High-level production of recombinant Aspergillus niger cinnamoyl esterase (FAEA) in the methylotrophic yeast Pichia pastoris

Expression of a synthetic gene encoding a Tribolium castaneum carboxylesterase in Pichia pastoris

This is the first report of an insect esterase efficiently expressed in the methylotrophic yeast Pichia pastoris (so far insect esterases have been produced only in the baculovirus system). Having isolated a Tribolium castaneum carboxylesterase cDNA (TCE), we were initially unable to express it in Escherichia coli or P. pastoris despite significant transcription levels. As codon usage bias is different in T. castaneum and P. pastoris, we assumed this was a possible explanation for the translational barrier observed in yeast. Accordingly, we designed and constructed by recursive PCR a synthetic TCE gene (synTCE) optimized for heterologous expression in P. pastoris, i.e., a gene in which certain TCE codons are replaced with synonymous codons 'preferred' in P. pastoris. When the altered gene was placed under the control of either the P. pastoris glyceraldehyde-3-phosphate dehydrogenase (GAP) promoter or the inducible alcohol oxidase (AOX1) promoter and introduced on an expression vector into P. pastoris, its product was produced intracellularly. We also successfully explored the possibility of obtaining a secreted product: P. pastoris cells expressing an in-frame fusion of synTCE with the alpha-factor secretion signal under the control of the GAP promoter were found to secrete the recombinant esterase into the external medium (to a concentration of 7 mg/L). In addition to this demonstration of TCE production in yeast, our results suggest that the GAP promoter could advantageously replace the AOX1 promoter as a driver of synTCE expression. TCE specific activity was approximately 5 U/mg when p-nitrophenyl acetate was used as substrate.

Biochemical characterization of recombinant acetyl xylan esterase from Aspergillus awamori expressed in Pichia pastoris: Mutational analysis of catalytic residues

We engineered an acetyl xylan esterase (AwaxeA) gene from Aspergillus awamori into a heterologous expression system in Pichia pastoris. Purified recombinant AwAXEA (rAwAXEA) displayed the greatest hydrolytic activity toward alpha-naphthylacetate (C2), lower activity toward alpha-naphthylpropionate (C3) and no detectable activity toward acyl-chain substrates containing four or more carbon atoms. Putative catalytic residues, Ser(119), Ser(146), Asp(168) and Asp(202), were substituted for alanine by site-directed mutagenesis. The biochemical properties and kinetic parameters of the four mutant enzymes were examined. The S119A and D202A mutant enzymes were catalytically inactive, whereas S146A and D168A mutants displayed significant hydrolytic activity. These observations indicate that Ser(119) and Asp(202) are important for catalysis. The S146A mutant enzyme showed lower specific activity toward the C2 substrate and higher thermal stability than wild-type enzyme. The lower activity of S146A was due to a combination of increased K(m) and decreased k(cat). The catalytic efficiency of S146A was 41% lower than that of wild-type enzyme. The synthesis of ethyl acetate was >10-fold than that of ethyl n-hexanoate synthesis for the wild-type, S146A and D168A mutant enzymes. However, the D202A showed greater synthetic activity of ethyl n-hexanoate as compared with the wild-type and other mutants.

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.

Production and applications of esterases

Esterase plays a major role in the degradation of natural materials and industrial pollutants, viz., cereal wastes, plastics, and other toxic chemicals. It is useful in the synthesis of optically pure compounds, perfumes, and antioxidants. The potential applications of esterase with reference to agriculture, food, and pharmaceutical industries, are discussed in this review. Promising applications in this avenue can be supported by appropriate production strategies.

Functional importance of Asp37 from a family 11 xylanase in the binding to two proteinaceous xylanase inhibitors from wheat

Aspergillus niger xylanase is a target enzyme of the two wheat proteinaceous inhibitors, XIP-I and TAXI-I. We previously suggested that the xylanase "thumb" region was XIP-I binding site. Here, we expressed the Asp37Ala mutant in Pichia pastoris and showed that the mutation abolished the enzyme capacity to interact with both inhibitors, suggesting a direct contact at the active site. The mutant pH profile was altered, confirming the key role of Asp37 in determining the pH optima of glycoside hydrolase family 11. The results are consistent with a competitive inhibition mode and underline the strategic importance of Asp37 in the inhibition mechanism.

