Enzymatic production of 3’-sialyllactose in milk

Human milk oligosaccharides (HMOs) are lactose-based glycan molecules present in human breast milk. HMOs are essentially not present in cow’s milk and hence not naturally available in infant formulas. HMOs possess several health and developmentally beneficial properties, and the sialylated HMOs are thought to play a particularly important role for infant brain development.

Enzymatic transsialylation directly in cow’s milk, involving enzyme catalyzed transfer of sialic acid from a sialic acid donor to an acceptor, is a novel route for producing sialylated HMOs for e.g. infant formulas. The transsialidase (EC 2.4.1.-) of Trypanosoma cruzi is linked to trypanosomatid pathogenicity, but certain hydrolytic sialidases (neuraminidases), EC 3.2.1.18, from non-pathogenic organisms, can actually catalyze transsialylation.

Here, we report enzymatic production of the HMO compound 3’-sialyllactose directly in cow’s milk using engineered transsialidases, Tr15 and Tr16, originating from the nonpathogenic Trypanosoma rangeli. Both Tr15 and Tr16 readily catalyzed transsialylation in milk at 5 °C - 40 °C using κ(kappa)-casein glycomacropeptide (cGMP) as sialyl donor substrate.

Tr15 was the most efficient as this enzyme produced 1160 mg/L (1.8 mM) 3’-sialyllactose in whole milk during 10 minutes of reaction at 5 °C. The activation energy values, Ea, of the enzymatic transsialylation reactions were similar in milk and in buffer solutions containing cGMP and lactose. The Ea of the Tr15 catalyzed transialylation reaction in milk was 16.5 kJ/mol, which was three times lower than the Ea of Tr16 (66 kJ/mol) and of T. cruzi transsialidase (50 kJ/mol), corroborating that Tr15 was the fastest of the three enzymes and a promising candidate for potential industrial production of 3’-sialyllactose in milk. 3’sialyllactose was stable during pasteurization (30 min. at 62.5 °C) and freeze-drying.