Loss-of-function mutation in bi-functional marine bacterial sialyltransferase.

An α2,3-sialyltransferase produced by Photobacterium phosphoreum JT-ISH-467 is a bi-functional enzyme showing both α2,3-sialyltransferase and α2,3-linkage specific sialidase activity. To date, the crystal structures of several sialyltransferases have been solved, but the roles of amino acid residues around the catalytic site have not been completely clarified. Hence we performed a mutational study using α2,3-sialyltransferase cloned from P. phosphoreum JT-ISH-467 as a model enzyme to study the role of the amino acid residues around the substrate-binding site. It was found that a mutation of the glutamic acid at position 342 in the sialyltransferase resulted in a loss of sialidase activity, although the mutant showed no decrease in sialyltransferase activity. Based on this result, it is strongly expected that the Glu342 of the enzyme is an important amino acid residue for sialidase activity.