A novel mechanism of xylan binding by a lectin-like module from Streptomyces lividans xylanase 10A

The C-terminal module of xylanase 10A from Streptomyces lividans is a famil y 13 carbohydrate-binding module (CBM13). CBM13 binds mono- and oligo-sacch arides with association constants of approximate to 1 x 10(2) M-1-l x 10(3) M-1. It appears to be specific only for pyranose sugars. CBM13 binds insol uble and soluble xylan, holocellulose, pachyman, lichenan, arabinogalactan and laminarin. The association constant for binding to soluble xylan is (6. 2 +/- 0.6) x 10(3)/mol of xylan polymer. Site-directed mutation indicates t he involvement of three functional sites on CBM13 in binding to soluble xyl an. The sites are similar in sequence, and are predicted to have similar st ructures, to the alpha, beta and gamma sites of ricin toxin B-chain, which is also in family 13. The affinity of a single binding site on CBM13 for so luble xylan is only approximate to (0.5 +/- 0.1) x 10(3)/mol of xylan. The binding of CBM13 to soluble xylan involves additive and co-operative intera ctions between the three binding sites. This mechanism of binding has not p reviously been reported for CBMs binding polysaccharides. CBM13 is the firs t bacterial module from family 13 to be described in detail.