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Structural and Kinetic Analysis of Substrate Binding to the Sialyltransferase Cst-II from Campylobacter jejuni*

Background: The transfer of sialic acids is catalyzed by a set of sialyltransferases with defined specificities. Results: We solved the ternary complex of the sialyltransferase Cst-II and kinetically characterized its mechanism. Conclusion: Our analysis gives insights into the acceptor specificity and proposes the iso-ordered Bi Bi mechanism. Significance: This work improves our understanding of sialyltransferase structure/function. Sialic acids play important roles in various biological processes and typically terminate the oligosaccharide chains on the cell surfaces of a wide range of organisms, including mammals and bacteria. Their attachment is catalyzed by a set of sialyltransferases with defined specificities both for their acceptor sugars and the position of attachment. However, little is known of how this specificity is encoded. The structure of the bifunctional sialyltransferase Cst-II of the human pathogen Campylobacter jejuni in complex with CMP and the terminal trisaccharide of its natural acceptor (Neu5Ac-α-2,3-Gal-β-1,3-GalNAc) has been solved at 1.95 Å resolution, and its kinetic mechanism was shown to be iso-ordered Bi Bi, consistent with its dual acceptor substrate specificity. The trisaccharide acceptor is seen to bind to the active site of Cst-II through interactions primarily mediated by Asn-51, Tyr-81, and Arg-129. Kinetic and structural analyses of mutants modified at these positions indicate that these residues are critical for acceptor binding and catalysis, thereby providing significant new insight into the kinetic and catalytic mechanism, and acceptor specificity of this pathogen-encoded bifunctional GT-42 sialyltransferase.