Enhancing the receptor affinity of the sialic acid-binding domain of <em>Vibrio cholerae</em> sialidase through multivalency

Many glycoside hydrolases possess carbohydrate-binding modules (CBMs) that help target these enzymes to appropriate substrates and increase their catalytic efficiency. The Vibrio cholerae sialidase contains two CBMs, one of which is designated as a family CBM40 module and has been shown through structural and calorimetry studies to recognize the alpha-anomer of sialic acid with a K-D of similar to 30 mu M at 37 degrees C. The affinity of this V. cholerae CBM40 module for sialic acid is one of the highest reported for recognition of a monosaccharide by a CBM. As Nature often increases a weak substrate affinity through multi-valency, we have explored the potential of developing reagents with an increased affinity for sialic acid receptors through linking CBM40 modules together. The V. cholerae CBM40 was sub-cloned and crystallized in the presence of sialyllactose confirming its ability to recognize sialic acid. Calorimetry revealed that this CBM40 demonstrated specificity to alpha(2,3)-, alpha(2,6)-, and alpha(2,8)-linked sialosides. Polypeptides containing up to four CBM40 modules in tandem were created to determine if an increase in affinity to sialic acid could be achieved through an avidity effect. Using SPR and a multivalent alpha(2,3)-sialyllactose ligand, we show that increasing the number of linked modules does increase the affinity for sialic acid. The four-CBM40 module protein has a 700- to 1500-fold increase in affinity compared with the single-CBM40 module. Varying the linker length of amino acids between each CBM40 module had little effect on the binding of these polypeptides. Finally, fluorescence-activated cell sorting analysis demonstrated that a green fluorescent protein fused to three CBM40 modules bound to subpopulations of human leukocytes. These studies lay the foundation for creating high affinity, multivalent CBMs that could have broad application in glycobiology.