Supplementation with phytase is an effective way to increase the availability of phosphorus in seed-based animal feed. The biochemical characteristics of an ideal phytase for this application are still largely unknown. To extend the biochemical characterization of wild-type phytases, the catalytic properties of a series of fungal phytases, as well as Escherichia coli phytase, were determined. The specific activities of the fungal phytases at 37°C ranged from 23 to 196 U · (mg of protein)−1, and the pH optima ranged from 2.5 to 7.0. When excess phytase was used, all of the phytases were able to release five phosphate groups of phytic acid (myo-inositol hexakisphosphate), which leftmyo-inositol 2-monophosphate as the end product. A combination consisting of a phytase and Aspergillus nigerpH 2.5 acid phosphatase was able to liberate all six phosphate groups. When substrate specificity was examined, the A. niger,Aspergillus terreus, and E. coli phytases were rather specific for phytic acid. On the other hand, theAspergillus fumigatus, Emericella nidulans, andMyceliophthora thermophila phytases exhibited considerable activity with a broad range of phosphate compounds, including phenyl phosphate, p-nitrophenyl phosphate, sugar phosphates, α- and β-glycerophosphates, phosphoenolpyruvate, 3-phosphoglycerate, ADP, and ATP. Both phosphate liberation kinetics and a time course experiment in which high-performance liquid chromatography separation of the degradation intermediates was used showed that all of themyo-inositol phosphates from the hexakisphosphate to the bisphosphate were efficiently cleaved by A. fumigatusphytase. In contrast, phosphate liberation by A. niger orA. terreus phytase decreased with incubation time, and themyo-inositol tris- and bisphosphates accumulated, suggesting that these compounds are worse substrates than phytic acid is. To test whether broad substrate specificity may be advantageous for feed application, phosphate liberation kinetics were studied in vitro by using feed suspensions supplemented with 250 or 500 U of eitherA. fumigatus phytase or A. niger phytase (Natuphos) per kg of feed. Initially, phosphate liberation was linear and identical for the two phytases, but considerably more phosphate was liberated by the A. fumigatus phytase than by the A. niger phytase at later stages of incubation.

Naturally-occurring phytases having the required level of thermostability for application in animal feeding have not been found in nature thus far. We decided to de novo construct consensus phytases using primary protein sequence comparisons. A consensus enzyme based on 13 fungal phytase sequences had normal catalytic properties, but showed an unexpected 15-22 degrees C increase in unfolding temperature compared with each of its parents. As a first step towards understanding the molecular basis of increased heat resistance, the crystal structure of consensus phytase was determined and compared with that of Aspergillus niger phytase. Aspergillus niger phytase unfolds at much lower temperatures. In most cases, consensus residues were indeed expected, based on comparisons of both three-dimensional structures, to contribute more to phytase stabilization than non-consensus amino acids. For some consensus amino acids, predicted by structural comparisons to destabilize the protein, mutational analysis was performed. Interestingly, these consensus residues in fact increased the unfolding temperature of the consensus phytase. In summary, for fungal phytases apparently an unexpected direct link between protein sequence conservation and protein stability exists.

