The Swiss Army Knife of Alginate Manipulation – A Gut Bacterium Alginate Lyase with Diverse Catalytic Activities

The alginate-degrading enzyme Bo PL38 of the human gut bacterium Bacteroides ovatus CP926 degrades the three polysaccharide structures found in alginate, a major constituent of brown macroalgae with numerous industrial applications. However, the detailed mechanisms of alginate-degrading enzymes remain unclear. Crystal structures of Bo PL38 complexes with alginate oligosaccharides, now shed light on the enzyme’s catalytic machinery. QM/MM simulations reveal distinct conformational and reaction pathways, highlighting different transition states for mannuronate and guluronate conversion. C5 proton abstraction at subsite +1 by Y298 and H243 facilitates syn- and anti -β-elimination reactions, respectively. Substrate recognition relies on R292 distorting the sugar at subsite +1 into a preactivated conformation, while stabilizing the active site tunnel through a salt bridge. Furthermore, NMR spectroscopy found that Bo PL38 also catalyze mannuronate to guluronate epimerization in addition to its lyase function, thereby paving the way for future enzymatic alginate modification.