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The Pseudomonas syringae genome encodes a combined mannuronan C-5-epimerase and O-acetylhydrolase, which strongly enhances the predicted gel-forming properties of alginates.

Authors
  • Bjerkan, Tonje M
  • Bender, Carol L
  • Ertesvåg, Helga
  • Drabløs, Finn
  • Fakhr, Mohamed K
  • Preston, Lori A
  • Skjak-Braek, Gudmund
  • Valla, Svein
Type
Published Article
Journal
Journal of Biological Chemistry
Publisher
American Society for Biochemistry & Molecular Biology (ASBMB)
Publication Date
Jul 09, 2004
Volume
279
Issue
28
Pages
28920–28929
Identifiers
PMID: 15123694
Source
Medline
License
Unknown

Abstract

Alginates are industrially important, linear copolymers of beta-d-mannuronic acid (M) and its C-5-epimer alpha-l-guluronic acid (G). The G residues originate from a postpolymerization reaction catalyzed by mannuronan C-5-epimerases (MEs), leading to extensive variability in M/G ratios and distribution patterns. Alginates containing long continuous stretches of G residues (G blocks) can form strong gels, a polymer type not found in alginate-producing bacteria belonging to the genus Pseudomonas. Here we show that the Pseudomonas syringae genome encodes a Ca(2+)-dependent ME (PsmE) that efficiently forms such G blocks in vitro. The deduced PsmE protein consists of 1610 amino acids and is a modular enzyme related to the previously characterized family of Azotobacter vinelandii ME (AlgE1-7). A- and R-like modules with sequence similarity to those in the AlgE enzymes are found in PsmE, and the A module of PsmE (PsmEA) was found to be sufficient for epimerization. Interestingly, an R module from AlgE4 stimulated Ps-mEA activity. PsmE contains two regions designated M and RTX, both presumably involved in the binding of Ca(2+). Bacterial alginates are partly acetylated, and such modified residues cannot be epimerized. Based on a detailed computer-assisted analysis and experimental studies another PsmE region, designated N, was found to encode an acetylhydrolase. By the combined action of N and A PsmE was capable of redesigning an extensively acetylated alginate low in G from a non gel-forming to a gel-forming state. Such a property has to our knowledge not been previously reported for an enzyme acting on a polysaccharide.

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