Capsule of Escherichia coli O5:K4:H4 is formed of a chondroitin-repeat disaccharide unit of glucuronic acid (GlcA)-N-acetylgalactosamine (GalNAc). This polysaccharide, commonly referred to as K4CP, is a potentially important source of precursors for chemoenzymatic or bioengineering synthesis of chondroitin sulfate. KfoA, encoded by a gene from region 2 of the K4 capsular gene cluster, shows high homology to the UDP-glucose-4-epimerase (GalE) from E. coli. KfoA is reputed to be responsible for uridine 5'-diphosphate-N-acetylgalactosamine (UDP-GalNAc) supply for K4CP biosynthesis in vivo, but it has not been biochemically characterized. Here, we probed the substrate specificity of KfoA by a capillary electrophoresis (CE)-based method. KfoA could epimerize both acetylated and non-acetylated substrates, but its k /K value for UDP-GlcNAc was approximately 1300-fold that for UDP-Glc. Recombinant KfoA showed a strong preference for acetylated substrates in vitro. The conclusion that KfoA is a higher efficiency UDP-GalNAc provider than GalE was supported by a coupled assay developed based on the donor-acceptor combination specificity of E. coli K4 chondroitin polymerase (KfoC). Furthermore, residue Ser-301, located near the UDP-GlcNAc binding pocket, plays an important role in the determination of the conversion ratio of UDP-GlcNAc to UDP-GalNAc by KfoA. Our results deepen the understanding of the mechanism of KfoA and will assist in the research into the metabolic engineering for chondroitin sulfate production.

Avibacterium paragallinarum is a Gram-negative bacterium that causes infectious coryza in chicken. It was reported that the capsule polysaccharides extracted from Av. paragallinarum genotype A contained chondroitin. Chondroitin synthase of Av. paragallinarum (ApCS) encoded by one gene within the presumed capsule biosynthesis gene cluster exhibited considerable homology to identified bacterial chondroitin synthases. Herein, we report the identification and characterization of ApCS. This enzyme indeed displays chondroitin synthase activity involved in the biosynthesis of the capsule. ApCS is a bifunctional protein catalyzing the elongation of the chondroitin chain by alternatively transferring the glucuronic acid (GlcA) and N-acetyl-D-galactosamine (GalNAc) residues from their nucleotide forms to the non-reducing ends of the saccharide chains. GlcA with a para-nitrophenyl group (pNP) could serve as the acceptor for ApCS; this enzyme shows a stringent donor tolerance when the acceptor is as small as this monosaccharide. Then, UDP-GalNAc and GlcA-pNP were injected sequentially through the chip-immobilized chondroitin synthases, and the surface plasmon resonance data demonstrated that the up-regulated extent caused by the binding of the donor is one possibly essential factor in successful polymerization reaction. This conclusion will, therefore, enhance the understanding of the mode of action of glycosyltransferase. Surprisingly, high activity at near-zero temperature as well as weak temperature dependence of this novel bacterial chondroitin synthase indicate that ApCS was a cold-active enzyme. From all accounts, ApCS becomes the fourth known bacterial chondroitin synthase, and the potential applications in artificial chondroitin sulfate and glycosaminoglycan synthetic approaches make it an attractive glycosyltransferase for further investigation.