Co‐operative binding of triplicate carbohydrate‐binding modules from a thermophilic xylanase

Summary Family 6 carbohydrate‐binding modules were am‐plified by polymerase chain reaction (PCR) from Clostridium stercorarium strain NCIB11754 genomic DNA as a triplet. Individually, these modules bound to xylooligosaccharides and cellooligosaccharides with affinities varying from ≈ 3 × 103 M–1 to ≈ 1 × 105 M–1. Tandem and triplet combinations of these modules bound co‐operatively to soluble xylan and insoluble cellulose to give ≈ 20‐ to ≈ 40‐fold increases in affinity relative to the individual modules. This co‐operativity was an avidity effect resulting from the modules within the tandems and triplet interacting simul‐ taneously with proximal binding sites on the polysac‐charides. This occurred by both intrachain and interchain interactions. The duplication or triplication of modules appears to be linked to the growth temperature of the organism; co‐operativity in these multiplets may compensate for the loss of affinity at higher temperatures.