Disulfide bonds are necessary for structure and activity in Aspergillus ficuum phytase

The effect of copper source on the stability and activity of α-tocopherol acetate, butylated hydroxytoulene and phytase

The supplementation of Copper (Cu) is essential for the optimum performance of physiological functions, including growth performance and immune function. Cu is usually formulated into animal premixes in the form of inorganic salts, such as sulphates, or organic minerals. Organic minerals are mineral salts that are either complexed or chelated to organic ligands such as proteins, amino acids, and polysaccharides. Cu is often formulated into premixes alongside other essential components such as vitamins, enzymes and synthetic antioxidants, all of which are susceptible to negative interactions with Cu which can detrimentally effect both their stability and activity. The aim of this study was to determine the effect of five different commercially available Cu sources in relation to their effect on the stability of α-tocopherol acetate and on the activity of Butylated Hydroxytoluene (BHT) and three commercially available phytases in vitro. The results determined that Cu source played a significant role in relation to limiting the interactions between Cu and each of the other components in vitro. There were significant differences (p ≤ 0.05), not only, between the inorganic and organic Cu sources but also between some of the individual organic Cu sources in relation to their effect on α-tocopherol acetate, BHT and phytase.

Thermostable phytase in feed and fuel industries

Phytase with wide ranging biochemical properties has long been utilized in a multitude of industries, even so, thermostability plays a crucial factor in choosing the right phytase in a few of the sectors. Mesophilic phytases are not considered to be a viable option in the feed industry owing to its limited stability in the required feed processing temperature. In the recent past, inclusion of thermostable phytase in fuel ethanol production from starch based raw material has been demonstrated with economic benefits. Therefore, considerable emphasis has been placed on using complementary approaches such as mining of extremophilic microbial wealth, encapsulation and using enzyme engineering for obtaining stable phytase variants. This article means to give an insight on role of thermostable phytases in feed and fuel industries and methods for its development, highlighting molecular determinants of thermostability.

Cloning, Sequencing, and In Silico Analysis of β-Propeller Phytase Bacillus licheniformis Strain PB-13

β-Propeller phytases (BPPhy) are widely distributed in nature and play a major role in phytate-phosphorus cycling. In the present study, a BPPhy gene from Bacillus licheniformis strain was expressed in E. coli with a phytase activity of 1.15 U/mL and specific activity of 0.92 U/mg proteins. The expressed enzyme represented a full length ORF “PhyPB13” of 381 amino acid residues and differs by 3 residues from the closest similar existing BPPhy sequences. The PhyPB13 sequence was characterized in silico using various bioinformatic tools to better understand structural, functional, and evolutionary aspects of BPPhy class by multiple sequence alignment and homology search, phylogenetic tree construction, variation in biochemical features, and distribution of motifs and superfamilies. In all sequences, conserved sites were observed toward their N-terminus and C-terminus. Cysteine was not present in the sequence. Overall, three major clusters were observed in phylogenetic tree with variation in biophysical characteristics. A total of 10 motifs were reported with motif “1” observed in all 44 protein sequences and might be used for diversity and expression analysis of BPPhy enzymes. This study revealed important sequence features of BPPhy and pave a way for determining catalytic mechanism and selection of phytase with desirable characteristics.

Atomistic details of effect of disulfide bond reduction on active site of Phytase B from Aspergillus niger: A MD Study

The molecular integrity of the active site of phytases from fungi is critical for maintaining phytase function as efficient catalytic machines. In this study, the molecular dynamics (MD) of two monomers of phytase B from Aspergillus niger, the disulfide intact monomer (NAP) and a monomer with broken disulfide bonds (RAP), were simulated to explore the conformational basis of the loss of catalytic activity when disulfide bonds are broken. The simulations indicated that the overall secondary and tertiary structures of the two monomers were nearly identical but differed in some crucial secondary-structural elements in the vicinity of the disulfide bonds and catalytic site. Disulfide bonds stabilize the β-sheet that contains residue Arg66 of the active site and destabilize the α-helix that contains the catalytic residue Asp319. This stabilization and destabilization lead to changes in the shape of the active-site pocket. Functionally important hydrogen bonds and atomic fluctuations in the catalytic pocket change during the RAP simulation. None of the disulfide bonds are in or near the catalytic pocket but are most likely essential for maintaining the native conformation of the catalytic site.

