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

An α2,3-Sialyltransferase from Photobacterium phosphoreum with Broad Substrate Scope: Controlling Hydrolytic Activity by Directed Evolution

Defined sialoglycoconjugates are important molecular probes for studying the role of sialylated glycans in biological systems. We show that the α2,3-sialyltransferase from Photobacterium phosphoreum JT-ISH-467 (2,3SiaTpph ) tolerates a very broad substrate scope for modifications in the sialic acid part, including bulky amide variation, C5/C9 substitution, and C5 stereoinversion. To reduce the enzyme's hydrolytic activity, which erodes the product yield, an extensive structure-guided mutagenesis study identified three variants that show up to five times higher catalytic efficiency for sialyltransfer, up to ten times lower efficiency for substrate hydrolysis, and drastically reduced product hydrolysis. Variant 2,3SiaTpph (A151D) displayed the best performance overall in the synthesis of the GM3 trisaccharide (α2,3-Neu5Ac-Lac) from lactose in a one-pot, two-enzyme cascade. Our study demonstrates that several complementary solutions can be found to suppress the common problem of undesired hydrolysis activity of microbial GT80 sialyltransferases. The new enzymes are powerful catalysts for the synthesis of a wide variety of complex natural and new-to-nature sialoconjugates for biological studies.

Active-Site His85 of Pasteurella dagmatis Sialyltransferase Facilitates Productive Sialyl Transfer and So Prevents Futile Hydrolysis of CMP-Neu5Ac

Sialyltransferases of the GT-80 glycosyltransferase family are considered multifunctional because of the array of activities detected. They exhibit glycosyl transfer, trans-sialylation, and hydrolysis activities. How these enzymes utilize their active-site residues in balancing the different enzymatic activities is not well understood. In this study of Pasteurella dagmatis α2,3sialyltransferase, we show that the conserved His85 controls efficiency and selectivity of the sialyl transfer. A His85→Asn variant was 200 times less efficient than wild-type for sialylation of lactose, and exhibited relaxed site selectivity to form not only the α2,3- but also the α2,6-sialylated product (21 %). The H85N variant was virtually inactive in trans-sialylation but showed almost the same CMP-Neu5Ac hydrolase activity as wild-type. The competition between sialyl transfer and hydrolysis in the conversion of CMP-Neu5Ac was dependent on the lactose concentration; this was characterized by a kinetic partition ratio of 85 m^-1 for the H85N variant, compared to 17 000 m^-1 for the wild-type enzyme. His85 promotes the productive sialyl transfer to lactose and so prevents hydrolysis of CMP-Neu5Ac in the reaction.