Studies on the non-pathogenicity of Chryseobacterium proteolyticum and on the safety of the enzyme: protein-glutaminase

A novel method for high level production of protein glutaminase by sfGFP tag in Bacillus subtilis

Protein glutaminase (PG; EC 3.5.1.44) is a novel deamidase that helps to improve functional properties of food proteins. Currently, the highest activated PG enzyme activity was 26 U/mg when recombinantly expressed via the twin-arginine translocation (Tat) pathway in Corynebacterium glutamicum. In this study, superfolder green fluorescent protein (sfGFP) was used to replace traditional signal peptides to facilitate efficient heterologous expression and secretion of Propeptide-Protein glutaminase (PP) in Bacillus subtilis. The fusion protein, sfGFP-PP, was secreted from 12 h of fermentation and reached its highest extracellular expression at 28 h, with a secretion efficiency of about 93 %. Moreover, when fusing sfGFP with PP at the N-terminus, it significantly enhances PG expression up to 26 U/mL by approximately 2.2-fold compared to conventional signal-peptides- guided PP with 11.9 U/mL. Finally, the PG enzyme activity increased from 26 U/mL to 36.9 U/mL after promoter and RBS optimization. This strategy not only provides a new approach to increase PG production as well as extracellular secretion but also offers sfGFP as an effective N-terminal tag for increased secreted production of difficult-to-express proteins.

The characteristics of protein-glutaminase from an isolated Chryseobacterium cucumeris strain and its deamidation application

Protein-glutaminase (PG), a deamidation enzyme commercially derived from Chryseobacterium proteolyticum, is used to improve the solubility and other functional properties of food proteins. In this study, a new PG-producing strain, Chryseobacterium cucumeris ZYF120413-7, was isolated from soil, and it had a high PG yield and a short culture time. It gave the maximum PG activity with 0.557 U/ml on Cbz-Gln-Gly after 12 h of culture, indicating that it was more suitable for PG production. The enzyme activity recovery and purification fold were 32.95% and 161.95-fold, respectively, with a specific activity of 27.37 U/mg. The PG was a pre-pro-protein with a 16 amino acids putative signal peptide, a pro-PG of 118 amino acids, and a mature PG of 185 amino acids. The amino acid sequence identity of PG from strain ZYF120413-7 was 74 and 45%, respectively, to that of PG from C. proteolyticum 9670T and BH-PG. The optimum reaction pH and temperature of PG was 6 and 60°C, respectively. Enzyme activity was inhibited by Cu2+. The optimum PG substrate was Cbz-Gln-Gly, and the Km and Vmax values were 1.68 mM and 1.41 μM mg protein−1 min−1, respectively. Degree of deamidation (DD) of soy protein isolate (SPI) treated by purified PG was 40.75% within the first 2 h and 52.35% after 18 h. These results demonstrated that the PG from C. cucumeris ZYF120413-7 was a promising protein-deamidating enzyme for improving the functionality of food proteins.

Protein deamidation to produce processable ingredients and engineered colloids for emerging food applications

With the ever-increasing demands for functional and sustainable foods from the general public, there is currently a paradigm shift in the food industry toward the production of novel protein-based diet. Food scientists are therefore motivated to search for natural protein sources and innovative technologies to modify their chemical structure for desirable functionality and thus utilization. Deamidation is a viable, efficient, and attractive approach for modifying proteins owing to its ease of operating, specificity, and cost-effective processes. Over the past three decades, the knowledge of protein deamidation for food applications has evolved drastically, including the development of novel approaches for deamidation, such as protein-glutaminase and ion exchange resin, and their practices in new protein substrate. Thanks to deamidation, enhanced functionalities of food proteins from cereals, legumes, milk, oil seeds and others, and thereby their processabilities as food ingredients have been achieved. Moreover, deamidated proteins have been used to fabricate engineered food colloids, including self-assembled protein particles, protein-metallic complexes, and protein-carbohydrate complexes, which have demonstrated tailored physicochemical properties to modulate oral perception, improve gastrointestinal digestion and bioavailability, and protect and/or deliver bioactive nutrients. Novel bioactivity, altered digestibility, and varied allergenicity of deamidated proteins are increasingly recognized. Therefore, deamidated proteins with novel techno-functional and biological properties hold both promise and challenges for future food applications, and a comprehensive review on this area is critically needed to update our knowledge and provide a better understanding on the protein deamidation and its emerging applications.

Complete Genome Sequence and Characterization of a Protein-Glutaminase Producing Strain, Chryseobacterium proteolyticum QSH1265

Recently, an enzyme named protein-glutaminase (PG) has been identified as a new type of enzyme with significant potential for deamidation of food proteins. The enzyme is shown to be expressed as a pre-pro-protein with a putative signal peptide of 21 amino acids, a pro-sequence of 114 amino acids, and a mature PG of 185 amino acids. The microbial enzyme PG specifically catalyzes deamidation of proteins without protein hydrolysis pretreatment and only reacts with glutamine residues in the side-chains of proteins or long peptides. All these attributes suggest that it has a great potential for food industrial applications. However, until recently, there have been relatively few studies of the PG-producing strains. A strain named Chryseobacterium proteolyticum QSH1265 which can produce PG was isolated from a soil sample collected in Songjiang, Shanghai, China. Its enzyme activity was about 0.34 ± 0.01 U/mL when using carboxybenzoxy-Gln-Gly as a substrate. The strain can produce acid from D-glucose, maltose, L-arabinose sucrose, glycerol, and mannitol but not fructose, and it is also positive for indole production and urease. Here we describe the complete genome sequence of this strain via PacBio RSII sequencing. The C. proteolyticum QSH1265 genome consists of a circular chromosome with total length of 4,849,803 bp without any plasmids. All of 4563 genes were predicted including 4459 genes for protein-coding and 104 RNA-relative genes with an average G+C content of 36.16%. The KEGG and COG annotation provide information for the specific function of proteins encoded in the genome, such as proteases, chromoproteins, stress proteins, antiporters, etc. A highly conserved hypothetical protein shares a promoter with the gene encoding the protein-glutaminase enzyme. The genome sequence and preliminary annotation provide valuable genetic information for further study of C. proteolyticum.

