ENDO-BETA-1,4-XYLANASE FAMILIES - DIFFERENCES IN CATALYTIC PROPERTIES

Microbial endo-beta-1,4-xylanases (EXs, EC 3.2.1.8) belonging to glyca nase families 10 (formerly F) and 11 (formerly G) differ in their acti on on 4-O-methyl-D-glucurono-D-xylan and rhodymenan, a beta-1,3-beta-1 ,4-xylan. Two high molecular mass EXs (family 10), the Cryptococcus al bidus EX and XlnA of Streptomyces lividans, liberate from glucuronoxyl an aldotetrauronic acid as the shortest acidic fragment, and from rhod ymenan an isomeric xylotriose of the structure Xyl beta 1-3Xyl beta 1- 4Xyl as the shortest fragment containing a beta-1,3-linkage. Low molec ular mass EXs (family 11), such as the Trichoderma reesei enzymes and XlnB and XlnC of S. lividans, liberate from glucuronoxylan an aldopent auronic acid as the shortest fragment, and from rhodymenan an isomeric xylotetraose as the shortest fragment containing a beta-1,3-linkage. The structure of the oligosaccharides was established by: NMR spectros copy, mass spectrometry of per-O-methylated compounds and enzymic hydr olysis by beta-xylosidase and EX, followed by analysis of products by chromatography. The structures of the fragments define in the polysacc harides the linkages attacked and non-attacked by the enzymes. EXs of family 10 require a lower number of unsubstituted consecutive beta-1,4 -xylopyranosyl units in the main chain and a lower number of consecuti ve beta-1,4-xylopyranosyl linkages in rhodymenan than EXs of family 11 . These results, together with a greater catalytic versatility of EXs of family 10, suggest that EXs of family 10 have substrate binding sit es smaller than those of EXs of family 11. This suggestion is in agree ment with the finding that EXs of family 10 show higher affinity for s horter linear beta-1,4-xylooligosaccharides than EXs of family 11. The results are discussed with relevant literature data to understand bet ter the structure-function relationship in this group of glycanases. ( C) 1997 Elsevier Science B.V.