Improvement of optimum pH and specific activity of pectate lyase from Bacillus RN.1 using loop replacement

Background: Alkaline pectate lyase plays an important role in papermaking, biological refining and wastewater treatment, but its industrial applications are largely limited owing to its low activity and poor alkali resistance. Methods: The alkaline pectate lyase BspPel from Bacillus RN.1 was heterologously expressed in Escherichia coli BL21 (DE3) and its activity and alkali resistance were improved by loop replacement. Simultaneously, the effect of R260 on enzyme alkaline tolerance was also explored. Results: Recombinant pectate lyase (BspPel-th) showed the highest activity at 60°C and pH 11.0, and showed significant stability over a wide pH range (3.0-11.0). The specific enzyme activity after purification was 139.4 U/mg, which was 4.4 times higher than that of the wild-type enzyme. BspPel-th has good affinity for apple pectin, since the V _max and K _m were 29 μmol/min. mL and 0.46 mol/L, respectively. Molecular dynamics simulation results showed that the flexibility of the loop region of BspPel-th was improved. Conclusion: The modified BspPel-th has considerable potential for industrial applications with high pH processes.

Improving the Catalytic Performance of Pectate Lyase Through Pectate Lyase/Cu3(PO4)2 Hybrid Nanoflowers as an Immobilized Enzyme

Pectate lyases (Pels) can be used in the textile industrial process for cotton scouring and ramie degumming, and its hydrolyzed products oligo galacturonic acid, are high-value added agricultural and health products. In our previous studies, an alkaline pectate lyase PEL168 mutant, PEL3, was obtained with improved specific activity and thermostability. Here, a facile and rapid method for preparing an immobilized PEL3-inorganic hybrid nanoflower was developed, as it could improve its biocatalytic performance. With 0.02 mg/mL (112.2 U/mL) PEL3 in PBS buffer, five different divalent ions, including Mn2+, Ca2+, Co2+, Zn2+, and Cu2+, were used as inorganic component. The results showed that PEL3/Cu3(PO4)2 hybrid nanoflowers presented the highest relative activity with 2.5-fold increase, compared to the free PEL3. X-ray diffraction analysis confirmed that the composition of PEL3/Cu3(PO4)2 hybrid nanoflowers were pectate lyase PEL3 and Cu3(PO4)2⋅5H2O. The optimum temperature and pH of PEL3/Cu3(PO4)2 hybrid nanoflowers were ascertained to be 55°C and pH 9.0, respectively, exhibiting subtle difference from the free PEL3. However, the PEL3/Cu3(PO4)2 hybrid nanoflowers maintained 33% residual activity after 24 h incubation at 55°C, while the free PEL3 completely lost its activity after 18 h incubation at 55°C. Furthermore, over 50% residual activity of the PEL3/Cu3(PO4)2 hybrid nanoflowers was remained, even after four times of repetitive utilization, demonstrating its promising stability for practical application.

Modeling the bioconversion of polysaccharides in a continuous reactor: A case study of the production of oligogalacturonates by Dickeya dadantii

In the quest for a sustainable economy of the Earth's resources and for renewable sources of energy, a promising avenue is to exploit the vast quantity of polysaccharide molecules contained in green wastes. To that end, the decomposition of pectin appears to be an interesting target because this polymeric carbohydrate is abundant in many fruit pulps and soft vegetables. To quantitatively study this degradation process, here we designed a bioreactor that is continuously fed with de-esterified pectin (PGA). Thanks to the pectate lyases produced by bacteria cultivated in the vessel, the PGA is depolymerized into oligogalacturonates (UGA), which are continuously extracted from the tank. A mathematical model of our system predicted that the conversion efficiency of PGA into UGA increases in a range of coefficients of dilution until reaching an upper limit where the fraction of UGA that is extracted from the bioreactor is maximized. Results from experiments with a continuous reactor hosting a strain of the plant pathogenic bacterium Dickeya dadantii and in which the dilution coefficients were varied quantitatively validated the predictions of our model. A further theoretical analysis of the system enabled an a priori comparison of the efficiency of eight other pectate lyase-producing microorganisms with that of D. dadantii Our findings suggest that D. dadantii is the most efficient microorganism and therefore the best candidate for a practical implementation of our scheme for the bioproduction of UGA from PGA.

