Figure 6.
Schematic depiction of the sequential degradation of TFLNH by pneumococcal GHs. GHs are indicated in bold next to the arrow for the reaction they catalyze; numbers in green refer to the gel lane in Fig. S4. SpGH95C and SpGH29C are required to remove the capping fucose residues from TFLNH and allow access to the oligosaccharide by other GHs. Treatment of TFLNH with SpGH29C results in removal of the α-(1→3)– and α-(1→4)–linked fucose units and allows BgaA and GH20C to degrade the arm proximal to the reducing end; however, without SpGH95C, the distal arm cannot be degraded. Treatment of TFLNH with SpGH95C results in partial removal of the α-(1→2)–linked fucose unit and a difucosylated oligosaccharide that cannot be acted upon by other GHs (except SpGH29C). If the α-(1→3)– and α-(1→4)–linked fucose units are removed by SpGH29C first, SpGH95C is able to fully remove the α-(1→2)–linked fucose unit from the distal arm to produce lacto-N-hexaose. This hexasaccharide can then be fully degraded into galactose and glucose by the combined actions of BgaA, BgaC, and GH20C. See Fig. S4 for experimental validation of this pathway.