Enzyme mechanism and catalytic property of β propeller phytase

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Title
Enzyme mechanism and catalytic property of β propeller phytase
Author(s)
Se Jeong Shin; Nan Chul Ha; Byung Chul Oh; Tae Kwang Oh; Byung Ha Oh
Bibliographic Citation
Structure, vol. 9, no. 9, pp. 851-858
Publication Year
2001
Abstract
Background: Phytases hydrolyze phytic acid (myo-inositol-hexakisphosphate) to less-phosphorylated myo-inositol derivatives and inorganic phosphate. Phytases are used in animal feed to reduce phosphate pollution in the environment. Recently, a thermostable, calcium-dependent Bacillus phytase was identified that represents the first example of the β propeller fold exhibiting phosphatase activity. We sought to delineate the catalytic mechanism and property of this enzyme. Results: The crystal structure of the enzyme in complex with inorganic phosphate reveals that two phosphates and four calcium ions are tightly bound at the active site. Mutation of the residues involved in the calcium chelation results in severe defects in the enzyme's activity. One phosphate ion, chelating all of the four calcium ions, is close to a water molecule bridging two of the bound calcium ions. Fluoride ion, which is expected to replace this water molecule, is an uncompetitive inhibitor of the enzyme. The enzyme is able to hydrolyze any of the six phosphate groups of phytate. Conclusions: The enzyme reaction is likely to proceed through a direct attack of the metal-bridging water molecule on the phosphorous atom of a substrate and the subsequent stabilization of the pentavalent transition state by the bound calcium ions. The enzyme has two phosphate binding sites, the "cleavage site", which is responsible for the hydrolysis of a substrate, and the "affinity site", which increases the binding affinity for substrates containing adjacent phosphate groups. The existence of the two nonequivalent phosphate binding sites explains the puzzling formation of the alternately dephosphorylated myo-inositol triphosphates from phytate and the hydrolysis of myo-inositol monophosphates.
Keyword
β propeller phytaseCrystal structureDirect phospho-transferEnzyme mechanismMetalloenzymePhosphatase
ISSN
0969-2126
Publisher
Elsevier-Cell Press
DOI
http://dx.doi.org/10.1016/S0969-2126(01)00637-2
Type
Article
Appears in Collections:
Division of Biomedical Research > Metabolic Regulation Research Center > 1. Journal Articles
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