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Biochemistry. 2016 Jun 14;55(23):3270-84. doi: 10.1021/acs.biochem.6b00209. Epub 2016 Jun 2.

Assembly of α-Glucan by GlgE and GlgB in Mycobacteria and Streptomycetes.

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Biological Chemistry Department, John Innes Centre , Norwich Research Park, Norwich NR4 7UH, United Kingdom.
Institute for Medical Microbiology and Hospital Hygiene, and Institute for Pharmaceutical Biology and Biotechnology, Heinrich-Heine-University Düsseldorf , Universitätsstrasse 1, 40225 Düsseldorf, Germany.
Cell and Developmental Biology Department, John Innes Centre , Norwich Research Park, Norwich NR4 7UH, United Kingdom.
School of Pharmacy, University of East Anglia , Norwich Research Park, Norwich NR4 7TJ, United Kingdom.


Actinomycetes, such as mycobacteria and streptomycetes, synthesize α-glucan with α-1,4 linkages and α-1,6 branching to help evade immune responses and to store carbon. α-Glucan is thought to resemble glycogen except for having shorter constituent linear chains. However, the fine structure of α-glucan and how it can be defined by the maltosyl transferase GlgE and branching enzyme GlgB were not known. Using a combination of enzymolysis and mass spectrometry, we compared the properties of α-glucan isolated from actinomycetes with polymer synthesized in vitro by GlgE and GlgB. We now propose the following assembly mechanism. Polymer synthesis starts with GlgE and its donor substrate, α-maltose 1-phosphate, yielding a linear oligomer with a degree of polymerization (∼16) sufficient for GlgB to introduce a branch. Branching involves strictly intrachain transfer to generate a C chain (the only constituent chain to retain its reducing end), which now bears an A chain (a nonreducing end terminal branch that does not itself bear a branch). GlgE preferentially extends A chains allowing GlgB to act iteratively to generate new A chains emanating from B chains (nonterminal branches that themselves bear a branch). Although extension and branching occur primarily with A chains, the other chain types are sometimes extended and branched such that some B chains (and possibly C chains) bear more than one branch. This occurs less frequently in α-glucans than in classical glycogens. The very similar properties of cytosolic and capsular α-glucans from Mycobacterium tuberculosis imply GlgE and GlgB are sufficient to synthesize them both.

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