Section 58
Chapter 57,237

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

Rashid, A.M.; Batey, S.F.D.; Syson, K.; Koliwer-Brandl, H.; Miah, F.; Barclay, J.E.; Findlay, K.C.; Nartowski, K.P.; Khimyak, Y.Z.; Kalscheuer, R.; Bornemann, S.

Biochemistry 55(23): 3270-3284


ISSN/ISBN: 1520-4995
PMID: 27221142
DOI: 10.1021/acs.biochem.6b00209
Accession: 057236919

Download citation:  

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.

PDF emailed within 0-6 h: $19.90