Chemical and thermal stability of ferulic acid esterase-III from Aspergillus niger

Expression of a fungal ferulic acid esterase in alfalfa modifies cell wall digestibility

BACKGROUND

Alfalfa (Medicago sativa) is an important forage crop in North America owing to its high biomass production, perennial nature and ability to fix nitrogen. Feruloyl esterase (EC 3.1.1.73) hydrolyzes ester linkages in plant cell walls and has the potential to further improve alfalfa as biomass for biofuel production.

RESULTS

In this study, faeB [GenBank:AJ309807] was synthesized at GenScript and sub-cloned into a novel pEACH vector containing different signaling peptides to target type B ferulic acid esterase (FAEB) proteins to the apoplast, chloroplast, endoplasmic reticulum and vacuole. Four constructs harboring faeB were transiently expressed in Nicotiana leaves, with FAEB accumulating at high levels in all target sites, except chloroplast. Stable transformed lines of alfalfa were subsequently obtained using Agrobacterium tumefaciens (LBA4404). Out of 136 transgenic plants regenerated, 18 independent lines exhibited FAEB activity. Subsequent in vitro digestibility and Fourier transformed infrared spectroscopy (FTIR) analysis of FAEB-expressing lines showed that they possessed modified cell wall morphology and composition with a reduction in ester linkages and elevated lignin content. Consequently, they were more recalcitrant to digestion by mixed ruminal microorganisms. Interestingly, delignification by alkaline peroxide treatment followed by exposure to a commercial cellulase mixture resulted in higher glucose release from transgenic lines as compared to the control line.

CONCLUSION

Modifying cell wall crosslinking has the potential to lower recalcitrance of holocellulose, but also exhibited unintended consequences on alfalfa cell wall digestibility due to elevated lignin content. The combination of efficient delignification treatment (alkaline peroxide) and transgenic esterase activity complement each other towards efficient and effective digestion of transgenic lines.

Enhanced thermostability of keratinase by computational design and empirical mutation

Keratinases are proteolytic enzymes capable of degrading insoluble keratins. The importance of these enzymes is being increasingly recognized in fields as diverse as animal feed production, textile processing, detergent formulation, leather manufacture, and medicine. To enhance the thermostability of Bacillus licheniformis BBE11-1 keratinase, the PoPMuSiC algorithm was applied to predict the folding free energy change (ΔΔG) of amino acid substitutions. Use of the algorithm in combination with molecular modification of homologous subtilisin allowed the introduction of four amino acid substitutions (N122Y, N217S, A193P, N160C) into the enzyme by site-directed mutagenesis, and the mutant genes were expressed in Bacillus subtilis WB600. The quadruple mutant displayed synergistic or additive effects with an 8.6-fold increase in the t 1/2 value at 60 °C. The N122Y substitution also led to an approximately 5.6-fold increase in catalytic efficiency compared to that of the wild-type keratinase. These results provide further insight into the thermostability of keratinase and suggest further potential industrial applications.

Identification of amino acid residues responsible for increased thermostability of feruloyl esterase A from Aspergillus niger using the PoPMuSiC algorithm

Feruloyl esterases are key enzymes involved in the complete hydrolysis of hemicellulose. In order to improve the thermostability of feruloyl esterase A (FaeA) from Aspergillus niger CIB 423.1, the PoPMuSiC algorithm was applied to predict the folding free energy change (ΔΔG) of amino acid substitutions. Four amino acid substitutions (S92A, D93G, D174A and S187F) were introduced into the enzyme by site-directed mutagenesis and the enzymes were produced in Pichia pastoris KM71. No obvious changes in thermal stability resulted from substitutions S92A and D174A, but, compared to the wild-type enzyme which has a half-life of inactivation of 8 min, the half-lives of enzymes with a D93G or S187F substitution increased to 9.4 and 60.5 min, respectively. The double mutant D93G/S187F displayed a synergistic effect with a t1/2 value of 77.0 min. It also displayed over 10-fold increase in catalytic turnover frequency. The result will benefit further investigation of the thermostability of feruloyl esterase A.

