Fig. 7

Model for the degradation and utilization of complex β-mannans in R. intestinalis. Intracellular degradation of an acetylated galactoglucomanno- oligosaccharide is used as an example. Sugars are represented as in Fig. 1. Initial depolymerization of acetylated galactoglucomannan (AcGalactoglucomannan) occurs at the outer surface of R. intestinalis by the activity of RiGH26 (green). The extracellular recruitment of β-mannan is facilitated by interactions with CBM27 and CBM23. Import of products occurs through the ABC transporter RiMnBP/RiMPP1/RiMPP2 (orange). Within the cytoplasm, the acetyl and galactosyl decorations are removed by the two acetyl esterases RiCE2 and RiCEX (pink) and the α-galactosidase RiGH36 (yellow). The two β-glucosidases RiGH3A and RiGH3B (blue) release glucose from the non-reducing end of the β-manno-oligosaccharide. In addition, the reducing end mannose-releasing exo-oligomannosidase RiGH113 (green) can catalyze the removal of mannose units from the decorated manno-oligosaccharides until it reaches a galactosyl substituent at the subsite −1. Once de-ornamented, the β-manno-oligosaccharides are saccharified by the exo-acting RiGH130_2 (light green) with accumulation of M₂. The M₂ undergoes subsequent epimerization and phosphorolysis by the concerted activity of RiMep - RiGH130_1 (light green), with release of glucose and M1P. These end products enter the glycolytic pathway either directly (for glucose) or after being converted into M6P and F6P by the phosphomannose mutase RiPgm (red) and the isomerase RiGH1_D2 (turquoise, purple domain). Released mannose is converted to M6P by a hexokinase and processed as described above. Galactose enters glycolysis after conversion to G1P via the Leloir pathway. The pyruvate generated from glycolysis is converted to acetyl-CoA and then butyrate. Black arrows show reactions demonstrated in this study. Green arrows indicate previously demonstrated steps for the generation of butyrate from monosaccharides fermentation⁶⁸ by R. intestinalis