ArySMA1 catalysis. This study provides guidance for agar desulfation and down-stream processing industry.

Multi-Trait Wheat Rhizobacteria from Calcareous Soil with Biocontrol Activity Promote Plant Growth and Mitigate Salinity Stress

Plant growth promoting rhizobacteria (PGPR) can be functional microbial fertilizers and/or biological control agents, contributing to an eco-spirit and safe solution for chemical replacement. Therefore, we have isolated rhizospheric arylsulfatase (ARS)-producing bacteria, belonging to Pseudomonas and Bacillus genus, from durum wheat crop grown on calcareous soil. These isolates harbouring plant growth promoting (PGP) traits were further evaluated in vitro for additional PGP traits, including indole compounds production and biocontrol activity against phytopathogens, limiting the group of multi-trait strains to eight. The selected bacterial strains were further evaluated for PGP attributes associated with biofilm formation, compatibility, salt tolerance ability and effect on plant growth. In vitro studies demonstrated that the multi-trait isolates, Bacillus (1.SG.7, 5.SG.3) and Pseudomonas (2.SG.20, 2.C.19) strains, enhanced the lateral roots abundance and shoots biomass, mitigated salinity stress, suggesting the utility of beneficial ARS-producing bacteria as potential microbial fertilizers. Furthermore, in vitro studies demonstrated that compatible combinations of multi-trait isolates, Bacillus sp. 1.SG.7 in a mixture coupled with 5.SG.3, and 2.C.19 with 5.SG.3 belonging to Bacillus and Pseudomonas, respectively, may enhance plant growth as compared to single inoculants.

Application of PTR-TOF-MS for the quality assessment of lactose-free milk: Effect of storage time and employment of different lactase preparations

Lactose-free dairy products undergo several chemical modifications during shelf life because of the reactivity of glucose and galactose produced by the lactose enzymatic hydrolysis. In this study, proton transfer reaction-mass spectrometry (PTR-MS), coupled with a time-of-flight (TOF) mass analyzer, was applied to get an insight on the phenomena occurring during the shelf life of ultrahigh-temperature (UHT) lactose-free milk (LFM). UHT LFMs produced by three different commercial lactase preparations were evaluated during storage at 20°C over a 150 days period, sampling the milk every 30 days. Production was repeated three times, on three consecutive weeks, in order to take milk variability into consideration. Principal component analysis applied to the whole "volatilome" data demonstrated the capability of PTR-TOF-MS in detecting the milk batch-to-batch variability: Freshly produced milk samples were distinguished based on the week of production at the beginning of shelf life. Additionally, a clear evolution of the volatiles organic compounds (VOCs) profiling during storage was highlighted. Further statistical analysis confirmed VOCs temporal evolution, mostly because of changes in methyl ketones concentration. Differences caused by the commercial lactases did not emerged, except for benzaldehyde. Altogether, data demonstrated PTR-TOF-MS analysis as a valuable and rapid method for the detection of changes in the VOCs profiling of UHT LFM.

Dietary Compounds Influencing the Sensorial, Volatile and Phytochemical Properties of Bovine Milk

The main aim of this study was to evaluate the volatile profile, sensory perception, and phytochemical content of bovine milk produced from cows fed on three distinct feeding systems, namely grass (GRS), grass/clover (CLV), and total mixed ration (TMR). Previous studies have identified that feed type can influence the sensory perception of milk directly via the transfer of volatile aromatic compounds, or indirectly by the transfer of non-volatile substrates that act as precursors for volatile compounds. In the present study, significant differences were observed in the phytochemical profile of the different feed and milk samples. The isoflavone formonoetin was significantly higher in CLV feed samples, but higher in raw GRS milk, while other smaller isoflavones, such as daidzein, genistein, and apigenin were highly correlated to raw CLV milk. This suggests that changes in isoflavone content and concentration in milk relate to diet, but also to metabolism in the rumen. This study also found unique potential volatile biomarkers in milk (dimethyl sulfone) related to feeding systems, or significant differences in the concentration of others (toluene, p-cresol, ethyl and methyl esters) based on feeding systems. TMR milk scored significantly higher for hay-like flavor and white color, while GRS and CLV milk scored significantly higher for a creamy color. Milk samples were easily distinguishable by their volatile profile based on feeding system, storage time, and pasteurization.

