Crystal Structures of d-Tagatose 3-Epimerase from Pseudomonas cichorii and Its Complexes with d-Tagatose and d-Fructose
| Autor: | Takeyori Nishitani, Shigehiro Kamitori, Ken Izumori, Mitsugu Yamada, Goro Takada, Hiromi Yoshida |
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| Rok vydání: | 2007 |
| Předmět: |
Stereochemistry
Dimer Fructose Isomerase Crystallography X-Ray Catalysis Substrate Specificity chemistry.chemical_compound Structural Biology Pseudomonas Molecular Biology Hexoses Pseudomonas cichorii Binding Sites biology Active site Hydrogen Bonding biology.organism_classification Sorbose Crystallography chemistry biology.protein Epimer Carbohydrate Epimerases Crystallization Tagatose |
| Zdroj: | Journal of Molecular Biology. 374:443-453 |
| ISSN: | 0022-2836 |
| Popis: | Pseudomonas cichoriii d -tagatose 3-epimerase (P. cichorii d -TE) can efficiently catalyze the epimerization of not only d -tagatose to d -sorbose, but also d -fructose to d -psicose, and is used for the production of d -psicose from d -fructose. The crystal structures of P. cichorii d -TE alone and in complexes with d -tagatose and d -fructose were determined at resolutions of 1.79, 2.28, and 2.06 A, respectively. A subunit of P. cichorii d -TE adopts a (β/α)8 barrel structure, and a metal ion (Mn2+) found in the active site is coordinated by Glu152, Asp185, His211, and Glu246 at the end of the β-barrel. P. cichorii d -TE forms a stable dimer to give a favorable accessible surface for substrate binding on the front side of the dimer. The simulated omit map indicates that O2 and O3 of d -tagatose and/or d -fructose coordinate Mn2+, and that C3–O3 is located between carboxyl groups of Glu152 and Glu246, supporting the previously proposed mechanism of deprotonation/protonation at C3 by two Glu residues. Although the electron density is poor at the 4-, 5-, and 6-positions of the substrates, substrate–enzyme interactions can be deduced from the significant electron density at O6. The O6 possibly interacts with Cys66 via hydrogen bonding, whereas O4 and O5 in d -tagatose and O4 in d -fructose do not undergo hydrogen bonding to the enzyme and are in a hydrophobic environment created by Phe7, Trp15, Trp113, and Phe248. Due to the lack of specific interactions between the enzyme and its substrates at the 4- and 5-positions, P. cichorii d -TE loosely recognizes substrates in this region, allowing it to efficiently catalyze the epimerization of d -tagatose and d -fructose (C4 epimer of d -tagatose) as well. Furthermore, a C3–O3 proton-exchange mechanism for P. cichorii d -TE is suggested by X-ray structural analysis, providing a clear explanation for the regulation of the ionization state of Glu152 and Glu246. |
| Databáze: | OpenAIRE |
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