Xianwei Liu

In vitro bacterial polysaccharide biosynthesis: defining the functions of Wzy and Wzz

Polysaccharides constitute a major component of bacterial cell surfaces and play critical roles in bacteria/host interactions. The biosynthesis of such molecules, however, has mainly been characterized through in vivo genetic studies, thus precluding discernment of the details of this pathway. Accordingly, we present a chemical approach which enabled reconstitution of the E. coli O-polysaccharide biosynthetic pathway in vitro. Starting with chemically prepared N-Acetyl-D-galactosamine-diphospho-undecaprenyl, the E. coli O86 oligosaccharide repeating unit was assembled via sequential enzymatic glycosylation. Successful expression of the putative polymerase Wzy via a chaperone co-expression system then allowed demonstration of polymerization in vitro using this substrate. Analysis of additional substrates revealed a defined mode of recognition for Wzy towards the lipid moiety. Specific polysaccharide chain length modality was furthermore demonstrated to result from the action of Wzz. Collectively, polysaccharide biosynthesis was chemically reconstituted in vitro, providing a well-defined system for further underpinning molecular details of this biosynthetic pathway.

Remodeling bacterial polysaccharides by metabolic pathway engineering

Introducing structural modifications into biomolecules represents a powerful approach to dissect their functions and roles in biological processes. Bacterial polysaccharides, despite their rich structural information and essential roles in bacterium-host interactions and bacterial virulence, have largely been unexplored for in vivo structural modifications. In this study, we demonstrate the incorporation of a panel of monosaccharide analogs into bacterial polysaccharides in a highly homogenous manner via metabolic engineering of a promiscuous sugar nucleotide biosynthetic pathway. In addition, the bioorthorgonal functional groups metabolically incorporated were exploited for cell surface labeling using in vitro selective chemical ligation reactions. In summary, our study presents a general, facile and effective approach for in vivo generation of novel tailor-made bacterial polysaccharides.

Histone Deacetylase Inhibitors through Click Chemistry

Histone deacetylase inhibitors (HDACi) are a relatively new class of chemotherapy agents. Herein, we report a click-chemistry based approach to the synthesis of HDACi. Fourteen agents were synthesized from the combination of two alkyne and seven azido precursors. The inhibition of HDAC1 and HDAC8 was then determined by in vitro enzymatic assays, after which the cytotoxicity was evaluated in the NCI human cancer cell line screen. A lead compound 5 g (NSC746457) was discovered that inhibited HDAC1 at an IC(50) value of 104 +/- 30 nM and proved quite potent in the cancer cell line screen with GI(50) values ranging from 3.92 microM to 10 nM. Thus, this click HDACi design has provided a new chemical scaffold that has not only revealed a lead compound, but one which is easily amendable to further structural modifications given the modular nature of this approach.

Over-Expression of the Endocan Gene in Endothelial Cells from Hepatocellular Carcinoma is Associated with Angiogenesis and Tumour Invasion

Endocan plays a role in tumour angiogenesis and tumour growth. The aim of this study was to detect the expression of endocan in hepatocellular carcinoma (HCC) tumour-associated endothelial cells and to correlate endocan expression with clinicopathological parameters and tumour angiogenesis. Tumour tissues and surrounding non-cancerous hepatic parenchyma from 42 primary HCC patients were studied. Endothelial cells were isolated using magnetic microbeads conjugated with anti-CD31 and endocan expression was evaluated by real-time reverse transcription-polymerase chain reaction, Western blotting and immunohistochemistry. Endocan was significantly over-expressed in endothelial cells isolated from HCC tumours compared with corresponding non-cancerous liver tissues. In addition, the endocan mRNA level was significantly correlated with the serum α-fetoprotein level, intra-tumoural microvessel density, vascular endothelial growth factor mRNA, and vascular and venous invasion. The over-expression of endocan in tumour endothelial cells was closely related to the process of angiogenesis and pathogenesis in HCC, and suggests that endocan might be a useful marker for HCC progression.

Identification of a new α1,2-fucosyltransferase involved in O-antigen biosynthesis of Escherichia coli O86: B7 and formation of H-type 3 blood group antigen

Escherichia coli O86 possesses high human blood group B activity because of its O-antigen structure, sharing the human blood group B epitope. In this study, the wbwK gene of E. coli O86:B7 was expressed and purified as the GST fusion protein. Thereafter, the wbwK gene was biochemically identified to encode an alpha1,2-fucosyltransferase through radioactivity assays, as well as mass spectrometry and NMR spectroscopy. WbwK shows strict substrate specificity and only recognizes Gal beta1,3GalNAc alpha-OR (T-antigen and derivatives) as the acceptor to generate the H-type 3 blood group antigen. In contrast to other alpha1,2-fucosyltransferases, WbwK does not display activity toward the simple substrate Gal beta-OMe. Comparison with another recently characterized alpha1,2-fucosyltransferase (WbsJ) of E. coli O128:B12 indicates a low level of amino acid identity between them; however, they share a common acceptor substrate, Gal beta1,3GalNAc alpha-OR. Domain swapping between WbwK and WbsJ revealed that the smaller variable domains located in the C-terminus determine substrate specificity, whereas the larger variable domain in the N-terminus might play a role in forming the correct conformation for substrate binding or for localization of the alpha1,2-fucosyltransferase involved in O-antigen biosynthesis. In addition, milligram scale biosynthesis of the H-type 3 blood group antigen was explored using purified recombinant WbwK. WbwK may have potential applications in masking T-antigen, the tumor antigen, in vivo.

