Structural rationale for the short branched substrate specificity of the glycogen debranching enzyme GlgX |
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Authors: | Hyung‐Nam Song Tae‐Yang Jung Jong‐Tae Park Byung‐Chul Park Pyung Keun Myung Winfried Boos Eui‐Jeon Woo Kwan‐Hwa Park |
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Affiliation: | 1. Medical Proteomics Research Center, Korea Institute of Bioscience and Biotechnology, Yuseong‐gu, Daejeon 305‐806;2. College of Pharmacy, Chungnam National University, Daejeon 305‐764, Korea;3. These authors contributed equally.;4. Department of Bio‐Analytical Science, University of Sciency and Technology, 113 Gwahangno, Yuseong‐gu, Daejeon 305‐333, Korea;5. Department of Biology, University of Incheon, Incheon 402‐749, Korea;6. Department of Biology, University of Konstanz, Konstanz 78457, Germany |
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Abstract: | Glycogen serves as major energy storage in most living organisms. GlgX, with its gene in the glycogen degradation operon, functions in glycogen catabolism by selectively catalyzing the debranching of polysaccharide outer chains in bacterial glycosynthesis. GlgX hydrolyzes α‐1,6‐glycosidic linkages of phosphorylase‐limit dextrin containing only three or four glucose subunits produced by glycogen phosphorylase. To understand its mechanism and unique substrate specificity toward short branched α‐polyglucans, we determined the structure of GlgX from Escherichia Coli K12 at 2.25 Å resolution. The structure reveals a monomer consisting of three major domains with high structural similarity to the subunit of TreX, the oligomeric bifunctional glycogen debranching enzyme (GDE) from Sulfolobus. In the overlapping substrate binding groove, conserved residues Leu270, Asp271, and Pro208 block the cleft, yielding a shorter narrow GlgX cleft compared to that of TreX. Residues 207–213 form a unique helical conformation that is observed in both GlgX and TreX, possibly distinguishing GDEs from isoamylases and pullulanases. The structural feature observed at the substrate binding groove provides a molecular explanation for the unique substrate specificity of GlgX for G4 phosphorylase‐limit dextrin and the discriminative activity of TreX and GlgX toward substrates of varying lengths. Proteins 2010. © 2010 Wiley‐Liss, Inc. |
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Keywords: | GlgX GDE x‐ray structure glycogen catabolism malto‐oligosaccharides |
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