The inhibition specificity of recombinant Penicillium funiculosum xylanase B towards wheat proteinaceous inhibitors

The filamentous fungus Penicillium funiculosum produces a mixture of modular and non-modular xylanases belonging to different glycoside hydrolase (GH) families. In the present study, we heterologously expressed the cDNA encoding GH11 xylanase B (XYNB) and studied the enzymatic properties of the recombinant enzyme. Expression in Escherichia coli led to the partial purification of a glutathione fusion protein from the soluble fraction whereas the recombinant protein produced in Pichia pastoris was successfully purified using a one-step chromatography. Despite O-glycosylation heterogeneity, the purified enzyme efficiently degraded low viscosity xylan [K(m)=40+/-3 g l(-1), V(max)=16.1+/-0.8 micromol xylose min(-1) and k(cat)=5405+/-150 s(-1) at pH 4.2 and 45 degrees C] and medium viscosity xylan [K(m)=34.5+/-3.2 g l(-1), V(max)=14.9+/-1.0 micromol xylose min(-1)k(cat)=4966+/-333 s(-1) at pH 4.2 and 45 degrees C]. XYNB was further tested for its ability to interact with wheat xylanase inhibitors. The xylanase activity of XYNB produced in P. pastoris was strongly inhibited by both XIP-I and TAXI-I in a competitive manner, with a K(i) of 89.7+/-8.5 and 2.9+/-0.3 nM, respectively, whereas no inhibition was detected with TAXI-II. Physical interaction of both TAXI-I and XIP-I with XYNB was observed using titration curves across a pH range 3-9.

The Crystal Structure of Feruloyl Esterase A from Aspergillus niger Suggests Evolutive Functional Convergence in Feruloyl Esterase Family

As a component of the array of enzymes produced by micro-organisms to deconstruct plant cell walls, feruloyl esterases hydrolyze phenolic groups involved in the cross-linking of arabinoxylan to other polymeric structures. This is important for opening the cell wall structure, making material more accessible to glycosyl hydrolases. Here, we describe the first crystal structure of the non-modular type-A feruloyl esterase from Aspergillus niger (AnFaeA) solved at 2.5A resolution. AnFaeA displays an alpha/beta hydrolase fold similar to that found in fungal lipases and different from that reported for other feruloyl esterases. Crystallographic and site-directed mutagenesis studies allow us to identify the catalytic triad (Ser133-His247-Asp194) that forms the catalytic machinery of this enzyme. The active-site cavity is confined by a lid (residues 68-80), on the analogy of lipases, and by a loop (residues 226-244) that confers plasticity to the substrate-binding site. The lid presents a high ratio of polar residues, which in addition to a unique N-glycosylation site stabilises the lid in an open conformation, conferring the esterase character to this enzyme. A putative model for bound 5,5'-diferulic acid-linked arabinoxylan has been built, pointing to the more relevant residues involved in substrate recognition. Comparison with structurally related lipases reveals that subtle amino acid and conformational changes within a highly conserved protein fold may produce protein variants endowed with new enzymatic properties, while comparison with functionally related proteins points to a functional convergence after evolutionary divergence within the feruloyl esterases family.

Production and characterization of the Talaromyces stipitatus feruloyl esterase FAEC in Pichia pastoris: identification of the nucleophilic serine

Feruloyl esterases constitute an interesting group of enzymes that have the potential for use over a broad range of applications in the agri-food industries. We report the over-expression and characterization of a novel feruloyl esterase exhibiting broad substrate specificity from Talaromyces stipitatus (FAEC) in Pichia pastoris. Using various gene constructions, we have investigated the use of alternative signal peptides to produce an authentic feruloyl esterase featuring the N-terminal sequence determined for the native enzyme. We demonstrate that additional amino acids at the N-terminus of the FAEC sequence do not influence the catalytic capacity of the enzyme, and that the nature of the signal sequence has a limited effect on the yield of the secreted enzyme, with the T. stipitatus FAEC signal sequence producing 297 mgL(-1), the Neurospora crassa Fae-1 260 mgL(-1), and the Saccharomyces cerevisiae alpha-factor secretion signal 214 mgL(-1). Mature FAEC contains two internal peptide sequences that correspond with the consensus motif G-X-S-X-G that contains the catalytic serine nucleophile, which is conserved in the esterase enzyme superfamily. The serine residues at the center of these peptide motifs have been independently mutated and the corresponding enzymes have been over-expressed in P. pastoris to identify the candidate nucleophilic residue responsible for catalyzing the enzymatic reaction. Purified recombinant FAEC containing S465A retained the esterase activity and appeared unaffected by the amino acid modification. In contrast, FAEC activity containing S166A was below the HPLC detection limit, suggesting that serine 166 constitutes the nucleophile.