Characterization and synthetic application of a novel β1,3-galactosyltransferase from Escherichia coli O55:H7

A beta1,3-galactosyltransferase (WbgO) was identified in Escherichia coli O55:H7. Its function was confirmed by radioactive activity assay and structure analysis of the disaccharide synthesized with the recombinant enzyme. WbgO requires a divalent metal ion, either Mn(2+) or Mg(2+), for its activity and is active between pH 6.0-8.0 with a pH optimum of 7.0. N-acetylglucosamine (GlcNAc) and oligosaccharides with GlcNAc at the non-reducing end were shown to be its preferred substrates and it can be used for the synthesis of type 1 glycan chains from these substrates. Together with a recombinant bacterial GlcNAc-transferase, benzyl beta-lacto-N-tetraoside was synthesized with the purified WbgO to demonstrate the synthetic utility of WbgO.

Overexpression and topology of bacterial oligosaccharyltransferase PglB.

Campylobacter jejuni contains a post-translational N-glycosylation system in which a STT3 homologue, PglB, functions as the oligosaccharyltransferase. Herein, we established a method for obtaining relatively large quantities of homogenous PglB proteins. PglB was overexpressed in Escherichia coli C43(DE3) at a level of 1 mg/L cell cultures. The activity of purified PglB was verified using a chemically synthesized sugar donor: N-acetylgalactosamine-diphospho-undecaprenyl (GalNAc-PP-Und) and a synthesized peptide acceptor. The result confirms that PglB is solely responsible for the oligosaccharyltransferase activity and complements the finding that PglB exhibits relaxed sugar substrate specificity. In addition, we performed the topology mapping of PglB using the PhoA/LacZ fusion method. The topological model shows that PglB possesses 11 transmembrane segments and two relatively large periplasmic regions other than the C-terminal domain, which is consistent with the proposal of the common N(cyt)-C(peri) topology with 11 transmembrane segments for the STT3 family proteins.

Combining carbochips and mass spectrometry to study the donor specificity for the Neisseria meningitidis β1,3-N-acetylglucosaminyltransferase LgtA.

A library of 11 UDP-N-acetylglucosamine analogs were rapidly screened for their activities as donors for the Neisseria meningitidis β1,3-N-acetylglucosaminyltransferase (LgtA) by direct on-chip reaction and detection with SAMDI-TOF mass spectrometry. Six of the analogs were active in this assay and were analyzed by SAMDI to characterize the kinetics toward LgtA. The analysis revealed that substitutions on C-2, C-4, and C-6 affect the activity of the donors, with bulky groups at these positions decreasing affinity of the donors for the enzyme, and also revealed that activity is strongly affected by the stereochemistry at C-3, but not C-4, of the donor. The study is also significant because it demonstrates that SAMDI can be used to both profile glycosyltransferase activities and to provide a quantitative assessment of enzyme activity.

Influence of N-glycosylation on Saccharomyces cerevisiae morphology: a golgi glycosylation mutant shows cell division defects.

The N-glycosylation mutants (mnn1 and mnn1 och1) show different morphological characteristics at the restrictive and nonpermissive temperature. We deleted the MNN1 to eliminate the terminal α1, 3-linked mannose of hypermannosylation and deleted the OCH1 to block the elongation of the main backbone chain. The mnn1 cells exhibited no observable change with respect to the wild-type strain at 28°C and 37°C, but the mnn1 och1 double mutant exhibited defects in cell cytokinesis, showed a slower growth rate, and became temperature-sensitive. Meanwhile, the mnn1 och1 mutant tended to aggregate, which was probably due to the glycolsylation defect. Loss of mannosyl-phosphate-accepting sites in this mutant migth result in reduced charge repulsion between cell surfaces. Pyridylaminated glycans were profiled and purified through an NH2 column by size-fractionation high-performance liquid chromatography. Matrix assisted laser desoption/ionization time of flight mass spectrometry (MALDI TOF/MS) analysis of the N-glycan structure of the mnn1 och1 mutant revealed that the main component is Man8GlcNAc2.

Systematic study on the broad nucleotide triphosphate specificity of the pyrophosphorylase domain of the N-acetylglucosamine-1-phosphate uridyltransferase from Escherichia coli K12

N-Acetylglucosamine-1-phosphate uridyltransferase (GlmU) from Escherichia coli K12 is a bifunctional enzyme that catalyzes both the acetyltransfer and uridyltransfer reactions in the prokaryotic UDP-GlcNAc biosynthetic pathway. In this study, we report the broad substrate specificity of the pyrophosphorylase domain of GlmU during its uridyltransfer reaction and the substrate priority is ranked in the following order: UTP > dUTP > dTTP >> CTP > dATP/dm(6) ATP. This pyrophosphorylase domain of GlmU is also a tool to synthesize UDP-GlcNAc analogs, two examples of which were synthesized herein in multiple mg scale in vitro.