An 1,4-α-glucosyltransferase defines a new maltodextrin catabolism scheme in Lactobacillus acidophilus

Abstract

The maltooligosaccharide (MOS) utilization locus in Lactobacillus acidophilus NCFM, a model for human small-intestine lactobacilli, encodes three glycoside hydrolases (GHs): a putative maltogenic α-amylase of family 13 subfamily 20 (LaGH13_20), a maltose phosphorylase of GH65 (LaGH65) and a family 13 subfamily 31 member (LaGH13_31B), annotated as a 1,6-α-glucosidase. Here, we reveal that LaGH13_31B is a 1,4-α-glucosyltransferase that disproportionates MOS of degree of polymerization (DP) ≥2, with preference for maltotriose. Kinetic analyses of the three GHs encoded by the MOS locus, revealed that the substrate preference of LaGH13_31B towards maltotriose, complements the about 40-fold lower k cat of LaGH13_20 towards this substrate, thereby enhancing the conversion of odd-numbered MOS to maltose. The concerted action of LaGH13_20 and LaGH13_31B confers the efficient conversion of MOS to maltose that is phosphorolysed by LaGH65. Structural analyses revealed the presence of a flexible elongated loop, which is unique for a previously unexplored clade of GH13_31 represented by LaGH13_31B. The identified loop insertion harbours a conserved aromatic residue that modulates the activity and substrate affinity of the enzyme, thereby offering a functional signature of this clade, which segregates from 1,6-α-glucosidases and sucrose isomerases previously described within GH13_31. Genomic analyses revealed that the LaGH13_31B gene is conserved in the MOS utilization loci of lactobacilli, including acidophilus cluster members that dominate the human small intestine. IMPORTANCE The degradation of starch in the small intestine generates short linear and branched α-glucans. The latter are poorly digestible by humans, rendering them available to the gut microbiota e.g lactobacilli adapted to the small intestine and considered as beneficial to health. This study unveils a previously unknown scheme of maltooligosaccharide (MOS) catabolism, via the concerted activity of an 1,4-α-glucosyltransferase together with a classical hydrolase and a phosphorylase. The intriguing involvement of a glucosyltransferase is likely to allow fine-tuning the regulation of MOS catabolism for optimal harnessing of this key metabolic resource in the human small intestine. The study extends the suite of specificities, which have been identified in GH13_31 and highlights amino acid signatures underpinning the evolution of 1,4-α-glucosyl transferases that have been recruited in the MOS catabolism pathway in lactobacilli.