Structural determination of Streptococcus pneumoniae repeat units in serotype 41A and 41F capsular polysaccharides to probe gene functions in the corresponding capsular biosynthetic loci

We report the repeating unit structures ofthe native capsular polysaccharidesof S. pneumoniaeserotypes 41A and 41F. Structuraldeterminationsyieldedsix carbohydrate units in the doubly branched repeating unit to givethe following structure for serotype 41A:The structure determinations were motivated (1) by anambitionto help close the remaining gaps in S. pneumoniaecapsular polysaccharide structures, and (2)by the attempt to derive functional annotationsof carbohydrate active enzymes in the biosynthesis of bacterial polysaccharides from the determined structures. Anactivitypresent in 41F but not 41Ais identified as an acetyltransferaseacting on therhamnopyranosoyl sidechain E. The genes encoding theformation of thesix glycosidic bondsin serogroup 41 weredetermined from thecapsular polysaccharide structuresofserotype 41A,41F and genetically related serotypes,in conjunction with corresponding genomic information and computational homology searches. In combination with complementary information, NMR spectroscopy considerably simplifiesthe functional annotation of carbohydrate active enzymes in the biosynthesis of bacterial polysaccharides