Assimilation of alternative sulfur sources in fungi

Fungi are well known for their metabolic versatility, whether it is the degradation of complex organic substrates or the biosynthesis of intricate secondary metabolites. The vast majority of studies concerning fungal metabolic pathways for sulfur assimilation have focused on conventional sources of sulfur such as inorganic sulfur ions and sulfur-containing biomolecules. Less is known about the metabolic pathways involved in the assimilation of so-called “alternative” sulfur sources such as sulfides, sulfoxides, sulfones, sulfonates, sulfate esters and sulfamates. This review summarizes our current knowledge regarding the structural diversity of sulfur compounds assimilated by fungi as well as the biochemistry and genetics of metabolic pathways involved in this process. Shared sequence homology between bacterial and fungal sulfur assimilation genes have lead to the identification of several candidate genes in fungi while other enzyme activities and pathways so far appear to be specific to the fungal kingdom. Increased knowledge of how fungi catabolize this group of compounds will ultimately contribute to a more complete understanding of sulfur cycling in nature as well as the environmental fate of sulfur-containing xenobiotics.

A natural variant of arylsulfatase from Kluyveromyces lactis shows no formylglycine modification and has no enzyme activity

Kluyveromyces lactis is a common fungal microorganism used for the production of enzyme preparations such as β-galactosidases (native) or chymosin (recombinant). It is generally important that enzyme preparations have no unwanted side activities. In the case of β-galactosidase preparations produced from K. lactis, an unwanted side activity could be the presence of arylsulfatase (EC 3.1.6.1). Due to the action of arylsulfatase, an unpleasant "cowshed-like" off-flavor would occur in the final product. The best choice to avoid this is to use a yeast strain without this activity. Interestingly, we found that certain natural K. lactis strains express arylsulfatases, which only differ in one amino acid at position 139. The result of this difference is that K. lactis DSM 70799 (expressing R139 variant) shows no arylsulfatase activity, unlike K. lactis GG799 (expressing S139 variant). After recombinant production of both variants in Escherichia coli, the R139 variant remains inactive, whereas the S139 variant showed full activity. Mass spectrometric analyses showed that the important posttranslational modification of C56 to formylglycine was not found in the R139 variant. By contrast, the C56 residue of the S139 variant was modified. We further investigated the packing and secondary structure of the arylsulfatase variants using optical spectroscopy, including fluorescence and circular dichroism. We found out that the inactive R139 variant exhibits a different structure regarding folding and packing compared to the active S139 variant. The importance of the amino acid residue 139 was documented further by the construction of 18 more variants, whereof only ten showed activity but always reduced compared to the native S139 variant.

Improvement thermostability of Pseudoalteromonas carrageenovora arylsulfatase by rational design

This study aimed to improve the thermostability of arylsulfatase from Pseudoalteromonas carrageenovora. A total of 10 single-site mutants were chosen using the PoPMuSiC program, and two mutants of K253N and P314T showed enhanced thermal stability. By saturation mutagenesis and thermostability analysis, K253H and P314T were the best mutants at the two sites. Combinational mutations of K253H, P314T and H260L were subsequently introduced, and the best mutant of K253H/H260L was selected. Thermal inactivation analysis showed the half-life (t_1/2) value at 55°C for K253H/H260L was 7.7-fold that of the wild-type enzyme (WT), meanwhile this mutant maintained the specific enzyme activity. Structure modeling demonstrated that the additional hydrogen bonds, optimization of surface charge-charge interactions, and increasing of hydrophobic interaction could account for the improved thermostability imparted by K253H/H260L.

