Unexpected and widespread connections between bacterial glycogen and trehalose metabolism

Abstract

Glycogen, a large α-glucan, is a ubiquitous energy storage molecule among bacteria, and its biosynthesis by the classical GlgC-GlgA pathway and its degradation have long been well understood – or so we thought. A second pathway of α-glucan synthesis, the four-step GlgE pathway, was recently discovered in mycobacteria. It requires trehalose as a precursor, and has been genetically validated as a novel anti-tuberculosis drug target. The ability to convert glycogen into trehalose was already known, so the GlgE pathway provides a complementary way of cycling these two metabolites. As well as containing cytosolic storage glycogen, mycobacteria possess an outer capsule containing a glycogen-like α-glucan that is implicated in immune system evasion, so the GlgE pathway might be linked to capsular α-glucan biosynthesis. Another pathway (the Rv3032 pathway) for α-glucan biosynthesis in mycobacteria generates a methylglucose lipopolysaccharide thought to be associated with fatty acid metabolism. A comparative genomic analysis was carried out to evaluate the occurrence and role of the classical pathway, the new GlgE pathway and the Rv3032 pathway across bacteria occupying very different ecological niches. The GlgE pathway is represented in 14 % of sequenced genomes from diverse bacteria (about half as common as the classical pathway), while the Rv3032 pathway is restricted with few exceptions to mycobacteria, and the GlgB branching enzyme, usually presumed to be associated with the classical pathway, correlates more strongly with the new GlgE pathway. The microbiological implications of recent discoveries in the light of the comparative genomic analysis are discussed.

Structure of Streptomyces Maltosyltransferase GlgE, a Homologue of a Genetically Validated Anti-tuberculosis Target

“…Relevance to GlgEs from Other Organisms-The glgE gene is widespread among bacteria (6). Most of these genes are similar in length to that of S. coelicolor, making it possible to generate homology models of GlgE proteins based on our structure.…”

“…This allows the structure of the S. coelicolor enzyme to be used to guide inhibitor design for the M. tuberculosis enzyme, which has been genetically validated as a potential novel drug target (3). Although the maltose site is largely hydrophilic, it includes two aromatic residues that sandwich maltose and provide potential hydrophobic surfaces to enhance the binding of inhibitors to the GlgE of M. tuberculosis and of other animal and plant pathogens (6). Importantly, the distinct configuration of the donor site of this GH13_3 enzyme provides the opportunity to develop inhibitors that do not target the many other GH13 subfamily enzymes present in mammals and plants.…”

“…The genes of this pathway are duplicated and separately and developmentally regulated in this organism, such that each is respectively associated with transient glycogen deposition at the initiation of aerial growth (phase I) and during the first stages of sporulation (phase II). The pathway is not restricted to actinomycetes and is remark-ably widespread (6). Fourteen percent of sequenced microbial genomes contain all of the GlgE pathway genes, which are usually clustered, making the pathway half as common as the more well known glycogen pathway involving GlgA and GlgC.…”

“…1) (3). M. tuberculosis is known to produce three ␣-glucans as follows: cytosolic glycogen, capsular ␣-glucan, and methylglucose lipopolysaccharide (6). These are either involved or implicated in the storage of carbon (7), evasion of the immune system (8 -11), and chaperoning/regulating fatty acid biosynthesis (12), respectively.…”

Mass Production of the Beneficial Nematode Heterorhabditis bacteriophora and Its Bacterial Symbiont Photorhabdus luminescens

“…Adult nematodes were seen (arrows) Fig. 11 P. luminescens on a NAT and b NBTA plates purposes as it is known to: (i) stabilize cellular constituents during environmental stresses such as high osmolarity and (ii) play a role in bacterial carbohydrate metabolism [59][60][61][62][63].…”