Size-Controlled Chemoenzymatic Synthesis of Homogeneous Oligosaccharides of W Capsular Polysaccharide.

(Nm) serogroup W (NmW) is one of the six meningococcal serogroups that cause majority of invasive meningococcal diseases (IMD). 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 the conjugate levels. Towards 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 NmSiaD in a sequential one-pot multienzyme (OPME) platform. Taking advantage of the obtained structurally defined synthetic oligosaccharides tagged with a hydrophobic chromophore, detailed biochemical characterization of NmSiaD has been achieved. While the catalytic efficiency of the galactosyltransferase activity of NmSiaD increases dramatically with the increase of the sialoside acceptor substrate size, the size difference of the galactoside acceptor substrate does not influence NmSiaD 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 NmSiaD capsular polysaccharide synthase and showcases an efficient chemoenzymatic synthetic platform for obtaining structurally defined NmW CPS oligosaccharides in a size-controlled manner.