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

Rashid, Abdul M., Batey, Sibyl F. D., Syson, Karl, Koliwer-Brandl, Hendrik, Miah, Farzana, Barclay, J. Elaine, Findlay, Kim C., Nartowski, Karol P., Khimyak, Yaroslav Z., Kalscheuer, Rainer and Bornemann, Stephen (2016) Assembly of α-Glucan by GlgE and GlgB in Mycobacteria and Streptomycetes. Biochemistry, 55 (23). pp. 3270-3284. ISSN 0006-2960

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Abstract

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.

Item Type: Article
Additional Information: This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited.
Faculty \ School: Faculty of Science > School of Biological Sciences
Faculty of Science > School of Pharmacy
Faculty of Science
Depositing User: Pure Connector
Date Deposited: 01 Jul 2016 10:00
Last Modified: 22 Jul 2020 00:56
URI: https://ueaeprints.uea.ac.uk/id/eprint/59638
DOI: 10.1021/acs.biochem.6b00209

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