Characterization of a novel carbohydrate-binding module specifically binding to the major structural units of porphyran

Porphyran is a promising bioactive polysaccharide majorly composed of 4-linked alpha-L-galactopyranose-6-sulfate (L6S) and 3-linked 13-D-galactopyranose (G) disaccharide repeating units. Carbohydrate-binding modules (CBMs) have been verified to be essential tools for investigating polysaccharides. However, no confirmed CBM binding to porphyran has been hitherto reported. In this study, an unknown domain with a predicted 13-sandwich fold from a potential GH86 porphyranase was discovered, and further recombinantly expressed. The CBM protein (named FvCBM99) presented a desired specificity for porphyran tetrasaccharide with an affinity constant of 1.9 x 10-4 M, while it could not bind to agarose tetrasaccharide. The sequence novelty and well-defined function of FvCBM99 and its homologs reveal a new CBM family, CBM99. Besides, the application potential of FvCBM99 in in situ visualization of porphyran was demonstrated. The discovery of FvCBM99 provides a favorable tool for future studies of porphyran.

Automated summary

In this study, a previously unknown domain with a predicted 13-sandwich fold structure was discovered in a potential GH86 porphyranase and successfully expressed in a CBM protein (FvCBM99). FvCBM99 specifically binds to porphyran tetrasaccharide and has potential applications in the in situ visualization of porphyran.