Unraveling the role of flexible coil near calcium binding site of levansucrase on thermostability and product profile via proline substitution and molecular dynamics simulations

Enhancing the Thermostability of Phospholipase C by Structural-Based Proline Incorporation to Improve Its Degumming Performance

Thermostability can be improved by introducing prolines into targeted sites, enhancing enzyme performance in specific reactions. In our present study, a novel fungal phospholipase C derived from Talaromyces islandicus (TiPLC) was first heterologously expressed in Pichia pastoris and biochemically characterized. Given the poor thermal stability of TiPLC, a structure-based proline incorporation strategy was used to enhance its thermostability further. Two single-site (E92P and A375P) mutants were selected from seven designs, exhibiting improved stability while retaining wild-type's basic properties (optimum reaction pH and temperature). Compared to the wild-type, the t1/2 of E92P and A375P under 40 °C extended by 1.62 and 1.27 times, respectively. Meanwhile, the E92P and A375P mutants exhibited a 20% increase in activity using p-NPPC as substrate. Moreover, double mutant E92P-A375P exhibited 2.43 times enhancement compared to the wild-type. Results of the oil degumming experiment further confirmed that the double mutant significantly improved the performance of TiPLC, with a reduction in residual phosphorus to 78 ppm, while for the wild-type, the residual phosphorus was 131 ppm under the same reaction. Molecular simulations indicated that proline incorporation into 92 and 375 sites significantly improved the rigidity of partial flexible regions, thus contributing to the increased thermostability.

Investigating the Potential of Phenolic Compounds and Carbohydrates as Acceptor Substrates for Levansucrase-Catalyzed Transfructosylation Reaction

This study characterizes the acceptor specificity of levansucrases (LSs) from Gluconobacter oxydans (LS1), Vibrio natriegens (LS2), Novosphingobium aromaticivorans (LS3), and Paraburkholderia graminis (LS4) using sucrose as fructosyl donor and selected phenolic compounds and carbohydrates as acceptors. Overall, V. natriegens LS2 proved to be the best biocatalyst for the transfructosylation of phenolic compounds. More than one fructosyl unit could be attached to fructosylated phenolic compounds. The transfructosylation of epicatechin by P. graminis LS4 resulted in the most diversified products, with up to five fructosyl units transferred. In addition to the LS source, the acceptor specificity of LS towards phenolic compounds and their transfructosylation products were found to greatly depend on their chemical structure: the number of phenolic rings, the reactivity of hydroxyl groups and the presence of aliphatic chains or methoxy groups. Similarly, for carbohydrates, the transfructosylation yield was dependent on both the LS source and the acceptor type. The highest yield of fructosylated-trisaccharides was Erlose from the transfructosylation of maltose catalyzed by LS2, with production reaching 200 g/L. LS2 was more selective towards the transfructosylation of phenolic compounds and carbohydrates, while reactions catalyzed by LS1, LS3 and LS4 also produced fructooligosaccharides. This study shows the high potential for the application of LSs in the glycosylation of phenolic compounds and carbohydrates.

Improving the thermostability and modulating the inulin profile of inulosucrase through rational glycine-to-proline substitution

The flexibility of protein structure plays a crucial role in enzyme stability and catalysis. Among the amino acids, glycine is particularly important in conferring flexibility to proteins. In this study, the effects of flexible glycine residues in Lactobacillus reuteri 121 inulosucrase (LrInu) on stability and inulin profile were investigated through glycine-to-proline substitutions. Molecular dynamics (MD) simulations were employed to discover the flexible glycine residues, and eight glycine residues, including Gly217, Gly298, Gly330, Gly416, Gly450, Gly624, Gly627, Gly629, were selected for site-directed mutagenesis. The results demonstrated significant changes in both thermostability and inulin profiles of the variants. Particularly, the G624P and G627P variants showed reduced production of long-chain oligosaccharides compared to the WT. This can be ascribed to the increased rigidity of the active site, which is crucial for the induction-fit mechanism. Overall, this study provides valuable insights into the role of flexible glycine residues in the activity, stability, and inulin synthesis of LrInu.