Heterologous expression in Pichia pastoris and biochemical characterization of the unmodified sulfatase from Fusarium proliferatum LE1

Sulfatases are a family of enzymes (sulfuric ester hydrolases, EC 3.1.6.-) that catalyze the hydrolysis of a wide array of sulfate esters. To date, despite the discovery of many sulfatase genes and the accumulation of data on numerous sulfated molecules, the number of characterized enzymes that are key players in sulfur metabolism remains extremely limited. While mammalian sulfatases are well studied due to their involvement in a wide range of normal and pathological biological processes, lower eukaryotic sulfatases, especially fungal sulfatases, have not been thoroughly investigated at the biochemical and structural level. In this paper, we describe the molecular cloning of Fusarium proliferatum sulfatase (F.p.Sulf-6His), its recombinant expression in Pichia pastoris as a soluble and active cytosolic enzyme and its detailed characterization. Gel filtration and native electrophoretic experiments showed that this recombinant enzyme exists as a tetramer in solution. The enzyme is thermo-sensitive, with an optimal temperature of 25°C. The optimal pH value for the hydrolysis of sulfate esters and stability of the enzyme was 6.0. Despite the absence of the post-translational modification of cysteine into Cα-formylglycine, the recombinant F.p.Sulf-6His has remarkably stable catalytic activity against p-nitrophenol sulfate, with kcat = 0.28 s-1 and Km = 2.45 mM, which indicates potential use in the desulfating processes. The currently proposed enzymatic mechanisms of sulfate ester hydrolysis do not explain the appearance of catalytic activity for the unmodified enzyme. According to the available models, the unmodified enzyme is not able to perform multiple catalytic acts; therefore, the enzymatic mechanism of sulfate esters hydrolysis remains to be fully elucidated.

Effect of Storage on Lactase-Treated β-Casein and β-Lactoglobulin with Respect to Bitter Peptide Formation and Subsequent in Vitro Digestibility

Using active lactose to hydrolyze lactose during storage is a common process to produce lactose-hydrolyzed (LH) milk. Proteolysis induced by residual proteases in commercial lactase was studied in a system using purified β-casein or β-lactoglobulin during a 60-day storage period at 22 or 38 °C. The proteolysis of β-casein by residual proteases occurred more extensively than that of β-lactoglobulin. Peptidomic analysis by LC-ESI-MS/MS revealed that Ile, Leu, Tyr, and Phe residues near the C-terminus of β-casein were the main sites of cleavage by the residual proteases, generating assumed bitter peptides. In the subsequent in vitro digestion study, proteolysis during storage was shown to greatly affect the subsequent digestibility of β-casein, leading to an elevated degree of hydrolysis and the formation of new digested peptides. This study highlights the potential influence of residual proteases in commercial lactase on the storage quality and digestibility of LH milk containing active lactase.

Detection, production, and application of microbial arylsulfatases

Arylsulfatases are enzymes which catalyze the hydrolysis of arylsulfate ester bonds to release a free sulfonate. They are widespread in nature and are found in microorganisms, most animal and human tissues, and plant seeds. However, this review focuses on arylsulfatases from microbial origin and gives an overview of different assays and substrates used to determine the arylsulfatase activity. Furthermore, the production of microbial arylsulfatases using wild-type organisms as well as the recombinant production using Escherichia coli and Kluyveromyces lactis as expression hosts is discussed. Finally, various potential applications of these enzymes are reviewed.

Production of Bioactive Recombinant Bovine Chymosin in Tobacco Plants

Chymosin (also known as rennin) plays an essential role in the coagulation of milk in the cheese industry. Chymosin is traditionally extracted from the rumen of calves and is of high cost. Here, we present an alternative method to producing bovine chymosin from transgenic tobacco plants. The CYM gene, which encodes a preprochymosin from bovine, was introduced into the tobacco nuclear genome under control of the viral 35S cauliflower mosaic promoter. The integration and transcription of the foreign gene were confirmed with Southern blotting and reverse transcription PCR (RT-PCR) analyses, respectively. Immunoblotting analyses were performed to demonstrate expression of chymosin, and the expression level was quantified by enzyme-linked immunosorbent assay (ELISA). The results indicated recombinant bovine chymosin was successfully expressed at an average level of 83.5 ng/g fresh weight, which is 0.52% of the total soluble protein. The tobacco-derived chymosin exhibited similar native milk coagulation bioactivity as the commercial product extracted from bovine rumen.