Identification of residues involved in active-site formation in Aspergillus ficuum phytase

Unfolding and refolding of Aspergillus niger PhyB phytase: role of disulfide bridges

The role of disulfide bridges in the folding of Aspergillus niger phytase pH 2.5-optimum (PhyB) was investigated using dynamic light scattering (DLS). Guanidinium chloride (GuCl) at 1.0 M unfolded phytase; however, its removal by dialysis refolded the protein. The thiol reagent tris(2-carboxyethyl)phosphine (TCEP) reduces the refolding activity by 68%. The hydrodynamic radius (R(H)) of PhyB phytase decreased from 5.5 to 4.14 nm when the protein was subjected to 1.0 M GuCl concentration. The active homodimer, 183 kDa, was reduced to a 92 kDa monomer. The DLS data taken together with activity measurements could indicate whether refolding took place or not in PhyB phytase. The correlation between molecular mass and the state of unfolding and refolding is a very strong one in fungal phytase belonging to histidine acid phosphatase (HAP). Unlike PhyA phytase, for which sodium chloride treatment boosted the activity at 0.5 M salt concentration, PhyB phytase activity was severely inhibited under identical condition. Thus, PhyA and PhyB phytases are structurally very different, and their chemical environment in the active site and substrate-binding domain may be different to elicit such an opposite reaction to monovalent cations.

Phytases: microbial sources, production, purification, and potential biotechnological applications

The review deals with phytase-producing microorganisms along with optimum conditions for its production. Various methods used for purifying phytases and their characteristics are discussed. Heterologous gene expression, cost-effective large-scale phytase production, and various biotechnological applications of the enzyme in animal feed and food industries are also discussed.

Biochemical characterization of cloned Aspergillus fumigatus phytase (phyA)

The gene for Aspergillus fumigatus phytase (phyA) was cloned and expressed in Pichia pastoris. The enzyme expressed was purified to near homogeneity using sequential ion-exchange chromatography and was characterized biochemically. Although A. fumigatus phytase shows 66.2% sequence homology with A. ficuum phytase, the most widely studied enzyme, the cloned phytase showed identical molecular weight and temperature optima profile to the benchmark phytase. The pH profile of activity and kinetic parameters, however, differed from A. ficuum phytase. The cloned enzyme contains the septapeptide RHGARYP motif, which is also identical to the active site motif of A. ficuum phytase. Chemical probing of the active site Arg residues using both cyclohexanedione and phenylglyoxal resulted in the inactivation of phytase. The cloned A. fumigatus phytase, however, was more resistant to phenylglyoxal-induced inactivation. Both cloned A. fumigatus and A. ficuum phytases were identically affected by cyclohexanedione. Both the thermal characterization data and kinetic parameters of cloned and expressed A. fumigatus phytase indicate that this biocatalyst is not superior to the benchmark enzyme. The sequence difference between A. fumigatus and A. ficuum phytase may explain why the former enzyme catalyzes poorly compared to the benchmark enzyme. In addition, differential sensitivity toward the Arg modifier, phenylglyoxal, indicates a different chemical environment at the active site for each of the phytases.

Identification of a histidine acid phosphatase (phyA)-like gene in Arabidopsis thaliana

A close examination of the protein sequence encoded by the Arabidopsis thaliana gene F21M12.26 reveals the gene product to be a phosphomonoesterase, acid optimum (EC 3.1.3.2). A subclass of this broad acid phosphatase is also known as 'histidine acid phosphatase. ' This is the first sequence-based evidence for a 'histidine acid phosphatase' in a dicotyledon. One important member of this class of enzymes is Aspergillus niger (ficuum) phytase, which came into prominence for its commercial application as a feed additive. The putative protein from A. thaliana gene F21M12.26 shares many important features of Aspergillus phytase, namely, size, active-site sequence, catalytic dipeptide and ten cysteine residues located in the key areas of the molecule, but lacks all nine N-glycosylation sites.

Phytase: sources, preparation and exploitation

This review deals with phytase (myo-inositol hexakisphosphate phosphohydrolase) and covers microbiological sources, phytase occurrence in plants and animals, its purification, physico-chemical and molecular properties. Protein engineering of phytase and potential enzyme applications are discussed.

