Featured Article - December 2010
Short description: Recent work gives insight into how microbes in our gut flora take advantage of passing nutrients, in this case, specific digestible carbohydrates.
Our intestines are home to many species of bacteria that make use of the nutrients passing through. Because these residents affect host health, both positively, by producing particular biochemicals, and adversely, by causing disease, the understanding this complex community has become a focus of metagenomics projects. Such data provide an excellent opportunity to follow adaptive changes and relative abundance of certain functions. For example the gut microbe genomes sequenced thus far show expanded representation of genes linked to carbohydrate metabolism and transport. Joachimiak and colleagues (PSI MCSG) have now biochemically and structurally characterized a Ruminococcus obeum α-glucosidase, dubbed Ro-α-G1. Ro-α-G1 structurally resembles the human maltase-glucoamylase (NtMGAM), a member of the glycosyl hydrolase GH31 family, but it forms a dimer in solution and in the crystal structure. The active sites are structurally very similar between human NtNGAM and Ro-α-G1, although there is less than 30% sequence identity in the catalytic domains. However, there is a key difference in the two active sites. The human NtMGAM has a tyrosine residue in the active site while the equivalent position in Ro-α-G1 is a Trp (W169). In substrate specificity assays, Ro-α-G1 does not act upon sucrose or lactose, but it does hydrolyze isomaltose more efficiently than it does maltose, suggesting that it prefers α(1-6) linkages over α(1-4) and cannot hydrolyze α(1-2) or β(1-4) linkages. A W169Y mutant shifts substrate specificity, now showing higher activity now displayed against maltose when compared to isomaltose. The authors also mutated other key catalytic residues, including the presumed nucleophile, D307, and D73, a residue from the N terminal domain that is present in the active site. These mutations kill hydrolase activity. In addition, the authors obtained the structure of the Ro-α-G1 D73A mutant, as well as the D307A mutant alone and in complex with isomaltose. Both these mutations confer reduced activity, and seem flexible in the apo-structure when compared to NtMGAM. However in the presence of substrate, these loops adopt a common structure that is similar to substrate-bound human NtMGAM. It may be that the α(1-6) linkage can be better accommodated in the Trp169 containing active site of Ro-α-G1, allowing the bacterium to potentially take full advantage of passing digestible carbohydrates.
K. Tan, C. Tesar, R. Wilton, L. Keigher, G. Babnigg & A. Joachimiak. Novel α-glucosidase from human gut microbiome: substrate specificities and their switch.
FASEB J. 24, 3939-3949 (2010). doi:10.1096/fj.10-156257