Mucins are highly glycosylated proteins present in the mucus layers of most organisms (e.g. the intestinal mucus). They contribute to protect us from external microorganisms (e.g. pathogenic bacteria), avoiding that they reach the epithelial cells.
Most bacteria that live in the intestinal mucus are able to eat fiber and starch, which they get from the food that we intake. Some of them can also consume sugars and mucins from our mucus. To do this they are equipped with specific enzymes, named mucinases, which break mucins into pieces before they can be correctly processed by the bacteria. These molecular scissors (mucinases) are not only important for the regeneration of our mucus, they also have a bad side, they are used by pathogenic bacteria, such as like enterohemorrhagic Escherichia coli, to penetrate in the protective mucus and reach the underlying epithelial cells.
The molecular mechanisms of mucin degradation by mucinases, i.e. how they bind to mucins (recognition) and how they cut them into pieces (catalysis), remains unknown. Recently, we have uncovered these mechanisms for AM0627, a mucinase secreted by the bacteria Akkermansia muciniphila, by means of a multidisciplinary work involving protein structure determination, synthetic biology and molecular simulations.
Our results show that AM0627 is able to recognize two contiguous aminoacids (residues) that are glycosylated, with preference for each of them containing two sugars of N-acetylgalactosamine and galactose (GalNAc-Gal, or T antigen) versus them having a single GalNAc (or Tn antigen). Structural comparison among mucinases identified a conserved tyrosine engaged in sugar-? interactions as responsible for the common activity of these two mucinases with bis-T/Tn antigens. Once the enzyme binds to these specific glycosylation sites, it cleaves the peptide bond between the two glycosylated residues via a concerted mechanism relying on a nucleophilic water molecule.
Our work illustrates how mucinases, through tremendous flexibility, adapt to the diversity in distribution and patterns of O-glycans on mucins and how they catalyze the chemical reaction at atomic detail.