ABBREVIATIONS

PhyB - 2.5 pH acid phophatese from Aspergillus niger, NAP - disulphide intact monomer of Phytase B, RAP - disulphide reduced monomer of Phytase B, Rg - radius of gyration, RMSD - root mean square deviation, MD - molecular dynamics.

Site-directed mutagenesis of disulfide bridges in Aspergillus niger NRRL 3135 phytase (PhyA), their expression in Pichia pastoris and catalytic characterization

Earlier studies have established the importance of five disulfide bridges (DBs) in Aspergillus niger phytase. In this study, the relative importance of each of the individual disulfide bridge is determined by its removal by site-directed mutagenesis of specific cysteines in the cloned A. niger phyA gene. Individually, these mutant phytases were expressed in a Pichia expression system and their product purified and characterized. The removal of disulfide bridge 2 yielded a mutant phytase with a complete loss of catalytic activity. The other disulfide mutants displayed a broad array of altered catalytic properties including a lower optimum temperature from 58 degrees C to 53 degrees C for bridge number 1, 37 degrees C for bridge number 3 and 4, and 42 degrees C for bridge number 5. The pH versus activity profile was also modified in the DB mutants. The pH profile of the wild-type phytase was modified by the DB mutations. In bridge number 1, 3, and 4, the second peak at pH 2.5 was abolished, and in bridge number 5, the peak at pH 5.0 was abolished completely leaving only the pH 2.5. While the K (m) was not affected drastically, the turnover number was lowered significantly in bridge number 3, 4, and 5.

Biochemical properties of a thermostable phytase from Neurospora crassa

A gene (Ncphy) encoding a putative phytase in Neurospora crassa was cloned and expressed in Pichia pastoris, and the biochemical properties of the recombinant protein were examined in relation to the phytic acid hydrolysis in animal feed. The recombinant phytase (rNcPhy) hydrolyzed phytic acid with a specific activity of 125 U mg-1, Km of 228 micromol L-1, Vmax of 0.31 nmol (phosphate) s-1 mg-1, a temperature optimum of 60 degrees C and a pH optimum of 5.5 and a second pH optimum of 3.5. The enzyme displayed pH stability around pH 3.5-9.5 and showed satisfactory thermostability at 80 degrees C. The phytase from N. crassa has potential for improving animal feed processing at higher temperatures.

Beta-propeller phytases in the aquatic environment

Phytate, which is one of the dominant organic phosphorus compounds in nature, is very stable in soils. Although a substantial amount of phytate is carried from terrestrial to aquatic systems, it is a minor component of organic phosphorus in coastal sediments. The ephemeral nature of phytate implies the rapid hydrolysis of phytate under aquatic conditions. Among the four classes of known phytases that have been identified in terrestrial organisms, only beta-propeller phytase-like sequences have been identified in the aquatic environment. A novel beta-propeller phytase gene (phyS), cloned from Shewanella oneidensis MR-1, was found to encode a protein with two beta-propeller phytase domains. The characterization of recombinant full-length PhyS and its domains demonstrated that Domain II was the catalytic domain responsible for phytate hydrolysis. The full-length PhyS displayed a K(m) of 83 microM with a kcat of 175.9 min(-1) and the Domain II displayed a K(m) of 474 microM with a kcat of 10.6 min(-1). These results confirm that the phyS gene encodes a functional beta-propeller phytase, which is expressed in S. oneidensis under phosphorus deficient condition. The presence of multiple sequences with a high similarity to phyS in aquatic environmental samples and the widespread occurrence of the Shewanella species in nature suggest that the beta-propeller phytase family is the major class of phytases in the aquatic environment, and that it may play an important role in the recycling of phosphorus.

Expression and characterization of extracellular fungal phytase in transformed sesame hairy root cultures

