Figure 7. Structure for S. oralis Uo5 LTA compared to S. pneumoniae LTA.

In the S. oralis Uo5 LTA, the repeating units (RUs) are composed of a pseudo-tetrasaccharide chain ((→ 4)-[3,6-O-di-P-Cho]-β-D-GalpNAc-(1 → 2)-Rib-ol-(1-P → 6)-(β-D-Galp-(1 → 3)-β-AATGalp-(1 →) (8 in Fig. 1)), RU 2 to n + 1 bear an additional α-1-linked D-Galp moiety at O-3 and an acetyl residue at O-4 of the β-D-Galp (9b in Fig. 1). Hydroxyl groups of the ribitol-1-P can be partially substituted with alanine. The chain length is 4 to 7 RUs. In S. pneumoniae LTA²³, all RUs comprise the pseudo-pentasaccharide ((→ 4)-6-O-P-Cho-α-D-GalpNAc-(1 → 3)-6-O-P-Cho-β-D-GalpNAc-(1 → 1)-Rib-ol-5-P-(O → 6)-β-D-Glcp-(1 → 3)-AATGalp-(1 →)) (4 in Fig. 1), the terminal RU can occur with or without 6-O-P-Cho-substitution (X = H or P-Cho). Known strains with the content of only one P-Cho per RU lack the P-Cho at β-D-GalpNAc (R′ = H). Hydroxyl groups of Rib-ol-5-P can be partially substituted with D-Ala. The chain length of pnLTA is 4 to 8 RUs in general. Notably, in pnLTA the first repeating unit is β-1-linked via the AATGalp to the lipid anchor, all other RUs are α-1-linked to the previous one. In S. oralis Uo5 LTA all AATGalp residues are β-configurated.