Cloning of the phytases from Emericella nidulans and the thermophilic fungus Talaromyces thermophilus

Phytases (EC 3.1.3.8) belong to the family of histidine acid phosphatases. We have cloned the phytases of the fungi Emericella nidulans and Talaromyces thermophilus. The putative enzyme encoded by the E. nidulans sequence consists of 463 amino acids and has a Mr of 51785. The protein deduced from the T. thermophilus sequence consists of 466 amino acids corresponding to a Mr of 51450. Both predicted amino acid sequences exhibited high identity (48% to 67%) to known phytases. This high level of identity allowed the modelling of all available fungal phytases based on the three-dimensional structure coordinates of the Aspergillus niger phytase. By this approach we identified 21 amino acids which are conserved in fungal phyA phytases and are part of the residues forming the substrate pocket. Furthermore, potential glycosylation sites were identified and compared between the aforementioned phytases and the A. niger phytase.

Crystal structure of phytase from Aspergillus ficuum at 2.5 A resolution

Phytase is a high molecular weight acid phosphatase. The structure has an alpha/beta-domain similar to that of rat acid phosphatase and an alpha-domain with a new fold.

Phytase

Of all the sources of phytase that have been studied (plant, animal, and microorganisms), the highest yields are produced by a wild-type strain A. niger NRRL 3135 (12.7 mg P/hr/ml = 6.8 microns P/ml/min = 113.9 nKat/ml) in a mineral salt medium in which total phosphate (4 mg %) is limiting for growth and cornstarch and glucose are the carbon sources. Synthesis of the enzyme is repressed by phosphate in the wild-type strain. Aspergillus niger NRRL 3135 produces two phytases one with pH optima at 2.5 and 5.5 (phyA) and one with an optimum at pH 2.0 (phyB). It also produces a pH 6.0 optimum phosphatase that has no phytase activity. These three glycoproteins have been purified to homogeneity, characterized, sequenced, and cloned. The sequences have been compared to each other, other phytases, and to known phosphatases. Their homology has been determined. The active sites of phytases show remarkable homology to the active site residues of the members of a particular class of acid phosphatase (histidine phosphatase). The most conserved sequence is RHGXRXP. Phytase has been covalently immobilized on Fractogel TSK HW-75 F and glutaraldehyde-activated silicate. It has been immobilized on agarose. Losses of activity have been noted on immobilization but these may be minimized by future research. It should be possible to commercially produce and recover penta-, tetra-, tri-, di-, and monoinositol phosphates using immobilized phytase if markets develop for those products. Phytase (phyA) from A. niger NRRL 3135 has been cloned into an A. niger glucoamylase producing strain CBS 513.88 using a construct that has a glucoamylae promoter and an A. niger NRRL 3135 leader sequence, and that is devoid of phosphate repression. The yield of the secreted enzyme was increased 52-fold above that of wild-type A. niger NRRL 3135. The bioengineered organism produces 270 microns P/ml/min (4500 nKat/ml) which is approximately 7.9 g/liter in the medium. The yield of the secreted enzyme was increased 1440-fold above that of wild type CBS 513.88. Commercial preparations of the cloned enzyme are available. Phytase (phyA) has been cloned into tobacco and canola. The enzyme is localized in the seed and expressed at high levels. Feeding of the seed to animals has made the phytin-P in the commercial diets available to the animals. The efficacy of feeding phytase to monogastric animals (poultry and swine) has been established. The amount of enzyme that is necessary to be added to commercial diets has been titred for broilers, layers, turkeys, ducks, and swine. The units of enzyme required are related to the phytin-P content in the diet. The use of the enzyme as a feed additive has been cleared in 22 countries. If phytase were used in the diets of all of the monogastric animals reared in the U.S., it would release phosphorus that has a value of $1.68 x 10(8) per year. The FDA has approved the enzyme preparation as GRAS. The effect of feeding phytase to animals enables assimilation of the P found in feed ingredients and diminishes the amount of phosphate in the manure and subsequently entering the environment. The effect of feeding phytase to animals on pollution has been quantitatively determined. If phytase were used in the diets of all of the monogastric animals reared in the United States, it would preclude 8.23 x 10(7) kg P from entering the environment.