A recombinant fungal phytase was produced by cultures of sesame hairy roots transformed with Agrobacterium rhizogenes, purified and its molecular properties were characterized. Its transcription level and the phytase production were rapidly increased after 4 weeks of the cultures, suggesting that its transcription and protein synthesis might concur. Western blot analysis provided evidence that the recombinant fungal phytase was secreted into the liquid culture medium of the hairy roots. The phytase enzyme secreted was purified by three steps of ultrafiltration, DEAE-Sepharose ion exchange chromatography, and Sephadex G-100 size-exclusion chromatography. As a result, one single band signal was observed with SDS-PAGE, indicating that the purification step was reasonable. The positive signs of both the zymogram and the PAS staining on SDS-PAGE suggested that the activity of the final product phytase was active and glycosylated. The optimal reaction temperature of the phytase was between 50 and 60 degrees C and at over 60 degrees C its activity was reduced by 30-90%, depending on the temperatures applied. Pre-incubation at temperatures of 20-50 degrees C showed stable catalytic activity, while at over 50 degrees C the phytase activity was gradually decreased by 90%. The optimal pH was between 4 and 5 pH values for the recombinant fungal phytase, while for native phytase it was at pH 5.0. Addition of iron ion inhibited the phytase activity but treatments of some cations, EDTA, and PMSF showed no effect on the activity or slightly stimulated it positively.

Conservation of cysteine residues in fungal histidine acid phytases

Amino acid sequence analysis of fungal histidine acid phosphatases displaying phytase activity has revealed a conserved eight-cysteine motif. These conserved amino acids are not directly associated with catalytic function; rather they appear to be essential in the formation of disulfide bridges. Their role is seen as being similar to another eight-cysteine motif recently reported in the amino acid sequence of nearly 500 plant polypeptides. An additional disulfide bridge formed by two cysteines at the N-terminus of all the filamentous ascomycete phytases was also observed. Disulfide bridges are known to increase both stability and heat tolerance in proteins. It is therefore plausible that this extra disulfide bridge contributes to the higher stability found in phytase from some Aspergillus species. To engineer an enhanced phytase for the feed industry, it is imperative that the role of disulfide bridges be taken into cognizance and possibly be increased in number to further elevate stability in this enzyme.

Monitoring of unfolding and refolding in fungal phytase (phyA) by dynamic light scattering

Role of disulfide bridges in phytase's unfolding-refolding was probed using dynamic light scattering. Phytase was unfolded by guanidinium chloride and then refolded by removing the denaturant by dialysis. Thiol reagents prevented refolding; thus, disulfide bridge formation is an integral step in phytase folding. Catalytic demise of phytase after unfolding and refolding in presence of Tris(2-carboxyethyl)phosphine (TCEP) indicates that disulfide bridges are necessary for refolding. The hydrodynamic radius (rh) of active and unfolded phytase is 4 and 14 nm, respectively. Removal of denaturant through dialysis refolds phytase; its rh shifts back to 4 nm. When TCEP remains in the refolding media, the rh remains high. The unfolded phytase when diluted in assay medium refolds as a function of time at 25 and 37 degrees C, but not at higher temperature. Monitoring rh under denaturing and renaturing condition gives an accurate measure of the folding status of phytase.

The role of disulfide bonds in the conformational stability and catalytic activity of phytase

Previous studies have predicted five disulfide bonds in Aspergillus niger phytase (phy A). To investigate the role of disulfide bonds, intrinsic fluorescence spectra, far-ultraviolet circular dichroism (CD) spectra, and an enzyme activity assay were used to compare the differences of catalytic activity and conformational stability of phytase during denaturation in urea in the presence and absence of dithiothreitol (DTT). In the presence of 2 mM DTT, the inactivation and unfolding were greatly enhanced at the same concentration of denaturant. The fluorescence emission maximum red shift and decreases of ellipticity at 222 nm were in accord with the changes of catalytic activity. The kinetics of the unfolding courses were a biphasic process consisting of two first-order reactions in the absence of DTT and a monophasic process of a first-order reaction in the presence of DTT. The results suggested that the loss of enzymatic activity was most likely because of a conformational change, and that disulfide bonds played an important role in three-dimensional structure and catalytic activity.

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.

