Mechanisms of cellulases and xylanases: a detailed kinetic study of the exo-beta-1,4-glycanase from Cellulomonas fimi.
The exoglucanase/xylanase from Cellulomonas fimi (Cex) has been subjected to a detailed kinetic investigation with a range of aryl beta-D-glycoside substrates. This enzyme hydrolyzes its substrates with net retention of anomeric configuration, and thus it presumably follows a double-displacement mechanism. Values of kcat are found to be invariant with pH whereas kcat/Km is dependent upon two ionizations of pKa = 4.1 and 7.7. The substrate preference of the enzyme increases in the order glucosides < cellobiosides < xylobiosides, and kinetic studies with a range of aryl glucosides and cellobiosides have allowed construction of Broensted relationships for these substrate types. A strong dependence of both kcat (beta 1g = -1) and kcat/Km (beta 1g = -1) upon leaving group ability is observed for the glucosides, indicating that formation of the intermediate is rate-limiting. For the cellobiosides a biphasic, concave downward plot is seenj for kcat, indicating a change in rate-determining step across the series. Pre-steady-state kinetic experiments allowed construction of linear Broensted plots of log k2 and log (k2/Kd) for the cellobiosides of modest (beta 1g = -0.3) slope. These results are consistent with a double-displacement mechanism in which a glycosyl-enzyme intermediate is formed and hydrolyzed via oxocarbonium ion-like transition states. Secondary deuterium kinetic isotope effects and inactivation experiments provide further insight into transition-state structures and, in concert with beta 1g values, reveal that the presence of the distal sugar moiety in cellobiosides results in a less highly charged transition state.(ABSTRACT TRUNCATED AT 250 WORDS)
Tull
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Department of Chemistry, University of British Columbia, Vancouver, Canada.
Withers
S G
SG
D000192:Actinomycetales / Q000201:enzymology*; D002475:Cellobiose / Q000031:analogs & derivatives* / Q000378:metabolism; D003903:Deuterium; D004789:Enzyme Activation; D004793:Enzyme Reactivators; D044643:Glucan 1,4-beta-Glucosidase; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D013379:Substrate Specificity; D001617:beta-Glucosidase / Q000378:metabolism*
D004793:Enzyme Reactivators; D002475:Cellobiose; D003903:Deuterium; D001617:beta-Glucosidase; D044643:Glucan 1,4-beta-Glucosidase