Assembly of α-Glucan by GlgE and GlgB in Mycobacteria and Streptomycetes
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- 作者：Abdul M. Rashid ; Sibyl F. D. Batey ; Karl Syson ; Hendrik Koliwer-Brandl ; Farzana Miah ; J. Elaine Barclay ; Kim C. Findlay ; Karol P. Nartowski ; Yaroslav Z. Khimyak ; Rainer Kalscheuer ; Stephen Bornemann
- 刊名：Biochemistry
- 出版年：2016
- 出版时间：June 14, 2016
- 年：2016
- 卷：55
- 期：23
- 页码：3270-3284
- 全文大小：741K
- 年卷期：0
- ISSN：1520-4995
文摘
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.