Advances in phytase research

Since its discovery in 1907, a complex of technological developments has created a potential $500 million market for phytase as an animal feed additive. During the last 30 years, research has led to increased use of soybean meal and other plant material as protein sources in animal feed. One problem that had to be overcome was the presence of antinutritional factors, including phytate, in plant meal. Phytate phosphorus is not digested by monogastric animals (e.g., hogs and poultry), and in order to supply enough of this nutrient, additional phosphate was required in the feed ration. Rock phosphate soon proved to be a cost-effective means of supplying this additional phosphorus, and the excess phytin phosphorus could be disposed of easily with the animals' manure. However, this additional phosphorus creates a massive environmental problem when the land's ability to bind it is exceeded. Over the last decade, numerous feed studies have established the efficacy of a fungal phytase, A. niger NRRL 3135, to hydrolyze phytin phosphorus in an animal's digestive tract, which benefits the animal while reducing total phosphorus levels in manure. The gene for phytase has now been cloned and overexpressed to provide a commercial source of phytase. This monomeric enzyme, a type of histidine acid phophatase (HAP), has been characterized and extensively studied. HAPs are also found in other fungi, plants, and animals. Several microbial and plant HAPs are known to have significant phytase activity. A second A. niger phytase (phyB), a tetramer, is known and, like phyA, has had its X-ray crystal structure determined. The model provided by this crystal structure research has provided an enhanced understanding of how these molecules function. In addition to the HAP phytase, several other phytases that lack the unique HAP active site motif RHGXRXP have been studied. The best known group of the non-HAPs is phytase C (phyC) from the genus Bacillus. While a preliminary X-ray crystallographic analysis has been initiated, no enzymatic mechanism has been proposed. Perhaps the pivotal event in the last century that created the need for phytase was the development of modern fertilizers after the Second World War. This fostered a transformation in agriculture and a tremendous increase in feed-grain production. These large quantities of cereals and meal in turn led to the transition of one segment of agriculture into "animal agriculture," with their its animal production capability. The huge volumes of manure spawned by these production units in time exceeded both the capacity of their crops and crop lands to utilize or bind the increased amount of phosphorus. Nutrient runoff from this land has now been linked to a number of blooms of toxin-producing microbes. Fish kills associated with these blooms have attracted public and governmental concern, as well as greater interest in phytase as a means to reduce this phosphorus pollution. Phytase research efforts now are focused on the engineering of an improved enzyme. Improved heat tolerance to allow the enzyme to survive the brief period of elevated temperature during the pelletization process is seen as an essential step to lower its cost in animal feed. Information from the X-ray crystal structure of phytase is also relevant to improving the pH optimum, substrate specificity, and enzyme stability. Several studies on new strategies that involve synergistic interactions between phytase and other hydrolytic enzymes have shown positive results. Further reduction in the production cost of phytase is also being pursued. Several studies have already investigated the use of various yeast expression systems as an alternative to the current production method for phytase using overexpression in filamentous fungi. Expression in plants is underway as a means to commercially produce phytase, as in biofarming in which plants such as alfalfa are used as "bioreactors," and also by developing plant cultivars that would produce enough transgenic phytase so that additional supplementation of their grain or meals is not necessary. Ultimately, transgenic poultry and hogs may produce their own digestive phytase. Another active area of current phytase research is expanding its usage. One area that offers tremendous opportunity is increasing the use of phytase in aquaculture. Research is currently centered on utilizing phytase to allow producers in this industry to switch to lower-cost plant protein in their feed formulations. Development of a phytase for this application could significantly lower production costs. Other areas for expanded use range from the use of phytase as a soil amendment, to its use in a bioreactor to generate specific myo-inositol phosphate species. The transformation of phytase into a peroxidase may lead to another novel use for this enzyme. As attempts are made to widen the use of phytase, it is also important that extended exposure and breathing its dust be avoided as prudent safety measures to avoid possible allergic responses. In expanding the use of phytase, another important consideration has been achieved. Conservation of the world's deposits of rock phosphate is recognized as important for future generations. Phosphorus is a basic component of life like nitrogen, but, unlike nitrogen, phosphorus does not have a cycle to constantly replenish its supply. It is very likely that the use of phytase will expand as the need to conserve the world's phosphate reserves increases.

Cloned and expressed fungal phyA gene in alfalfa produces a stable phytase

The phyA gene from Aspergillus ficuum that codes for a 441-amino-acid full-length phosphomonoesterase (phytase) was cloned and expressed in Medicago sativa (alfalfa) leaves. The expressed enzyme from alfalfa leaves was purified to homogeneity and biochemically characterized, and its catalytic properties were elucidated. The expressed phytase in alfalfa leaves retained all the biochemical properties of the benchmark A. ficuum phytase. Although the characteristic bi-hump pH optima were retained in the cloned phytase, the optimal pH shifted downward from 5.5 to 5.0. Also, the recombinant phytase was inhibited by the pseudo-substrate myo-inositol hexasulfate and also by antibody raised against a 20-mer peptide belonging to fungal phytase. The expressed phytase in alfalfa could also be modified by phenylglyoxal. Taken together, the results indicate that fungal phytase when cloned and expressed in alfalfa leaves produces stable and catalytically active phytase while retaining all the properties of the benchmark phytase. This affirms our view that "molecular biofarming" could be an alternative means of producing stable hydrolytic enzymes such as phytase.

