A phytase would be considered ideal for feed application if it were catalytically effective, proteolysis-resistant, thermostable, and cheap. The catalytic efficiency and protease susceptibility of any given phytase decide its ability to release phytate-phosphorus in the digestive tract. The thermostability of phytase determines its feasibility in feed pelleting, and the overall cost to produce the enzyme ranks its final acceptance by industry. Although there are significant differences in these features among various naturally occurring phy-tases, no single wild-type enzyme possesses all of the desired properties. With advances of biotechnology, there are three ways to develop effective phytases with improved properties. First, site-directed mutagenesis, based on crystal structure of phytases, has been applied to improve pH profile, thermostability, and catalytic efficiency. Second, synthetic phytases such as the experimental consensus phytase have been generated based on homologous sequences of multiple phytases. Last, new phytases can be produced by directed evolution with efficient selections. A number of heterolo-gous expression systems have been used for phytase production. The expression hosts include plants, bacteria, fungi, and yeast.[2'9] Recently, transgenic pigs over-expressing a bacterial phytase in salivary gland have been generated. If approved by regulatory agencies, this approach may serve as a sustainable and economical delivery of phytase.
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