Studies of a mycobacterial α-(1→4)-mannosyltransferase involved in 3-O-methyl-mannose polysaccharide biosynthesis

• Author / Creator

  Xia, Li

• Mycobacteria produce a number of unique carbohydrates, the majority of which are found in the cell wall of the organism. In addition to these extracellular glycans, mycobacteria also produce intracellular glycans. One class of them are the 3-O-methy-lmannose polysaccharides (MMPs), whose physiological functions and biosynthesis have not been fully established.
  An α-(1→4)-mannosyltransferase (ManT), one of the enzymes involved in MMP biosynthesis, incorporates mannose via an unusual α-(1→4)-linkage. However, to date, there is only one report studying this uncommon glycosyltransferase. In this thesis, we sought to expand upon this study to fully characterize the activity, specificity, and identity of ManT.
  We hypothesized that access to synthetic MMP fragments would lead to a greater understanding of the substrate specificity of ManT. When using synthetic MMP analogs to probe this enzyme, an unexpected activity for ManT was discovered. This enzyme was previously reported to recognize only substrates having a terminal methylated residue. However, we demonstrated that ManT also recognizes substrates without a methyl group on this terminal mannose residue, and with even better affinity. This finding was supported through careful structural elucidation of the isolated enzymatic products with various techniques including glycosidase digestion, Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry (MALDI-MS) and Nuclear Magnetic Resonance (NMR) spectroscopy, and a comparison of the kinetic parameters of two representative tetrasaccharide substrates.
  To further our understanding of MMP biosynthesis we sought to obtain pure ManT enzyme. Our first attempt to purify ManT using synthetic MMP-functionalized affinity chromatography was not successful. Therefore, a bioinformatic approach was used. Following this approach, we identified a putative ManT gene from M. smegmatis, and expressed this gene recombinantly in E. coli. The activity of the gene product was found to be identical to the native ManT from M. smegmatis. This will facilitate further studies of MMP biosynthesis.
  In the course of synthesizing MMP analogs for these studies, we developed a four-step methodology to quickly install several methyl groups directly on oligosaccharides. The key reaction involves n-Bu2SnCl2-mediated simultaneous activation of multiple cis-diols. By tuning protecting groups on the substrates, we were able to functionalize multiple cis-diols in a consistent and highly regioselective manner.

• Subjects / Keywords

  • 3-O-methyl-mannose polysaccharide
  • Α-(1→4)-mannosyltransferase
  • MMP biosynthesis
  • Regioselective polymethylation

• Graduation date

  Spring 2013

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R36688T0W

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.