Improving the catalytic performance of peroxidases in organic synthesis

Peroxidases are ubiquitous enzymes that catalyze a variety of enantioselective oxygen-transfer reactions with hydrogen peroxide (H2O2). Although they have enormous potential, their industrial application is hampered by their high price and low operational stability. Recent developments, such as the controlled addition and in situ formation of the oxidant, protein engineering and the rational design of semi-synthetic peroxidases, aim to improve the operational stability of peroxidases.

Site-directed mutagenesis improves catalytic efficiency and thermostability of Escherichia coli pH 2.5 acid phosphatase/phytase expressed in Pichia pastoris

Escherichia coli pH 2.5 acid phosphatase gene (appA) and three mutants were expressed in Pichia pastoris to assess the effect of strategic mutations or deletion on the enzyme (EcAP) biochemical properties. Mutants A131N/ V134N/D207N/S211N, C200N/D207N/S211N, and A131N/ V134N/C200N/D207N/S211N had four, two, and four additional potential N-glycosylation sites, respectively. Extracellular phytase and acid phosphatase activities were produced by these mutants and the intact enzyme r-AppA. The N-glycosylation level was higher in mutants A131N/V134N/D207N/S211N (48%) and A131N/V134N/ C200N/D207N/S211N (89%) than that in r-AppA (14%). Despite no enhancement of glycosylation, mutant C200N/ D207N/S211N was different from r-AppA in the following properties. First, it was more active at pH 3.5-5.5. Second, it retained more (P < 0.01) phytase activity than that of r-AppA. Third, its specific activity of phytase was 54% higher. Lastly, its apparent catalytic efficiency kcat/Km for either p-nitrophenyl phosphate (5.8 x 10(5) vs 2.0 x 10(5) min(-1) M(-1)) or sodium phytate (6.9 x 10(6) vs 1.1 x 10(6) min(-1) M(-1)) was improved by factors of 1.9- and 5.3-fold, respectively. Based on the recently published E. coli phytase crystal structure, substitution of C200N in mutant C200N/D207N/S211N seems to eliminate the disulfide bond between the G helix and the GH loop in the alpha-domain of the protein. This change may modulate the domain flexibility and thereby the catalytic efficiency and thermostability of the enzyme.

Biocatalytic and biomimetic oxidations with vanadium

Approaches to the rational design of vanadium-based semi-synthetic enzymes and biomimetic models as catalysts for enantioselective oxidations are reviewed. Incorporation of vanadate ion into the active site of phytase (E.C. 3.1.3.8), which in vivo mediates the hydrolysis of phosphate esters, afforded a semi-synthetic peroxidase. It catalyzed the enantioselective oxidation of prochiral sulfides with H2O2 affording the S-sulfoxide, e.g. in 66% ee at quantitative conversion of thioanisole. Under the reaction conditions the semi-synthetic vanadium peroxidase was stable for more than 3 days with only a slight decrease in turnover frequency. Amongst the transition-metal oxoanions that are known to be potent inhibitors of phosphatases, only vanadate resulted in a semi-synthetic peroxidase when incorporated into phytase. In a biomimetic approach, vanadium complexes of chiral Schiff base complexes were encapsulated in the super cages of a hydrophobic zeolite Y. Unfortunately, these ship-in-a-bottle complexes afforded only racemic sulfoxide in the catalytic oxidation of thioanisole with H2O2.

Characterization of recombinant fungal phytase (phyA) expressed in tobacco leaves

The phyA gene from Aspergillus ficuum coding for a 441-amino-acid full-length phytase was expressed in Nicotiana tabacum (tobacco) leaves. The expressed phytase was purified to homogeneity using ion-exchange column chromatography. The purified phytase was characterized biochemically and its kinetic parameters were determined. When the recombinant phytase was compared with its counterpart from Aspergillus ficuum for physical and enzymatic properties, it was found that catalytically the recombinant protein was indistinguishable from the native phytase. Except for a decrease in molecular mass, the overexpressed recombinant phytase was virtually the same as the native fungal phytase. While the temperature optima of the recombinant protein remain unchanged, the pH optima shifted from pH 5 to 4. The results are encouraging enough to open the possibility of overexpressing phyA gene from Aspergillus ficuum in other crop plants as an alternative means of commercial production of this important enzyme.

