Pectin methylesterase. The representative structure is BT1017 Q8A900 a methylesterases (6-O-methyl esterase)essential for pectin rhamnogalacturonan II metabolism from the gut bacterium Bacteroides thetaiotaomicron. This family is exclusivly bacterial . Plant pectin methylesterase are not alpha/bet hydrolases. In many members C-term is DUF3826 a putative sugar-binding domain. Some members are close to Acetyl-esterase_deacetylase or Abhydrolase_7. Carbohydrate Esterase Family 19 (CE_19) of CAZY
Pectins are a major dietary nutrient source for the human gut microbiota (HGM). The prominent gut microbe Bacteroides thetaiotaomicron was recently shown to encode the founding member (BT1017) of a new family of pectin methylesterases (PMEs) essential for the metabolism of the complex pectin rhamnogalacturonan-II (RG-II). However, biochemical and structural knowledge of this family is lacking. Here, we showed that BT1017 is critical for the metabolism of an RG-II-derived oligosaccharide deltaBT1017oligoB generated by a BT1017 deletion mutant (deltaBT1017) during growth on carbohydrate extract from apple juice. Structural analyses of deltaBT1017oligoB using a combination of enzymatic, mass spectrometric and nuclear magnetic resonance approaches revealed that it is a bi-methylated nona-oligosaccharide GlcA-beta1,4-(2-O-Me-Xyl-alpha1,3)-Fuc-alpha1,4-(GalA-beta1,3)-Rha-alpha1,3-Api-beta1,2-(Araf-alpha1,3)-(GalA-alpha1,4)-GalA containing components of the RG-II backbone and its side chains. We showed that the catalytic module of BT1017 adopts an alpha/beta (alpha/beta) hydrolase fold, consisting of a central twisted 10-stranded beta-sheet sandwiched by several alpha-helices. This constitutes a new fold for PMEs, which are predominantly right-handed beta-helical proteins. Bioinformatics analyses revealed that the family is dominated by sequences from the prominent genera of the HGM, including Bacteroides and Prevotella Our results not only highlight the critical role played by this family of enzymes in pectin metabolism but provide new insights into the molecular basis of the adaptation of B. thetaiotaomicron to the human gut.
The metabolism of carbohydrate polymers drives microbial diversity in the human gut microbiota. It is unclear, however, whether bacterial consortia or single organisms are required to depolymerize highly complex glycans. Here we show that the gut bacterium Bacteroides thetaiotaomicron uses the most structurally complex glycan known: the plant pectic polysaccharide rhamnogalacturonan-II, cleaving all but 1 of its 21 distinct glycosidic linkages. The deconstruction of rhamnogalacturonan-II side chains and backbone are coordinated to overcome steric constraints, and the degradation involves previously undiscovered enzyme families and catalytic activities. The degradation system informs revision of the current structural model of rhamnogalacturonan-II and highlights how individual gut bacteria orchestrate manifold enzymes to metabolize the most challenging glycan in the human diet.
