Characterization of sialate-O-acetylesterases in the gut commensal bacterium Bacteroides thetaiotamicron

The sialic acids are a family of acid sugars that are widely distributed in mammal cells and are found typically linked to the distal end of glycan chains in mammalian glycoproteins, including the mucin proteins found on mucosal surfaces. The most common sialic acid, N-acetyl neuraminic acid or Neu5Ac is often found with additional O-linked acetyl group s when located on gut epithelial cell surfaces. In Escherichia coli the enzyme NanS functions as a sialate O-acetylesterase, to remove the additional acetyl groups and allow use of these O-acetylated sialic acids for nutrition. Although NanS has a cytoplasmic location in E. coli, human faecal extracts have been found to contain extracellular sialate-O-acetylesterase activity, which is thought to correlate to the presence of bacteria from the Bacteroides genes. We have identified in silico the genes encoding potential sialate O- acetylesterases across a range of Bacteroides species and for Bacteroides thetaiotamicron, the most abundant commensal species of Bacteroides in the gut, there are 10 potential candidates. Preliminary expression experiments indicated that these proteins are highly insoluble after expression in E. coli conventional vectors and increased solubility was dependent on the expression of the target protein with a range of different protein fusion partners. Basic acetylesterase enzyme assays, using p-nitrophenylacetate as a standard substrate, demonstrated that a number of these proteins have detectable acetyl esterase activity. A subset of these proteins are predicted to be secreted and hence could be responsible for the previously reported sialate O-acetylesterase activity present in the gut. The identification of this activity is potentially significant as alteration in levels of O-acetylation of gut proteins are known to be connected to progression of gastrointestinal cancer.