Inhibitory effects of substrate analogues on enzyme activity and substrate specificities of mushroom tyrosinase

Optimizing Skim Milk Yogurt Properties: Combined Impact of Trans-glutaminase and Protein-Glutaminase

The lack of fat in yogurt can lead to alterations in taste and whey separation, reducing consumer acceptance. In this study, the feasibility of enhancing the quality of skim milk yogurt through a combination of transglutaminase (TG) and protein-glutaminase (PG) was investigated. The combination of TG and PG resulted in simultaneous cross-linking and deamidated of casein micelles, with PG deamidation taking priority over TG cross-linking, leading to higher solubility and lower turbidity of milk proteins compared with TG alone. When 0.06 U/mL TG and 0.03 U/mL PG were added, firmness and viscosity indexes significantly increased by 38.26 and 78.59%, respectively as compared with the control. Microscopic images revealed increased cross-linking with casein and filling of cavities by smaller sub-micelles in the combination of TG and PG treatment. Furthermore, the combination of TG and PG resolved issues of rough taste and whey separation, leading to improved overall liking. This study highlights the benefits of using both enzymes in dairy production and has important implication for future research.

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.

Characterization of Tyrosinase Inhibitors in Dryopteris crassirhizoma Rhizome Using a Combination of High-Speed Counter-Current Chromatography, Affinity-Based Ultrafiltration, and Liquid Chromatography-Tandem Mass Spectrometry

Dryopteris crassirhizoma rhizome (DCR) inhibits melanin production in B16F10 melanoma cells and tyrosinase activity. The melanin content and tyrosinase activity of DCR-treated zebrafish embryos were determined to evaluate the in vivo inhibitory effect of DCR on melanogenesis. Moreover, an off-line hyphenated method combining the high-speed counter-current chromatography, affinity-based ultrafiltration, and liquid chromatography-tandem mass spectrometry was used to identify and characterize the DCR compounds with tyrosinase inhibitory activity. Our results indicated that DCR significantly decreased the melanin content and tyrosinase activity in zebrafish embryos in a dose-dependent manner; moreover, 22 compounds in DCR presented tyrosinase inhibitory activity. In silico molecular docking prediction data indicated that the 22 compounds in DCR can form stable conformations in the active site pocket of tyrosinase.

Inhibition kinetics of chlorobenzaldehyde thiosemicarbazones on mushroom tyrosinase

2-Chlorobenzaldehyde thiosemicarbazone (2-Cl-BT) and 4-chlorobenzaldehyde thiosemicarbazone (4-Cl-BT) were synthesized, and their inhibitory kinetics on the activity of mushroom tyrosinase were investigated. Results showed that these compounds exhibited significant inhibitory potency on both monophenolase activity and diphenolase activity of tyrosinase. For the monophenolase activity, both compounds could decrease the steady-state activity of the enzyme sharply, without any influence on the lag period. The IC50 values of them were estimated to be 15.4 μM and 6.7 μM, respectively. For the diphenolase activity, both compounds belonged to reversible inhibitors, but their mechanisms were different: 2-Cl-BT was a noncompetitive type inhibitor, while 4-Cl-BT was a mixed-type inhibitor. Their inhibition constants were determined and compared.

Inhibitory effects of methyl trans-cinnamate on mushroom tyrosinase and its antimicrobial activities

The control of food browning and growth of disease-causing microorganisms is critical to maintaining the quality and safety of food. Tyrosinase is the key enzyme in food browning. The inhibitory effect of methyl trans-cinnamate on the activity of tyrosinase has been investigated. Methyl trans-cinnamate can strongly inhibit both monophenolase and diphenolase activity of mushroom tyrosinase. When the concentration of methyl trans-cinnamate reached 2.5 mM, the lag time was lengthened from 32 to 160 s and the steady-state activity was lost about 65%. The IC(50) value was 1.25 mM. For the diphenolase activity, the inhibition of methyl trans-cinnamate displayed a reversible and noncompetitive mechanism. The IC(50) value was 1.62 mM, and the inhibition constant (K(I)) was determined to be 1.60 mM. Moreover, the antibacterial activity against Escherichia coli, Bacillus subtilis and Staphyloccocus aureus and antifungal activity against Candida albicans were tested. The results showed that methyl trans-cinnamate possessed an antimicrobial ability.