Active Site-directed Inhibition by Optically Pure Epoxyalkyl Cellobiosides Reveals Differences in Active Site Geometry

1,31,4-@-~-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) 8-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 8-cellobioside diastereoisomers. The (3s)epoxybutyl 8-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 8-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.