Journal
BIOCHEMISTRY
Volume 46, Issue 23, Pages 6710-6722Publisher
AMER CHEMICAL SOC
DOI: 10.1021/bi700356u
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Funding
- NCI NIH HHS [T32 CA009582] Funding Source: Medline
- NIEHS NIH HHS [P30 ES00267, T32 ES07028] Funding Source: Medline
- NIGMS NIH HHS [R01 GM30910] Funding Source: Medline
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The enzyme 2-hydroxychromene-2-carboxylic acid (HCCA) isomerase catalyzes the glutathione (GSH)-dependent interconversion (K-eq = 1.5) of HCCA and trans-o-hydroxybenzylidene pyruvic acid (tHBPA) in the naphthalene catabolic pathway of Pseudomonas putida. The dimeric protein binds one molecule of GSH very tightly (K-d approximate to nM) and a second molecule of GSH with much lower affinity (K-d approximate to 2 to 11 mu M). The enzyme is unstable in the absence of GSH. The turnover number in the forward direction (47 s(-1) at 25 degrees C) greatly exceeds off rates for GSH (k(off) approximate to 10(-3) to 10(-2) s(-1) at 10 degrees C), suggesting that GSH acts as a tightly bound cofactor in the reaction. The crystal structure of the enzyme at 1.7 angstrom resolution reveals that the isomerase is closely related to class kappa GSH transferases. Diffraction quality crystals could only be obtained in the presence of GSH and HCCA/tHBPA. Clear electron density is seen for GSH. Electron density for the organic substrates is located near the GSH and is best modeled to include both HCCA and tHBPA at occupancies of 0.5 for each. Although there is no electron density connecting the sulfur of GSH to the organic substrates, the sulfur is located very close (2.78 angstrom) to C7 of HCCA. Taken together, the results suggest that the isomerization reaction involves a short-lived covalent adduct between the sulfur of GSH and C7 of the substrate.
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