Feed Additives - Vemo 99

The different types of phytase

Phytic acid is the stock form of phosphorus in plants. In relation to human and animal nutrition, the following two facts about phytic acid are essential:

1.    monogastric animals have low levels of phytate degrading enzymes in their digestive tract and phytic acid itself cannot be absorbed. Food for pigs and poultry are usually enriched with inorganic phosphate in order to meet the animals needs of phosphorus

2.    phytic acid is an anti-nutritional factor, because it forms complexes with proteins and various metal ions and thus reduces the nutritional value of food.

Because of these problems, there is growing interest in phytate degrading enzymes. Phytases are a subfamily of histidine acid phosphatases and are found in plants, microorganisms and fungi. Phosphatases as a class were not previously well characterized biochemically. In order to give a clear definition of this class of enzymes, several fungal phytases were cloned and over-expressed. In this article we describe the mold phytases structural and biophysical characteristics and the results of the comparison with prokaryotic phytase from E.coli.

Wild type phytases from different fungi strains have been over produced and purified, and their biophysical properties were compared with those of phytase from Escherichia coli. All phytases examined were monomeric. Escherichia coli phytase is a non-glycosylated enzyme, mold phytases are glycosylated differently, depending on the type of phytase, given that they were expressed in different systems. Furthermore, the degree of glycosylation of phytases is modeled in fungi, where energy is consumed. Also, the extent of glycosylation had no effect on the specific activity, thermostability, or the restoration capabilities of different phytases. Emitted from various Aspergillus niger, phytases were susceptible to the action of proteases present in the culture supernatant. N-terminal sequencing of the fragments revealed that cleavage invariably occurred at open sections (loops) on the surface of the molecule. Site-directed mutagenesis of phytase from A. fumigatus and E. nidulans, resulted in mutants, which are significantly more resistant to proteolytic attack. Processing of open (uncovered) surface loops may be a good strategy for improving phytase stability during feed processing in the digestive tract.