Chryseobacterium artocarpi sp. nov., isolated from the rhizosphere soil of Artocarpus integer

A bacterial strain, designated UTM-3T, isolated from the rhizosphere soil of Artocarpus integer (cempedak) in Malaysia was studied to determine its taxonomic position. Cells were Gram-stain-negative, non-spore-forming rods, devoid of flagella and gliding motility, that formed yellow-pigmented colonies on nutrient agar and contained MK-6 as the predominant menaquinone. Comparative analysis of the 16S rRNA gene sequence of strain UTM-3T with those of the most closely related species showed that the strain constituted a distinct phyletic line within the genus Chryseobacterium with the highest sequence similarities to Chryseobacterium lactis NCTC 11390T, Chryseobacterium viscerum 687B-08T, Chryseobacterium tructae 1084-08T, Chryseobacterium arthrosphaerae CC-VM-7T, Chryseobacterium oncorhynchi 701B-08T, Chryseobacterium vietnamense GIMN1.005T, Chryseobacterium bernardetii NCTC 13530T, Chryseobacterium nakagawai NCTC 13529T, Chryseobacterium gallinarum LMG 27808T, Chryseobacterium culicis R4-1AT, Chryseobacterium flavum CW-E2T, Chryseobacterium aquifrigidense CW9T, Chryseobacterium ureilyticum CCUG 52546T, Chryseobacterium indologenes NBRC 14944T, Chryseobacterium gleum CCUG 14555T, Chryseobacterium jejuense JS17-8T, Chryseobacterium oranimense H8T and Chryseobacterium joostei LMG 18212T. The major whole-cell fatty acids were iso-C15 : 0 and iso-C17 : 1ω9c, followed by summed feature 4 (iso-C15 : 0 2-OH and/or C16 : 1ω7t) and iso-C17 : 0 3-OH, and the polar lipid profile consisted of phosphatidylethanolamine and several unknown lipids. The DNA G+C content strain UTM-3T was 34.8 mol%. On the basis of the phenotypic and phylogenetic evidence, it is concluded that the isolate represents a novel species of the genus Chryseobacterium , for which the name Chryseobacterium artocarpi sp. nov. is proposed. The type strain is UTM-3T ( = CECT 8497T = KCTC 32509T).

Soy protein hydrolysis with microbial protease to improve antioxidant and functional properties

Soybean proteins are widely used as nutritional and functional food ingredients. This investigation evaluated through a 2(3) central composite design the effect of three variables (pH, temperature and enzyme/substrate (E/S) ratio) on the production of soy protein isolate (SPI) hydrolysates with a microbial protease. Soluble peptides, antioxidant activity, and foaming and emulsifying capabilities of the hydrolysates were analyzed. All variables, as well as their interactions, were significant for the soluble peptides content of SPI hydrolysates. Optimal conditions for obtaining soluble peptides were around 30-35 °C, pH 6.5-9.5, and E/S ratios of 1,650-6,300 U g(-1). SPI hydrolysates produced at 30-45 °C, pH 8.0-9.5, and E/S ratios of 4,000-8,000 U g(-1) showed higher capacity to scavenge the 2,2'-azino-bis-(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) radical. Models for soluble peptides and ABTS activity of hydrolysates were obtained. In the range studied, the variables had not significant influence on the ability of hydrolysates to scavenge the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical. SPI hydrolysates also presented reducing power and ability to chelate iron. Hydrolysis temperature was significant for the Fe(2+)-chelating ability of hydrolysates. Temperature of hydrolysis was significant for the foaming capacity of hydrolysates, with higher values observed at 45 °C and 8,000 U g(-1). For emulsifying capacity, only E/S ratio presented a significant effect. Temperature and E/S ratio appeared to be more significant variables influencing the properties of the SPI hydrolysates. The results of this study indicate that specific hydrolysis conditions should be selected to obtain SPI hydrolysates with preferred characteristics.

Crystal structures of protein glutaminase and its pro forms converted into enzyme-substrate complex

Protein glutaminase, which converts a protein glutamine residue to a glutamate residue, is expected to be useful as a new food-processing enzyme. The crystal structures of the mature and pro forms of the enzyme were refined at 1.15 and 1.73 Å resolution, respectively. The overall structure of the mature enzyme has a weak homology to the core domain of human transglutaminase-2. The catalytic triad (Cys-His-Asp) common to transglutaminases and cysteine proteases is located in the bottom of the active site pocket. The structure of the recombinant pro form shows that a short loop between S2 and S3 in the proregion covers and interacts with the active site of the mature region, mimicking the protein substrate of the enzyme. Ala-47 is located just above the pocket of the active site. Two mutant structures (A47Q-1 and A47Q-2) refined at 1.5 Å resolution were found to correspond to the enzyme-substrate complex and an S-acyl intermediate. Based on these structures, the catalytic mechanism of protein glutaminase is proposed.