This is the crystal structure of the dodecasodium salt of myoinositol hexaphosphate, also known as sodium phytate. Metal salts of phytate are the major storage forms of phosphorus in seeds and cereal grains and the predominant forms of organic phosphorus in soils. The six membered carbon ring, (inositol), at the center of each phytate molecule tends to adopt one of two chair conformations. In crystaline form, the perfered chair conformation results in five of the six phosphate groups in axial positions relative to the plane of the carbon ring and the sixth phosphate group in an equitorial position. Upon disolution, the carbon ring can rapidly flip between the two chair conformations. This conversion changes the orientation of the phosphate groups from axial to equitorial and equitorial to axial. The conformation found in the crystal structure is thought predominate under natural conditions. Very slight twisting or bending of any given chair conform results in changes in distance between phosphate groups. This enables phytate to bond strongly with many metals of different size and valence.
Biological utilization of the phosphate groups of phytate requires hydrolysis of the C-O-P ester bonds. Both acid and alkali can hydrolyze the ester bond chemically but enzymatic hydrolysis by phytases predominates in nature. Phytase, or myo-inositol hexakisphosphate phosphohydrolase (EC 3.1.3.8) was first reported in 1962. This enzyme is ubiquitos in nature, found in higher plants, animal tissues, and microorganisms. The estimated molecular mass for the various phytases range from 35-700 kDa. In general, plant phytases are smaller than those from microorganisms, optimal activity occurs withing the pH range 4.5-6.0, and the optimal temperature for enzyme activity is between 45-60 oC. Several phytases have been cloned and characterized, including Aspergillus ficuum, Escherichia coli, and a mammalian phytase from rat liver. The most conserved feature across all domains is the Arg-His-Gly tripeptide sequence found at the active site. It is Thought that the histidine residue acts as a nucleophile forming a covalent phosphohistidine intermediate.
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This page was constructed by T. J. Boerth as a class project for "Soil Mineralogy", Soil Sci. 875, under the direction of Drs. P. Barak and E. Nater, and was contributed to the Virtual Museum of Minerals and Molecules