Steve Ealick's Research Group


Klebsiella pneumoniae Allantoin Racemase


PDB files:

3QVJ - unliganded
3QVK - allantoin complex:
3QVL - 5-acetyl hydantoin complex

Description:

The oxidative catabolism of uric acid produces 5-hydroxyisourate (HIU), which is further degraded to (S)-allantoin by two enzymes, HIU hydrolase (HpxT) and 2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline (OHCU) decarboxylase (HpxQ).  The intermediates of the latter two reactions, HIU and OHCU, are unstable in solution and decay nonstereospecifically to allantoin.  In addition, non-enzymatic racemization of allantoin has been shown to occur at physiological pH.  Since the further breakdown of allantoin is catalyzed by allantoinase, an enzyme that is specific for (S)-allantoin, an allantoin racemase is necessary for complete and efficient catabolism of uric acid.  In this work we determined the structure of a gene product from Klebsiella pneumoniae, KpHpxA, which functions as an allantoin racemase.

KpHpxA is an α/β protein with two domains that form a V shape in the protomer. The two domains share a high degree of similarity (RMSD 3.1 Å)

 


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 When crystallographic symmetry related molecules were considered, it appeared that KpHpxA formed a stable hexamer in the crystal. The hexamer, which was corroborated by size exclusion chromatography, is made up of a trimer of dimers, with the three identical dimer interfaces occurring at the groove between domains.


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The hydantoin ring of allantoin is positioned in the active site through a series of interactions as shown at the right.  Mutation of either of the active site cysteine residues to serine completely abolishes the ability of the enzyme to catalyze the racemization of allantoin. 


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Reference:

French JB, Neau DB, and Ealick SE. Characterization of the Structure and Function of Klebsiella pneumoniae Allantoin Racemase. J. Mol. Biol. 410: 447-460. PubMed



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