A non-modular type B feruloyl esterase from Neurospora crassa exhibits concentration-dependent substrate inhibition

Feruloyl esterases, a subclass of the carboxylic acid esterases (EC 3.1.1.1), are able to hydrolyse the ester bond between the hydroxycinnamic acids and sugars present in the plant cell wall. The enzymes have been classified as type A or type B, based on their substrate specificity for aromatic moieties. We show that Neurospora crassa has the ability to produce multiple ferulic acid esterase activities depending upon the length of fermentation with either sugar beet pulp or wheat bran substrates. A gene identified on the basis of its expression on sugar beet pulp has been cloned and overexpressed in Pichia pastoris. The gene encodes a single-domain ferulic acid esterase, which represents the first report of a non-modular type B enzyme (fae-1 gene; GenBank accession no. AJ293029). The purified recombinant protein has been shown to exhibit concentration-dependent substrate inhibition (K(m) 0.048 mM, K (i) 2.5 mM and V(max) 8.2 units/mg against methyl 3,4-dihydroxycinnamate). The kinetic behaviour of the non-modular enzyme is discussed in terms of the diversity in the roles of the feruloyl esterases in the mobilization of plant cell wall materials and their respective modes of action.

Specific characterization of substrate and inhibitor binding sites of a glycosyl hydrolase family 11 xylanase from Aspergillus niger

The importance of aromatic and charged residues at the surface of the active site of a family 11 xylanase from Aspergillus niger was evaluated using site-directed mutagenesis. Ten mutant proteins were heterologously produced in Pichia pastoris, and their biochemical properties and kinetic parameters were determined. The specific activity of the Y6A, Y10A, Y89A, Y164A, and W172A mutant enzymes was drastically reduced. The low specific activities of Y6A and Y89A were entirely accounted for by a change in k(cat) and K(m), respectively, whereas the lower values of Y10A, Y164A, and W172A were due to a combination of increased K(m) and decreased k(cat). Tyr(6), Tyr(10), Tyr(89), Tyr(164), and Trp(172) are proposed as substrate-binding residues, a finding consistent with structural sequence alignments of family 11 xylanases and with the three-dimensional structure of the A. niger xylanase in complex with the modeled xylobiose. All other variants, D113A, D113N, N117A, E118A, and E118Q, retained full wild-type activity. Only N117A lost its sensitivity to xylanase inhibitor protein I (XIP-I), a protein inhibitor isolated from wheat, and this mutation did not affect the fold of the xylanase as revealed by circular dichroism. The N117A variant showed kinetics, pH stability, hydrolysis products pattern, substrate specificity, and structural properties identical to that of the wild-type xylanase. The loss of inhibition, as measured in activity assays, was due to abolition of the interaction between XIP-I and the mutant enzyme, as demonstrated by surface plasmon resonance and electrophoretic titration. A close inspection of the three-dimensional structure of A. niger xylanase suggests that the binding site of XIP-I is located at the conserved "thumb" hairpin loop of family 11 xylanases.

Functional expression of human liver cytosolic beta-glucosidase in Pichia pastoris. Insights into its role in the metabolism of dietary glucosides

Human tissues such as liver, small intestine, spleen and kidney contain a cytosolic beta-glucosidase (CBG) that hydrolyses various beta-d-glycosides, but whose physiological function is not known. Here, we describe the first heterologous expression of human CBG, a system that facilitated a detailed assessment of the enzyme specificity towards dietary glycosides. A full-length CBG cDNA (cbg-1) was cloned from a human liver cDNA library and expressed in the methylotrophic yeast Pichia pastoris at a secretion yield of approximately 10 mg x L-1. The recombinant CBG (reCBG) was purified from the supernatant using a single chromatography step and was shown to be similar to the native enzyme isolated from human liver in terms of physical properties and specific activity towards 4-nitrophenyl-beta-D-glucoside. Furthermore, the reCBG displayed a broad specificity with respect to the glycone moiety of various aryl-glycosides (beta-D-fucosides, alpha-L-arabinosides, beta-D-glucosides, beta-D-galactosides, beta-L-xylosides, beta-D-arabinosides), similar to the native enzyme. For the first time, we show that the human enzyme has significant activity towards many common dietary xenobiotics including glycosides of phytoestrogens, flavonoids, simple phenolics and cyanogens with higher apparent affinities (K(m)) and specificities (k(cat)/K(m)) for dietary xenobiotics than for other aryl-glycosides. These data indicate that human CBG hydrolyses a broad range of dietary glucosides and may play a critical role in xenobiotic metabolism.