Article révisé par les pairs
Résumé : The structure of Xylose isomerase (X.I.) from Actinoplanes missouriensis has been solved to 2.8 Angstroms resolution. Phases were determined from a single Eu3+ derivative and from the noncrystallographic 22 symmetry of the tetrameric molecule. An atomic model was built and subjected to restrained crystallographic refinement. The resulting model is shown to be closely similar to the recently reported X.I.'s structures from three other bacterial sources. Each monomer is found to be composed of an eight‐stranded α/β “T.I.M.” barrel forming an N‐terminal domain of 328 residues followed by a large loop of 66 residues embracing an adjacent subunit. Analysis of intersubunit packing shows that the X.I. tetramer is an assembly of two tight dimers. The β barrel fits a simple hyperboloid model as other T.I.M. barrels do. The active site, identified as the binding site for the inhibitor xylitol, is located at the carboxyl end of the beta strands in the barrel next to a pair binding site for Eu3+ ions, which are assumed to the sites for the divalent ions involved in catalysis. Active sites in the tetramer are oriented towards the interface between dimmers. It is suggested that subunit interfaces might stabilize the active site region and this might explain the oligomeric nature of the other α/β barrel enzymes. Copyright © 1988 Alan R. Liss, Inc.

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