Edited by Judit OvádiKeywords: a2,3-SialyltransfeaseCrystal structure JT-ISH-467 Sialic acid Broad substrate specificity Active conformation Photobacterium phosphoreum a b s t r a c t a/b-Galactoside a2,3-sialyltransferase produced by Photobacterium phosphoreum JT-ISH-467 is a unique enzyme that catalyzes the transfer of N-acetylneuraminic acid residue from cytidine monophosphate N-acetylneuraminic acid to acceptor carbohydrate groups. The enzyme recognizes both mono-and di-saccharides as acceptor substrates, and can transfer Neu5Ac to both a-galactoside and b-galactoside, efficiently. To elucidate the structural basis for the broad acceptor substrate specificity, we determined the crystal structure of the a2,3-sialyltransferase in complex with CMP. The overall structure belongs to the glycosyltransferase-B structural group. We could model a reasonable active conformation structure based on the crystal structure. The predicted structure suggested that the broad substrate specificity could be attributed to the wider entrance of the acceptor substrate binding site.

Heparan sulfate is a ubiquitous glycosaminoglycan in the extracellular matrix of most animals. It interacts with various molecules and exhibits important biological functions. K5 antigen produced by Escherichia coli strain K5 is a linear polysaccharide N-acetylheparosan consisting of GlcUA ␤1-4 and GlcNAc ␣1-4 repeating disaccharide, which forms the backbone of heparan sulfate. Region 2, located in the center of the K5-specific gene cluster, encodes four proteins, KfiA, KfiB, KfiC, and KfiD, for the biosynthesis of the K5 polysaccharide. Here, we expressed and purified the recombinant KfiA and KfiC proteins and then characterized these enzymes. Whereas the recombinant KfiC alone exhibited no GlcUA transferase activity, it did exhibit GlcUA transferase and polymerization activities in the presence of KfiA. In contrast, KfiA had GlcNAc transferase activity itself, which was unaffected by the presence of KfiC. The GlcNAc and GlcUA transferase activities were analyzed with various truncated and point mutants of KfiA and KfiC. The point mutants replacing aspartic acid of a DXD motif and lysine and glutamic acid of an ionic amino acid cluster, and the truncated mutants deleting the C-terminal and N-terminal sites, revealed the essential regions for GlcNAc and GlcUA transferase activity of KfiC and KfiA, respectively. The interaction of KfiC with KfiA is necessary for the GlcUA transferase activity of KfiC but not for the enzyme activity of KfiA. Together, these results indicate that the complex of KfiA and KfiC has polymerase activity to synthesize N-acetylheparosan, providing a useful tool toward bioengineering of defined heparan sulfate chains. Heparan sulfate (HS)3 is a linear polysaccharide of alternating hexuronic acid (D-glucuronic acid (GlcUA) or L-iduronic acid (IdoUA)) and D-glucosamine (GlcN) residues carrying sulfogroups at various sites of sugar residues. Usually, HS chains are covalently attached to a core protein in the form of proteoglycans and are present ubiquitously on the cell surface and in the extracellular matrices of animals. HS chains interact with cytokines, growth factors, coagulation factors, proteases and their inhibitors, and other molecules and contribute to several biological processes, including development, morphogenesis, cell proliferation and differentiation, and cancer cell invasion (1, 2).The biosynthesis of HS begins with the synthesis of the linkage tetrasaccharide, GlcUA-Gal-Gal-Xyl, on the serine residues of core proteins. The backbone of HS polysaccharide is then synthesized onto the linkage tetrasaccharide by alternating addition of monosaccharide units of GlcNAc and GlcUA with ␣1-4 and ␤1-4 bonds, respectively, using UDP-sugar donors with desired GlcNAc transferase (GlcNAc-T) and GlcUA transferase (GlcA-T). The elongation reaction is performed by HS co-polymerases, EXT1 and EXT2, in the Golgi apparatus (3, 4). The backbone is modified by several reactions, including N-deacetylation and N-sulfonation of GlcN by N-deacetylase/ N-sulfotransferase, C5-epimerization of GlcUA to f...