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.