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Sialyltransferase is a key enzymes involved in the biosynthesis of sialic acid-containing structure and potential therapeutic targets. Pasteurella multocida is a multifunctional sialyltransferase (PST) which transfer sialic acid from cytidine 5’-monophosphonuraminic acid (CMP-NeuAc) to an acceptor sugar. Multifunction of PST was controlled by temperature and pH. PST transferred a NeuAc residue from CMP-NeuAc to lactose to form α2,3-sialyl linkages efficiently in a pH range of 7.5-9.0 with low temperature. On the other hand, α2,6-sialyl linkage was formed at pH 4.5-7.0 over 24℃. Multifunction of PST is associated with its conformational
change and binding with lactose acceptor according to pH and temperature. We selected 3 amino acids through the modelling and generated mutants by saturation mutagenesis. To select mutants having an increased activity, pH-color assay method is used. Screening is based on the color changes of the pH indicator cresol red when proton is released during the transfer
of NeuAc from CMP-NeuAc to acceptor substrate. By using the method, mutants were selected and characterized.