Sialyltransferases of marine bacteria efficiently utilize glycosphingolipid substrates.

Bacterial sialyltransferases (STs) from marine sources were characterized using glycosphingolipids (GSLs). Bacterial STs were found to be beta-galacotoside STs. There were two types of STs: (1) ST obtained from strains such as ishi-224, 05JTC1 (#1), ishi-467, 05JTD2 (#2), and faj-16, 05JTE1 (#3), which form alpha 2-3 sialic acid (Sia) linkages, named alpha 2-3ST, (2) ST obtained from strains such as ISH-224, N1C0 (#4), pda-rec, 05JTB2 (#5), and pda-0160, 05JTA2 (#6), which form alpha 2-6 Sia linkages, named alpha 2-6ST. All STs showed affinity to neolacto- and lacto-series GSLs, particularly in neolactotetraosyl ceramide (nLc(4)Cer). No large differences were observed in the pH and temperature profiles of enzyme activities. Kinetic parameters obtained by Lineweaver-Burk plot analysis showed that #3 and #4 STs had practical synthetic activity and thus it became easily possible to achieve large-scale ganglioside synthesis (100-300 mu M) using these recombinant enzymes. Gangliosides synthesized from nLc(4)Cer by alpha 2-3 and alpha 2-6STs were structurally characterized by several analytical and immunological methods, and they were identified as IV3 alpha NeuAc-nLc(4)Cer(S2-3PG) and IV6 alpha NeuAc-nLc(4)Cer (S2-6PG), respectively. Further characterization of these STs using lactotetraosylceramide (Lc(4)Cer), neolactohexaosylceramide (i antigen), and IV(6)kladoLc(8)Cer (I antigen) showed the synthesis of corresponding gangliosides as well. Synthesized gangliosides showed binding activity to the influenza A virus [A/panama/2007/99 (H3N2)] at a similar level to purified S2-3PG and S2-6PG from mammalian sources. The above evidence suggests that these STs have unique features, including substrate specificities restricted to lacto- and neolactoseries GSLs, as well as catalytic potentials for ganglioside synthesis. This demonstrates that efficient in vitro ganglioside synthesis could be a valuable tool for selectively synthesizing Sias modifications, thereby permitting the exploration of unknown functions.