Modified phytases

A process for preparing a modified Aspergillus phytase with a specific activity improved over the specific activity of an unmodified Aspergillus phytase which comprises:

• (a) determining the three dimensional structure of the unmodified Aspergillus phytase and of a second phytase which has the improved specific activity by aligning the amino acid sequences of said phytases with the amino acid sequence of a third phytase which is the phytase of Aspergillus niger and using the three dimensional structure of the phytase of Aspergillus niger as a template based on the alignment to determine said three dimensional structures;
• (b) determining from the structures of step (a) the amino acids of the active sites of the unmodified Aspergillus phytase and of the second phytase having the improved specific activity which active site provides the improved specific activity and comparing the amino acids which form the active sites to identify which amino acids are different in the active site of the second phytase from the amino acids in the active site of the unmodified Aspergillus phytase;
• (c) constructing a DNA sequence coding for the modified phytase by obtaining the DNA sequence of the unmodified Aspergillus phytase and changing the nucleotides coding for the active site which provides the improved specific activity for said unmodified Aspergillus phytase so that at least one of the amino acids in the active site which provides the improved specific activity is substituted by one of the amino acids which was identified as being different in step (b);
• (d) integrating such a DNA sequence into a vector capable of expression in a suitable host cell;
• and
• (e) transforming the suitable host cell by the DNA sequence of step (c) or the vector of step (d), growing said host cell under suitable growth conditions and isolating the modified phytase from the host cell or the culture medium.