Enzymatic production of HMO mimics by the sialylation of galacto-oligosaccharides.

Human milk oligosaccharides (HMOs) are a family of structurally diverse unconjugated glycans that exhibit a wide range of biological activities. In this report, we describe an efficient, Multi-Enzyme One-Pot strategy to produce HMO mimics by the sialylation of galacto-oligosaccharides (GOSs), which are often added to infant formula as an inexpensive alternative to HMOs. In this system, the sialyltransferase donor, cytidine-5'-monophospho-N-acetylneuraminic acid (CMP-Neu5Ac), was generated in situ using a CMP-sialic acid synthetase. The sialylated GOSs were obtained by one-step purification after digesting CMP using the alkaline phosphatase PhoA to cytidine and inorganic phosphate. Although the synthesized α2,3-, α2,6- and α2,3/8-sialyl-GOSs exhibit different sialylation levels and patterns, all of these mixtures can be fermented by Bifidobacterium longum subsp. infantis ATCC 15697 but not by Bifidobacterium adolescentis ATCC 15703. The sialidase NanH2, which is unique to the former strain, hydrolyzed all of the synthesized HMO mimics.