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J. Biol. Chem., Vol. 281, Issue 41, 30650-30659, October 13, 2006

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Soluble Aldose Sugar Dehydrogenase from Escherichia coli

A HIGHLY EXPOSED ACTIVE SITE CONFERRING BROAD SUBSTRATE SPECIFICITY^*

From the Centre for Metalloprotein Spectroscopy and Biology, School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, United Kingdom

A water-soluble aldose sugar dehydrogenase (Asd) has been purified for the first time from Escherichia coli. The enzyme is able to act upon a broad range of aldose sugars, encompassing hexoses, pentoses, disaccharides, and trisaccharides, and is able to oxidize glucose to gluconolactone with subsequent hydrolysis to gluconic acid. The enzyme shows the ability to bind pyrroloquinoline quinone (PQQ) in the presence of Ca²⁺ in a manner that is proportional to its catalytic activity. The x-ray structure has been determined in the apo-form and as the PQQ-bound active holoenzyme. The -propeller fold of this protein is conserved between E. coli Asd and Acinetobacter calcoaceticus soluble glucose dehydrogenase (sGdh), with major structural differences lying in loop and surface-exposed regions. Many of the residues involved in binding the cofactor are conserved between the two enzymes, but significant differences exist in residues likely to contact substrates. PQQ is bound in a large cleft in the protein surface and is uniquely solvent-accessible compared with other PQQ enzymes. The exposed and charged nature of the active site and the activity profile of this enzyme indicate possible factors that underlie a low affinity for glucose but generic broad substrate specificity for aldose sugars. These structural and catalytic properties of the enzymes have led us to propose that E. coli Asd provides a prototype structure for a new subgroup of PQQ-dependent soluble dehydrogenases that is distinct from the A. calcoaceticus sGdh subgroup.

Received for publication, February 24, 2006 , and in revised form, July 24, 2006.

^* This work was supported by Diabetes UK Grant BDA:RD04/0002927. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. The atomic coordinates and structure factors (code 2G8S) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).

The on-line version of this article (available at http://www.jbc.org) contains supplemental Fig. 1.

² Present address: N.V. Organon, Molecular Design and Informatics, P.O. Box 20, 5340 BH Oss, The Netherlands.

¹ To whom correspondence should be addressed. E-mail: Stacey.Southall{at}icr.ac.uk.

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