A polysaccharide lyase from Stenotrophomonas maltophilia with a unique, pH-regulated substrate specificity.

Polysaccharide lyases (PLs) catalyze the depolymerization of anionic polysaccharides via a β-elimination mechanism. PLs also play important roles in microbial pathogenesis, participating in bacterial invasion and toxin spread into the host tissue via degradation of the host extracellular matrix, or in microbial biofilm formation often associated with enhanced drug resistance. Stenotrophomonas maltophilia is a Gram-negative bacterium that is among the emerging multidrug-resistant organisms associated with chronic lung infections as well as with cystic fibrosis patients. A putative alginate lyase (Smlt1473) from S. maltophilia was heterologously expressed in Escherichia coli, purified in a one-step fashion via affinity chromatography, and activity as well as specificity determined for a range of polysaccharides. Interestingly, Smlt1473 catalyzed the degradation of not only alginate, but poly-β-D-glucuronic acid and hyaluronic acid as well. Furthermore, the pH optimum for enzymatic activity is substrate-dependent, with optimal hyaluronic acid degradation at pH 5, poly-β-D-glucuronic acid degradation at pH 7, and alginate degradation at pH 9. Analysis of the degradation products revealed that each substrate was cleaved endolytically into oligomers comprised predominantly of even numbers of sugar groups, with lower accumulation of trimers and pentamers. Collectively, these results imply that Smlt1473 is a multifunctional PL that exhibits broad substrate specificity, but utilizes pH as a mechanism to achieve selectivity.