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.

Previously, we calculated a consensus amino acid sequence from 13 homologous fungal phytases. A synthetic gene was constructed and recombinantly expressed. Surprisingly, consensus phytase-1 was 15-26 degrees C more thermostable than all parent phytases used in its design [Lehmann et al. (2000)Protein Eng., 13, 49-57]. In the present study, inclusion of six further phytase sequences in the amino acid sequence alignment resulted in the replacement of 38 amino acid residues in either one or both of the new consensus phytases-10 and -11. Since consensus phytase-10, again, was 7.4 degrees C more thermostable than consensus phytase-1, the thermostability effects of most of the 38 amino acid substitutions were tested by site-directed mutagenesis. Both stabilizing and destabilizing mutations were identified, but all affected the stability of the enzyme by <3 degrees C. The combination of all stabilizing amino acid exchanges in a multiple mutant of consensus phytase-1 increased the unfolding temperature from 78.0 to 88.5 degrees C. Likewise, back-mutation of four destabilizing amino acids and introduction of an additional stabilizing amino acid in consensus phytase-10 further increased the unfolding temperature from 85.4 to 90.4 degrees C. The thermostabilization achieved is the result of a combination of slight improvements from multiple amino acid exchanges rather than being the effect of a single or of just a few dominating mutations that have been introduced by chance. The present findings support the general validity of the consensus concept for thermostability engineering of proteins.

Phytases catalyse the hydrolysis of phytate (myo-inositol hexakisphosphate) to myo-inositol and inorganic phosphate. In this study genes encoding novel phytases from two different filamentous fungi, Aspergillus terreus strain 9A-1 and Myceliophthora thermophila were isolated. The encoded PhyA phytase proteins show 60% (A. terreus) and 489'0 (M. thermophila) identity, respectively, to the PhyA of Aspergillus niger and have 21-29% identity compared to other histidine acid phosphatases. All three PhyA proteins, in contrast to the A. niger pH 25-optimum acid phosphatase, prefer phytic acid as substrate and show enzyme activity at a broad range of acidic pH values. Based on their enzyme characteristics and protein sequence homology, the phytases form a novel subclass of the histidine acid phosphatase family.

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.

The nuclear gene for manganese-containing superoxide dismutase (MnSOD; superoxide:superoxide oxidoreductase, EC 1.15.1.1) of yeast mitochondria was mapped on chromosome VIII and inactivated by gene disruption. The resulting mutant lacked any protein cross-reacting with antiMnSOD antibodies, and its mitochondria exhibited less than 1% of the cyanide-insensitive superoxide dismutase activity found in mitochondria of the wild-type parent strain. In the absence of oxygen, the mutant grew as rapidly as the wild-type parent. However, increasing concentrations of oxygen led to a progressive inhibition of growth. Experiments implicated superoxide in reactions such as DNA breakage, hyaluronate depolymerization, linoleate oxidation, erythrocyte lysis, and bacterial killing (6). Prolonged exposure of animal, plant, or bacterial cells to increased oxygen concentrations causes an increase in their superoxide dismutase activity, and the increased activity correlates with an increased ability to tolerate the higher oxygen concentrations (2, 7-10). In addition, exposure of bacteria to redox dyes that promote the formation of superoxide radicals causes a dramatic induction of MnSOD (11,12).Whether superoxide dismutases indeed contribute to the cell's protection against superoxide radicals, is, however, still under debate (for reviews, see refs. 1, 2, and 13). For example, it has been argued that the superoxide-decomposing activity of superoxide dismutases is artifactual and that the true function of these enzymes might well involve metal storage or some other, as yet unknown, reaction (1 MATERIALS AND METHODSStrains and Culture Conditions. The Saccharomyces cerevisiae strains DL1 (a, leu2-3, 2-112, his3-11, 3-15, ura3-251, 3-372, 3-328) and CHL2881-1C (a, ura3, leu2, trpl, Abbreviations: MnSOD, manganese-containing superoxide dismutase; kb, kilobase(s). 3820The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.