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Substrate specificity of endoglucanase A from Cellulomonas fimi: Fundamental differences between endoglucanases and exoglucanases from family 6

TitleSubstrate specificity of endoglucanase A from Cellulomonas fimi: Fundamental differences between endoglucanases and exoglucanases from family 6
Publication TypeJournal Article
Year of Publication1996
AuthorsDamude, HG, Ferro, V, Withers, SG, WARREN, RAJ
JournalBIOCHEMICAL JOURNAL
Volume315
Pagination467-472
Date PublishedAPR 15
ISSN0264-6021
Abstract

Values of k(cat). and K-m for the hydrolysis of cellotetraose, cellotriose, beta-cellobiosyl fluoride and various beta-aryl cellobiosides by endoglucanase A (CenA) from Cellulomonas funi indicate that specific binding interactions between the reducing-end glucose residues of cellotetraose and cellotriose and the enzyme at the transition state provide enormous stabilization, endowing glucose with the `effective leaving group ability' of 2,4-dinitrophenol. As has been seen with several other inverting glycosidases, CenA hydrolyses the `wrong' anomer of its glycosyl fluoride substrate, alpha-cellobiosyl fluoride, according to non-Michaelian kinetics. This indicates that CenA carries out this hydrolysis by a mechanism involving binding of two substrate molecules in the active site {[}Hehre, Brewer and Genghof (1979) J. Biol. Chem. 254, 5942-5950] in contrast with that reported for cellobiohydrolase II, another family-6 enzyme {[}Konstantinidis, Marsden and Sinnott (1993) Biochem. J. 291, 833-838]. The pH profiles for wild-type CenA indicate that k(cat.) for CenA depends on the presence of both a protonated group and a deprotonated group for full activity, consistent with the presence of an acid and a base catalyst at the active site. By contrast, the profile for the Asp252Ala mutant of CenA shows a dependence only on a base-catalytic group, thereby confirming the role of Asp-252 as an acid catalyst. These results show that hydrolysis by CenA occurs by a typical inverting mechanism involving both acid and base catalysis, as first proposed by Koshland. It also suggests that endoglucanases from family 6 may function by fundamentally different mechanisms from exoglucanases in this family.