Sunday, July 29, 2007
P5

Optimizing the pH profile of fungal phytase through protein engineering also enhances activity

Edward J. Mullaney1, Abul H. J. Ullah1, Kandan Sethumadhavan1, and Xin Gen Lei2. (1) SRRC-ARS-USDA, 1100 Robert E. Lee Blvd., New Orleans, LA 70124, (2) Department of Animal Science, Cornell University, Ithaca, NY 14853

Phytic acid is the primary phosphorus storage compound in plants.  As the amount of plant derived protein in animal feed has increased a problem developed.  The concern is that monogastric animals lack a phytase in their digestive tract to hydrolyze this compound that is found in soybean and other plant meals.  Today, the principal phytase marketed as a feed additive is a native enzyme whose gene has been cloned and overexpressed to make it commercially viable. While this enzyme does make some of this phosphate available to the animal, its pH profile does not permit optimal activity at the pH environment found in the digestive tract of swine.  Recent research has provided information on the catalytic mechanism of this enzyme, a histidine acid phosphatase, and efforts have now focused on defining the role of the individual amino acids that constitute the substrate specificity site (SSS).  Specific changes in the SSS amino acid residues can result in alterations of the pH profile of phytase. In Aspergillus niger NRRL 3135 phytase (PhyA), specific activity of the enzyme in the mid acidic pH range 3.0-4.0 was increased when site-directed mutagenesis of the phyA gene was employed to change individual SSS amino acid residues.  This alteration of the pH profile enhanced the enzyme activity in the pH environment of the digestive tract. A swine feed trial further confirmed the efficacy of this SSS mutant PhyA in the stomach by yielding significant weight gain over the same diet supplemented with un-engineered wild type PhyA.

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