Food-Grade Expression and Characterization of a Dextranase from Chaetomium gracile Suitable for Sugarcane Juice Clarification

The microbial production of dextranase using cheap carbon sources is beneficial to solve the economic loss caused by the accumulation of dextran in syrup. A food-grade microbial cell factory was constructed by introducing the dextranase encoding gene DEX from Chaetomium gracile to the chromosome of Bacillus subtilis, and the antibiotic resistance marker gene was subsequently deleted via the Cre/loxP strategy. The dual-promoter system with a sequentially arranged constitutive P43 promoter resulted in an 85 % increase in DEX expression. Under the optimal fermentation conditions of 10 g/L maltose, 15 g/L casein, 1 g/L Na₂ HPO₄ , 1 g/L FeSO₄ and 8 g/L NaCl, DEX activity was increased from 2.625 to 64.34 U/mL. Recombinant DEX was purified 5.98-fold with a recovery ratio of 26.67 % and specific activity of 3935.02 U/mg. Enzyme activity was optimal at 55 °C and pH 5.0 and remained 80.34 % and 71.36 % of the initial activity at 55 °C and pH 4.0 after 60 min, respectively. The enzyme possessed high activity in the presence of Co²⁺ , while Ag⁺ showed the strongest inhibition ability. The optimal substrate was 20 g/L dextran T-2000. The findings could facilitate the low-cost, large-scale production of food-grade DEX for use in the sugar industry.

Two steps purification, biochemical characterization, thermodynamics and structure elucidation of thermostable alkaline serine protease from Nocardiopsis alba strain OM-5

The Nocardiopsis alba strain OM-5 showed maximum protease production in submerged culture. The OM-5 protease was purified by hydrophobic interaction chromatography. The purified protease of 68 kDa showed maximum activity (3312 ± 1.64 U/mL) at 70 °C and was quite stable at 80 °C up to 4 M NaCl (w/v) at pH 9. The purified protease showed significant activity and stability in different cations, denaturing agents, metal ions, and osmolytes. The thermodynamic parameters including deactivation rate constant (K_d) and half lives (t_1/2) at 50-80 °C were in the range of 2.50 × 10^-3 to 5.50 × 10^-3 and 277.25-111.25 min respectively at 0-4 M NaCl. The structural stability of the OM-5 protease under various harsh conditions was elucidated by circular dichroism (CD) spectroscopy followed by K2D3 analysis revealed that the native structure of OM-5 protease was stable even in sodium dodecyl sulfate and Tween 20 indicated by increased α-helices content assisted with decreased β-sheets content.

On the indirect relationship between protein dynamics and enzyme activity

The behaviors of simple thermal systems have been well studied in physical chemistry and the principles obtained from such studies have been applied to complex thermal systems, such as proteins and enzymes. But the simple application of such principles is questionable and may lead to mistakes under some circumstances. In enzymology, the transition state theory of chemical reactions has been accepted as a fundamental theory, but the role of protein dynamics in enzyme catalysis is controversial in the context of transition state theory. By studying behaviors of complex thermal systems, we have revised the Arrhenius equation and transition state theory and our model is validated in enzymology. Formally speaking, the revised Arrhenius equation is apparently similar to a conventional Arrhenius equation, but the physical meanings of its parameters differ from that of traditional forms in principle. Within this model, the role of protein dynamics in enzyme catalysis is well defined and quantified.