Pytate is a small compound responsible for storing more than 80% of the total phosporus in cereals and legumes. Many commercially important animals, such as pigs, poultry, and fish cannot metabolize phytate, and thus excrete a lot of phosphorus, creating environmental problems. Addition of a phytase to animal feed helps alleviate the problem. Unfortunately, animal feed must be processed at 58 deg. C, so only a highly stable enzyme will do the job. Such a protein has been isolated from Bacillus amyloliquefaciens, prompting the Ha et al the crystallize it to find out what makes it so stable.

This protein represents a novel scaffold for phytase activity. It is predominantly a beta structure. Ha et al liken its appearance to a propeller with six blades. Each blade is composed of four or five curved beta sheets, and the topology of each blade is identical. One of the features of the protein that may contribute to its stability is a structural element that has been termed double clasp. The N-terminal segment of the protein forms part of the two C-terminal blades. In this manner it is proposed to stabilize the circular arrangent of blade domains.

There are extensive hydrophobic interactions observed between the blades.

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Backbone
Polar side chain
Hydrophobic side chain
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Solvent-filled Channel

Down the shaft of the propeller that is a solvent-filled channel.

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Calcium Binding
Six calcium ions are bound by the protein. Three, the one in the center and two in the double clasp area, are tightly bound. They are not removed by extensive dialysis. The two involved in the double clasp are important for the stability of the protein. The other three are termed low-affinity ions, because they are more easily removed. However, they are required for enzyme activity, suggesting that their binding location, one mouth of the channel, is the active site. They are proposed to activate the protein by altering the electrostatic potential so that phytate may bind, and may be direcyly involved in catalysis as well. One calcium is bound in the center
Hexa-coordinated calcium
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Ca is coordinated by resides far apart in primary sequence
N-terminus -------------> C-terminus
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See electrostatic surface with no calcium bound