Polysaccharide utilization loci from Bacteroidota encode CE15 enzymes with possible roles in cleaving pectin-lignin bonds.

The plant cell wall is a highly complex matrix, and while most of its polymers interact non-covalently, there are also covalent bonds between lignin and carbohydrates. Bonds between xylan and lignin are known, such as the glucuronoyl ester bonds that are cleavable by CE15 enzymes. Our work here indicates that enzymes from CE15 may also have other activities, as we have discovered enzymes in PULs proposed to target other polysaccharides, including pectin. Our study represents the first investigation of such enzymes. Our first hypothesis that the enzymes would act as pectin methylesterases was shown to be false, and we instead propose that they may cleave other esters on complex pectins such as rhamnogalacturonan II. The work presents both the characterization of five novel enzymes and can also provide indirect information about the components of the cell wall itself, which is a highly challenging material to chemically analyze in fine detail.