In silico studies on the substrate specificity of an l-arabinose isomerase from Bacillus licheniformis

Improved substrate specificity for D-galactose of L-arabinose isomerase for industrial application

L-Arabinose isomerase isolated from Geobacillus stearothermophilus (GSAI) was modified to improve its substrate specificity for D-galactose for the production of D-tagatose, a potential reduced-energy sweetener. Among the selected residues, mutation at residue 18 produced a mutant strain, H18T, which exhibited increased activity for D-galactose compared with the wild-type (WT) enzyme. Analysis of the substrate specificity of H18T showed a 45.4% improvement for D-galactose. Replacing histidine with threonine at residue 18 resulted in approximately 2.7-fold and 1.8-fold higher substrate binding and catalytic efficiency, respectively, for D-galactose. Further enhancement of the specific activity and catalytic efficiency of H18T for D-galactose by up to 2.7-fold and 4.3-fold, respectively, was achieved by adding borate during L-arabinose isomerase catalysis. Moreover, H18T showed thermostability and no destabilization was detected, which is promising for the industrial production of D-tagatose.

Characterization of an L-arabinose isomerase from Bacillus coagulans NL01 and its application for D-tagatose production

Background

L-arabinose isomerase (AI) is a crucial catalyst for the biotransformation of D-galactose to D-tagatose. In previous reports, AIs from thermophilic bacterial strains had been wildly researched, but the browning reaction and by-products formed at high temperatures restricted their applications. By contrast, AIs from mesophilic Bacillus strains have some different features including lower optimal temperatures and lower requirements of metallic cofactors. These characters will be beneficial to the development of a more energy-efficient and safer production process. However, the relevant data about the kinetics and reaction properties of Bacillus AIs in D-tagatose production are still insufficient. Thus, in order to support further applications of these AIs, a comprehensive characterization of a Bacillus AI is needed.

Results

The coding gene (1422 bp) of Bacillus coagulans NL01 AI (BCAI) was cloned and overexpressed in the Escherichia coli BL21 (DE3) strain. The enzymatic property test showed that the optimal temperature and pH of BCAI were 60 °C and 7.5 respectively. The raw purified BCAI originally showed high activity in absence of outsourcing metallic ions and its thermostability did not change in a low concentration (0.5 mM) of Mn²⁺ at temperatures from 70 °C to 90 °C. Besides these, the catalytic efficiencies (k_cat/K_m) for L-arabinose and D-galactose were 8.7 mM^-1 min^-1 and 1.0 mM^-1 min^-1 respectively. Under optimal conditions, the recombinant E. coli cell containing BCAI could convert 150 g L^-1 and 250 g L^-1 D-galactose to D-tagatose with attractive conversion rates of 32 % (32 h) and 27 % (48 h).

Conclusions

In this study, a novel AI from B. coagulans NL01was cloned, purified and characterized. Compared with other reported AIs, this AI could retain high proportions of activity at a broader range of temperatures and was less dependent on metallic cofactors such as Mn²⁺. Its substrate specificity was understood deeply by carrying out molecular modelling and docking studies. When the recombinant E. coli expressing the AI was used as a biocatalyst, D-tagatose could be produced efficiently in a simple one-pot biotransformation system.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-016-0286-5) contains supplementary material, which is available to authorized users.

Improving the Thermostability and Catalytic Efficiency of the d-Psicose 3-Epimerase from Clostridium bolteae ATCC BAA-613 Using Site-Directed Mutagenesis

d-Psicose is a highly valuable rare sugar because of its excellent physiological properties and commercial potential. d-Psicose 3-epimerase (DPEase) is the key enzyme catalyzing the isomerization of d-fructose to d-psicose. However, the poor thermostability and low catalytic efficiency are serious constraints on industrial application. To address these issues, site-directed mutagenesis of Tyr68 and Gly109 of the Clostridium bolteae DPEase was performed. Compared with the wild-type enzyme, the Y68I variant displayed the highest substrate-binding affinity and catalytic efficiency, and the G109P variant showed the highest thermostability. Furthermore, the double-site Y68I/G109P variant was generated and exhibited excellent enzyme characteristics. The Km value decreased by 17.9%; the kcat/Km increased by 1.2-fold; the t1/2 increased from 156 to 260 min; and the melting temperature (Tm) increased by 2.4 °C. Moreover, Co(2+) enhanced the thermostability significantly, including the t1/2 and Tm values. All of these indicated that the Y68I/G109P variant would be appropriate for the industrial production of d-psicose.