Cloning, sequencing, and expression of an Escherichia coli acid phosphatase/phytase gene (appA2) isolated from pig colon

Bacterial strains were isolated from the pig colon to screen for phytase and acid phosphatase activities. Among 93 colonies, Colony 88 had the highest activities for both enzymes and was identified as an Escherichia coli strain. Using primers derived from the E. coli pH 2.5 acid phosphatase appA sequence (Dassa et al. (1990), J. Bacteriol. 172, 5497-5500), we cloned a 1482 bp DNA fragment from the isolate. In spite of 95% homology between the sequenced gene and the appA, 7 amino acids were different in their deduced polypeptides. To characterize the properties and functions of the encoded protein, we expressed the coding region of the isolated DNA fragment and appA in Pichia pastoris, respectively, as r-appA2 and r-appA. The recombinant protein r-appA2, like r-appA and the r-phyA phytase expressed in Aspergillus niger, was able to hydrolyze phosphorus from sodium phytate and p-nitrophenyl phosphate. However, there were distinct differences in their pH profiles, Km and Vmax for the substrates, specific activities of the purified enzymes, and abilities to release phytate phosphorus in soybean meal. In conclusion, the DNA fragment isolated from E. coli in pig colon seems to encode for a new acid phosphatase/phytase and is designated as E. coli appA2.

Molecular characterization and expression of a phytase gene from the thermophilic fungus Thermomyces lanuginosus

The phyA gene encoding an extracellular phytase from the thermophilic fungus Thermomyces lanuginosus was cloned and heterologously expressed, and the recombinant gene product was biochemically characterized. The phyA gene encodes a primary translation product (PhyA) of 475 amino acids (aa) which includes a putative signal peptide (23 aa) and propeptide (10 aa). The deduced amino acid sequence of PhyA has limited sequence identity (ca. 47%) with Aspergillus niger phytase. The phyA gene was inserted into an expression vector under transcriptional control of the Fusarium oxysporum trypsin gene promoter and used to transform a Fusarium venenatum recipient strain. The secreted recombinant phytase protein was enzymatically active between pHs 3 and 7.5, with a specific activity of 110 micromol of inorganic phosphate released per min per mg of protein at pH 6 and 37 degrees C. The Thermomyces phytase retained activity at assay temperatures up to 75 degrees C and demonstrated superior catalytic efficiency to any known fungal phytase at 65 degrees C (the temperature optimum). Comparison of this new Thermomyces catalyst with the well-known Aspergillus niger phytase reveals other favorable properties for the enzyme derived from the thermophilic gene donor, including catalytic activity over an expanded pH range.

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.

Myo-inositol hexasulfate is a potent inhibitor of Aspergillus ficuum phytase

Myo-inositol hexasulfate (MIHS), a structural analog of the substrate myo-inositol hexaphosphate, is a potent competitive inhibitor of both phyA and phyB enzymes. The Ki of inhibition for the phyA and phyB proteins were estimated to be 4.6 and 0.2 microM, respectively. Thus, the phyB protein is 23-fold more sensitive to MIHS inhibition than the phyA protein. The active-site geometry of phyB protein is presumed to be very different from the phyA protein as deduced by chemical probing of the enzymes by Arg-specific modifiers, i.e., 1,2-cyclohexanedione and phenylglyoxal. Probing the catalytic site of the same proteins by this newly developed specific inhibitor also gives a similar conclusion.

Differences in the active site environment of Aspergillus ficuum phytases

While Aspergillus ficuum phytaseA (phyA) was rapidly inactivated by 1,2-cyclohexanedione and phenylglyoxal, both specific modifiers of arginine, phytaseB (phyB) showed a markedly different behavior. First, phyB was totally insensitive to 1,2-cyclohexanedione even in the presence of 0.2 M guanidinium hydrochloride; second, the enzyme showed a great deal of resistance to inactivation by phenylglyoxal. Taken together, these results indicate that the chemical environment of the active site of phyB is very different from that of the active site of phyA. Despite sequence similarities of the active site region in these two proteins, their differential behavior to arginine modifiers indicates that other parts of the protein play a role in the active site formation. We expected some differences in the structure since the proteins have dissimilar kinetic parameters and pH optima.

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.