Dickeya dadantii pectic enzymes necessary for virulence are also responsible for activation of the Arabidopsis thaliana innate immune system

Soft-rot diseases of plants attributed to Dickeya dadantii result from lysis of the plant cell wall caused by pectic enzymes released by the bacterial cell by a type II secretion system (T2SS). Arabidopsis thaliana can express several lines of defence against this bacterium. We employed bacterial mutants with defective envelope structures or secreted proteins to examine early plant defence reactions. We focused on the production of AtrbohD-dependent reactive oxygen species (ROS), callose deposition and cell death as indicators of these reactions. We observed a significant reduction in ROS and callose formation with a bacterial mutant in which genes encoding five pectate lyases (Pels) were disrupted. Treatment of plant leaves with bacterial culture filtrates containing Pels resulted in ROS and callose production, and both reactions were dependent on a functional AtrbohD gene. ROS and callose were produced in response to treatment with a cellular fraction of a T2SS-negative mutant grown in a Pels-inducing medium. Finally, ROS and callose were produced in leaves treated with purified Pels that had also been shown to induce the expression of jasmonic acid-dependent defence genes. Pel catalytic activity is required for the induction of ROS accumulation. In contrast, cell death observed in leaves infected with the wild-type strain appeared to be independent of a functional AtrbohD gene. It was also independent of the bacterial production of pectic enzymes and the type III secretion system (T3SS). In conclusion, the work presented here shows that D. dadantii is recognized by the A. thaliana innate immune system through the action of pectic enzymes secreted by bacteria at the site of infection. This recognition leads to AtrbohD-dependent ROS and callose accumulation, but not cell death.

The structure and mode of action of Caldicellulosiruptor bescii family 3 pectate lyase in biomass deconstruction

The unique active site of the Caldicellulosiruptor bescii family 3 pectate lyase catalytic module (PL3-cat) has been structurally described and synergistic digestion studies with C. bescii cellulase A have been performed on unpretreated biomass. The X-ray structure of PL3-cat was determined at 1.6 Å resolution (PDB entry 4ew9) in complex with the products of trigalacturonic acid. Comparison with family 1 pectate lyase (PL1) structures shows that the active site of the PL3 catalytic module is considerably different. However, on superimposing the identical sugar rings at the -2 subsites conserved interactions could be identified. Interestingly, only one catalytic residue, the lysine that donates the proton to the carboxylate group in the β-elimination reaction of PL1 (Lys108 in PL3-cat), is conserved in PL3 and there is no arginine to abstract the proton from the C5 carbon of the galactouronate ring. This suggests that the reaction mechanism of PL3 requires different catalytic residues. Most interestingly, comparison with other proton-abstraction reactions reveals that in PL3 the α-proton is abstracted by a lysine, in a striking similarity to enolases. These observations led us to propose that in PL3-cat Lys108 is the catalytic base, Glu84 is the catalytic acid and an acidified water molecule completes the anti β-elimination reaction by protonating the O4 atom of the substrate. Also, our digestion experiments with unpretreated switchgrass show that the loadings of C. bescii cellobiohydrolase A (CelA) can be lowered by the addition of PL3 to the reaction mixture. This result suggests that PL3 can significantly improve the deconstruction of unpretreated biomass by allowing other enzymes to better access their preferred substrates.

A 1.5 Å resolution X-ray structure of the catalytic module of Caldicellulosiruptor bescii family 3 pectate lyase

A 1.5 Å resolution X-ray structure of the catalytic module of Caldicellulosiruptor bescii family 3 pectate lyase is reported (PDB entry 3t9g). The resulting structure was refined to an R factor of 0.143 and an R(free) of 0.178. Structural analysis shows that this new structure is very similar to the previously solved structure of a family 3 pectate lyase from Bacillus sp. strain KSM-P15 (PDB entry 1ee6), with a root-mean-square deviation of 0.93 Å and a sequence identity of 53%. This structural similarity is significant considering that C. bescii is a hyperthermophile and Bacillus sp. is a mesophile.