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Phytases catalyze the hydrolysis of phosphomonoester bonds of phytate (myo-inositol hexakisphosphate), thereby creating lower forms of myo-inositol phosphates and inorganic phosphate. In this study, cDNA expression libraries were constructed from four basidiomycete fungi (Peniophora lycii, Agrocybe pediades, a Ceriporia sp., and Trametes pubescens) and screened for phytase activity in yeast. One full-length phytaseencoding cDNA was isolated from each library, except for the Ceriporia sp. library where two different phytase-encoding cDNAs were found. All five phytases were expressed in Aspergillus oryzae, purified, and characterized. The phytases revealed temperature optima between 40 and 60°C and pH optima at 5.0 to 6.0, except for the P. lycii phytase, which has a pH optimum at 4.0 to 5.0. They exhibited specific activities in the range of 400 to 1,200 U ⅐ mg, of protein ؊1 and were capable of hydrolyzing phytate down to myo-inositol monophosphate. Surprisingly, 1 H nuclear magnetic resonance analysis of the hydrolysis of phytate by all five basidiomycete phytases showed a preference for initial attack at the 6-phosphate group of phytic acid, a characteristic that was believed so far not to be seen with fungal phytases. Accordingly, the basidiomycete phytases described here should be grouped as 6-phytases (EC 3.1.3.26).Phytases (myo-inositol hexakisphosphate phosphohydrolases) belong to the family of histidine acid phosphatases sharing the sequence consensus pattern(27; http://www.expasy.ch /cgi-bin/get-prodoc-entry?PDOC00538). They are capable of catalyzing the hydrolysis of phosphomonoester bonds of phytate (salts of myo-inositol hexakisphosphate or myo-inositol 1,2,3,4,5,6-hexakis dihydrogen phosphate), thereby creating lower forms of myo-inositol phosphates and inorganic phosphate (22,24). Phytases are grouped according to the specific position of the phosphate ester group on the phytate molecule at which hydrolysis is initiated, i.e., as 3-phytases (EC 3.1.3.8) or as 6-phytases (EC 3.1.3.26).Phytate is the primary source of inositol and the primary storage form of phosphate in plant seeds (23). Seeds, cereal grains, and legumes are important components of food and, in particular, of animal feed preparations. However, monogastric animals such as poultry and swine are incapable of utilizing the phosphorus bound in phytate due to low levels of phytase activity in the digestive tract. Furthermore, phytate acts as an antinutrient by chelating divalent cations and preventing the uptake of minerals, e.g., Zn (9). Thus, phytases are used as a cereal feed additive that enhances the phosphorus and mineral uptake in monogastric animals and reduces the level of phosphate output in their manure. Recently, there have been several reports on the cloning of fungal PhyA phytases from Aspergillus niger (16,20, 28), Aspergillus fumigatus (19), Aspergillus terreus, Myceliophthora thermophila (14), Emericella nidulans, Talaromyces thermophilus (18), and Thermomyces lanuginosus (2). Based on their characteristics and on their s...

Beta,beta-carotene 15,15'-dioxygenase cleaves beta,beta-carotene into two molecules of retinal, and is the key enzyme in the metabolism of beta,beta-carotene to vitamin A. The enzyme has been known for more than 40 years, yet all attempts to purify the protein to homogeneity have failed. Recently, the successful cloning and sequencing of an enzyme with beta,beta-carotene 15,15'-dioxygenase activity from chicken, as well as from Drosophila, has been reported. Here, we describe in detail our attempt to enrich the chicken beta,beta-carotene 15,15'-dioxygenase to such an extent as to allow determination of partial amino acid sequences, which were then used to design degenerate oligonucleotides. Screening of a chicken duodenal expression library yielded a full-length clone containing a coding sequence of 1578 bp. Functional expression in Escherichia coli and in eukaryotic cell lines confirmed that we had cloned the first vertebrate dioxygenase that cleaves beta,beta-carotene at the central 15,15'-double bond. By performing a sequence homology search, the cDNA sequence of the mouse homologue was found as an expressed sequence tag (EST) in the gene bank. At the amino-acid level, the degree of homology between the chicken and mouse sequences is 81%. Thus beta,beta-carotene 15,15'-dioxygenase can be considered as being an enzyme that is evolutionarily rather well conserved. We established the expression pattern of beta,beta-carotene 15,15'-dioxygenase in chicken and mouse tissues with a combination of Northern blots and in situ hybridization. The mRNA for beta,beta-carotene 15,15'-dioxygenase was localized primarily in duodenal villi, as well as in liver and in tubular structures of lung and kidney. These new findings demonstrate that beta,beta-carotene 15,15'-dioxygenase is also expressed in epithelial structures, where it serves to provide the tissue-specific vitamin A supply.