Feruloyl esterases as biotechnological tools: current and future perspectives

Feruloyl esterases represent a diverse group of hydrolases catalyzing the cleavage and formation of ester bonds between plant cell wall polysaccharide and phenolic acid. They are widely distributed in plants and microorganisms. Besides lipases, a considerable number of microbial feruloyl esterases have also been discovered and overexpressed. This review summarizes the latest research on their classification, production, and biophysicochemical properties. Special emphasis is given to the importance of that type of enzyme and their related phenolic ferulic acid compound in biotechnological processes, and industrial and medicinal applications.

Ferulic acid: an antioxidant found naturally in plant cell walls and feruloyl esterases involved in its release and their applications

Ferulic acid is the most abundant hydroxycinnamic acid in the plant world and maize bran with 3.1% (w/w) ferulic acid is one of the most promising sources of this antioxidant. The dehydrodimers of ferulic acid are important structural components in the plant cell wall and serve to enhance its rigidity and strength. Feruloyl esterases are a subclass of the carboxylic acid esterases that hydrolyze the ester bond between hydroxycinnamic acids and sugars present in plant cell walls and they have been isolated from a wide range of microorganisms, when grown on complex substrates such as cereal brans, sugar beet pulp, pectin and xylan. These enzymes perform a function similar to alkali in the deesterification of plant cell wall and differ in their specificities towards the methyl esters of cinnamic acids and ferulolylated oligosaccharides. They act synergistically with xylanases and pectinases and facilitate the access of hydrolases to the backbone of cell wall polymers. The applications of ferulic acid and feruloyl esterase enzymes are many and varied. Ferulic acid obtained from agricultural byproducts is a potential precursor for the production of natural vanillin, due to the lower production cost.

Molecular imprinting: a dynamic technique for diverse applications in analytical chemistry

Continuous advances in analyzing complex matrices, improving reliability and simplicity, and performing multiple simultaneous assays with extreme sensitivity are increasing. Several techniques have been developed for the quantitative assays of analytes at low concentrations (e.g., high-pressure liquid chromatography, gas chromatography, immunoassay and the polymerase chain reaction technique). To achieve highly specific and sensitive analysis, high affinity, stable, and specific recognition agents are needed. Although biological recognition agents are very specific and sensitive they are labile and/or have a low density of binding sites. During the past decade molecular imprinting has emerged as an attractive and highly accepted tool for the development of artificial recognition agents. Molecular imprinting is achieved by the interaction, either noncovalent or covalent, between complementary groups in a template molecule and functional monomer units through polymerization or polycondensation. These molecularly imprinted polymers have been widely employed for diverse applications (e.g., in chromatographic separation, drug screening, chemosensors, catalysis, immunoassays etc.) owing to their specificity towards the target molecules and high stability against physicochemical perturbations. In this review the advantages, applications, and recent developments in molecular imprinting technology are highlighted.

Catalytically active polymers obtained by molecular imprinting and their application in chemical reaction engineering

Molecular imprinting is a way of creating polymers bearing artificial receptors. It allows the fabrication of highly selective plastics by polymerizing monomers in the presence of a template. This technique primarily had been developed for the generation of biomimetic materials to be used in chromatographic separation, in extraction approaches and in sensors and assays. Beyond these applications, in the past few years molecular imprinting has become a tool for producing new kinds of catalysts. For catalytic applications, the template must be chosen, so that it is structurally comparable with the transition state (a transition state analogue, TSA) of a reaction, or with the product or substrate. The advantage of using these polymeric catalysts is obvious: the backbone withstands more aggressive conditions than a bio material could ever survive. Results are presented showing the applicability of a molecularly imprinted catalyst in different kinds of chemical reactors. It is demonstrated that the catalysts can be utilized not only in batch but also in continuously driven reactors and that their performance can be improved by means of chemical reaction engineering.