No abstract

Neisseria meningitidis (Nm) serogroup W (NmW) is one of the six meningococcal serogroups that cause majority of invasive meningococcal diseases. Its capsular polysaccharide (CPS) is a virulence factor and is a key component in NmW CPS-protein conjugate vaccines. The current clinically used NmW CPS-protein conjugate vaccines are effective, but the costs are high and the products are heterogeneous at both the CPS and conjugate levels. Toward the development of potentially better NmW CPS vaccines, herein we report the synthesis of homogeneous oligosaccharides of NmW CPS in a size-controlled manner using polysaccharide synthase NmSiaDW in a sequential one-pot multienzyme platform. Taking advantage of the obtained structurally defined synthetic oligosaccharides tagged with a hydrophobic chromophore, detailed biochemical characterization of NmSiaDW has been achieved. While the catalytic efficiency of the galactosyltransferase activity of NmSiaDW increases dramatically with the increase of the sialoside acceptor substrate size, the size difference of the galactoside acceptor substrate does not influence NmSiaDW sialyltransferase activity significantly. The ratio of donor and acceptor substrate concentrations, but not the size of the acceptor substrates, has been found to be the major determining factor for the sizes of the oligosaccharides produced. NmW CPS oligosaccharides with a degree of polymerization (DP) higher than 65 have been observed. The study provides a better understanding of NmSiaDW capsular polysaccharide synthase and showcases an efficient chemoenzymatic synthetic platform for obtaining structurally defined NmW CPS oligosaccharides in a size-controlled manner.

While multiple α 1–2‐mannosidases are necessary for glycoprotein N‐glycan maturation in vertebrates, a single bacterial α1–2‐mannosidase can be sufficient to cleave all α1–2‐linked mannose residues in host glycoprotein N‐glycans. We report here the characterization and crystal structure of a new α1–2‐mannosidase (EfMan‐I) from Enterococcus faecalis, a Gram‐positive opportunistic human pathogen. EfMan‐I catalyzes the cleavage of α1–2‐mannose from not only oligomannoses but also high‐mannose‐type N‐glycans on glycoproteins. Its 2.15 Å resolution crystal structure reveals a two‐domain enzyme fold similar to other CAZy GH92 mannosidases. An unexpected potassium ion was observed bridging two domains near the active site. These findings support EfMan‐I as an effective catalyst for in vitro N‐glycan modification of glycoproteins with high‐mannose‐type N‐glycans.

O-Acetylated sialic acid has been found in the Neisseria meningitidis serogroup W (NmW) capsular polysaccharide (CPS) and is a required structural component of clinically used NmW CPS-based polysaccharide and polysaccharide–conjugate vaccines. The role of sialic acid O-acetylation in NmW CPS, however, is not clearly understood. This is partially due to the lack of a precise control of the percentage and the location of O-acetylation which is labile and susceptible to migration. We explore chemoenzymatic synthetic strategies for preparing N-acetylated analogues of O-acetylated NmW CPS oligosaccharides which can serve as structurally stable probe mimics. Substrate specificity studies of NmW CPS polymerase (NmSiaDW) identified 4-azido-4-deoxy-N-acetylmannosamine (ManNAc4N3) and 6-azido-6-deoxy-N-acetylmannosamine (ManNAc6N3) as suitable chemoenzymatic synthons for synthesizing N-acetyl analogues of NmW CPS oligosaccharides containing 7-O-acetyl-N-acetylneuraminic acid (Neu5,7Ac2) and/or 9-O-acetyl-N-acetylneuraminic acid (Neu5,9Ac2). The synthesis was achieved by NmSiaDW-dependent sequential one-pot multienzyme (OPME) strategy with in situ generation of the corresponding sugar nucleotides from simple monosaccharides or derivatives to form N3-oligosaccharides which were converted to the desired NAc-oligosaccharides by an efficient one-step chemical transformation.

With the continuing advancement of carbohydrate chemical synthesis, bacterial glycomes have become increasingly attractive and accessible synthetic targets. While bacteria also produce carbohydrate-containing secondary metabolites, our review here will cover recent chemical synthetic efforts on bacterial surface glycans. The obtained compounds are excellent candidates for the development of improved structurally defined glycoconjugate vaccines against bacterial infection. They are also important probes for investigating glycan-protein interactions. Glycosylation strategies applied for the formation of some challenging glycosidic bonds of various uncommon sugars in a number of recently synthesized bacterial surface glycans are highlighted.