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J. Biol. Chem., Vol. 266, Issue 18, 11628-11631, Jun, 1991

Active site-directed inhibition by optically pure epoxyalkyl cellobiosides reveals differences in active site geometry of two 1,3- 1,4-beta-D-glucan 4-glucanohydrolases. The importance of epoxide stereochemistry for enzyme inactivation

PB Hoj, EB Rodriguez, JR Iser, RV Stick and BA Stone
Department of Biochemistry, La Trobe University, Bundoora, Victoria, Australia.

1,3-1,4-beta-D-Glucan 4-glucanohydrolases (EC 3.2.-1.73) from Bacillus subtilis and barley (Hordeum vulgare) with identical substrate specificities but unrelated primary structures have been probed with (R,S)-epoxyalkyl (-propyl, -butyl, -pentyl) beta-cellobiosides and with optically pure (3S)- and (3R)-3,4-cellobiosides as active site-directed inhibitors. The optimal aglycon length for inactivation differs for the two enzymes, and they are differentially inhibited by the pure epoxybutyl beta-cellobioside diastereoisomers. The (3S)-epoxybutyl beta- cellobioside inactivates the B. subtilis enzyme much more efficiently than does the (3R)-isomer, whereas the reverse is true for the barley enzyme. Both enzymes are inactivated by a mixture of the stereoisomers at a rate intermediate of that observed with the individual isomers. The two beta-glucan endohydrolases may therefore employ sterically different mechanisms to achieve glycoside bond hydrolysis in their common substrate. The efficiency and specificity of epoxide-based "suicide" inhibitors may be enhanced significantly by the use of inhibitors bearing only one stereoisomeric form of the epoxide group.
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