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| '''THE ROLE OF THE DIVALENT METAL ION IN SUGAR BINDING, RING OPENING, AND ISOMERIZATION BY D-XYLOSE ISOMERASE: REPLACEMENT OF A CATALYTIC METAL BY AN AMINO-ACID'''
| | ===THE ROLE OF THE DIVALENT METAL ION IN SUGAR BINDING, RING OPENING, AND ISOMERIZATION BY D-XYLOSE ISOMERASE: REPLACEMENT OF A CATALYTIC METAL BY AN AMINO-ACID=== |
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| ==Overview==
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| The distinct roles of the two magnesium ions essential to the activity of D-xylose isomerase from Streptomyces olivochromogenes were examined. The enzyme-magnesium complex was isolated, and the stoichiometry of cation binding determined by neutron activation analysis to be 2 mol of magnesium per mole of enzyme. A plot of Mg2+ added versus Mg2+ bound to enzyme is consistent with apparent KD values of < or = 0.5-1.0 mM for one Mg2+ and < or = 2-5 mM for the second. A site-directed mutant of D-xylose isomerase was designed to remove the tighter, tetracoordinated magnesium binding site (site 1, Mg-1); Glu180 was replaced with Lys180. The stoichiometry of metal binding to this mutant, E180K, is 1 mol of magnesium per mole of enzyme. Ring-opening assays with 1-thioglucose (H2S released upon ring opening) show E180K catalyzes the opening of the sugar ring at 20% the rate of the wild-type, but E180K does not catalyze isomerization of glucose to fructose. Thus, the magnesium bound to Glu180 is essential for isomerization but not essential for ring opening. The X-ray crystallographic structures of E180K in the absence of magnesium and in the presence and absence of 250 mM glucose were obtained to 1.8-A resolution and refined to R factors of 17.7% and 19.7%, respectively. The wild-type and both E180K structures show no significant structural differences, except the epsilon-amino group of Lys180, which occupies the position usually occupied by the Mg-1.(ABSTRACT TRUNCATED AT 250 WORDS) | | The line below this paragraph, {{ABSTRACT_PUBMED_7906142}}, adds the Publication Abstract to the page |
| | (as it appears on PubMed at http://www.pubmed.gov), where 7906142 is the PubMed ID number. |
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| | {{ABSTRACT_PUBMED_7906142}} |
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| ==About this Structure== | | ==About this Structure== |
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| [[Category: Petsko, G A.]] | | [[Category: Petsko, G A.]] |
| [[Category: Ringe, D.]] | | [[Category: Ringe, D.]] |
| ''Page seeded by [http://oca.weizmann.ac.il/oca OCA ] on Sat May 3 15:40:13 2008'' | | |
| | ''Page seeded by [http://oca.weizmann.ac.il/oca OCA ] on Mon Jul 28 06:29:56 2008'' |