Analyzing the Active Site of Selenomonas ruminantium Phytase by Site-directed Mutagenesis and X-ray Crystallography
Date Issued
2004
Date
2004
Author(s)
Hsu, Ching-Yi
DOI
zh-TW
Abstract
Phytase can hydrolyzes phytate（myo-inositol hexaphosphate）during plant germination to produce phosphate（Pi）and inositol polyphasphate derivatives. In poultry and pig farms, phytase and phytate are fed to livestock as additives to provide Pi. In this study, we analyzed the active site of Selenomous ruminantium phytase . It exhibits higher catalytic activity than many other phytases. We wish our analysis could improve its applicability in the livestock industry.
Most phytases can be classified into two big families: Histidine acid phytase family and Alkaline phytase family. S. ruminantium phytase, however, belongs to neither of them. Through structural alignment we have found that the active site of S. ruminantium phytase greatly resembles to members of the dual specificity phosphatase family in that it contains conserved Cys241 in the primary catalytic site, P-loop, and Asp212 and Pro216 in the auxiliary site, WPD-loop. To study the functions of these conserved amino acid residues, we employed site-directed mutagenesis to change Cys241 into Ser241 or Ala241, and Asp212 and Pro216 into Ala212 and Gly216, respectively, and measured their catalytic activity of these mutant forms. We found that the activity of P216G decreases as the temperatures increases, whereas mutations in Cys241 and Asp212 abrogate the activity of S. ruminantium phytase. These results suggest that these sites are very important for the function of S. ruminantium phytase. We also examined CD spectra of these mutant forms to test whether the increase in temperature causes conformational changes. To better understand the structural properties of phytase and its interaction with the substrates, C241A recombinant protein is over expressed in E. coli, and crystallized by using sodium malonate as precipitant, and subject to X-ray crystallography. We found that malonate binds into the active site of C214A. This observation has gain insight that how the competitive inhibition of polycarboxyl acid acts to the phytase active site.
Subjects
活性區域
點突變
X光結晶學
植酸
Active Site
Site-directed Mutagenesis
X-ray Crystallography
Phytase
Type
other
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