Ligand-bound structures and site-directed mutagenesis identify the acceptor and secondary binding sites of Streptomyces coelicolor maltosyltransferase GlgE.

GlgE is a maltosyltranferase involved in alpha-glucan biosynthesis in bacteria that has been genetically validated as a target for tuberculosis therapies. Crystals of the Mycobacterium tuberculosis enzyme diffract at low resolution so most structural studies have been with the very similar Streptomyces coelicolor GlgE isoform 1. Although the donor binding site for alpha-1-phosphate had been previously structurally defined, the acceptor site had not. Using mutagenesis, kinetics and protein crystallography of the S. coelicolor enzyme, we have now identified the +1 to +6 sub-sites of the acceptor/product, which overlap with the known cyclodextrin binding site. The sugar residues in the acceptor sub-sites +1 to +5 are oriented such that they disfavour the binding of malto-oligosaccharides that bear branches at their 6 positions, consistent with the known acceptor chain specificity of GlgE. A secondary binding site remote from the catalytic centre was identified that is distinct from one reported for the M. tuberculosis enzyme. This new site is capable of binding a branched alpha-glucan, and is most likely involved in guiding acceptors towards the donor site because its disruption kinetically compromises the ability of GlgE to extend polymeric substrates. However, disruption of this site, which is conserved in the S. venezuelae GlgE enzyme, did not affect the growth of S. venezuelae or the structure of the polymeric product. The acceptor sub-sites +1 to +4 in the S. coelicolor enzyme are well conserved in the M. tuberculosis enzyme so their identification could help inform the design of inhibitors with therapeutic potential.