Substrate recognition by amyloglucosidase: evaluation of conformationally biased isomaltosides

Amyloglucosidase catalyzes the hydrolysis of methyl beta-maltoside (1) 30-50 times more rapidly than methyl alpha-isomaltoside (2). It is established that OH-6', OH-4', and OH-4 which are involved in key polar interactions with the enzyme in the case of isomaltoside. Conformational analyses based on HSEA calculations indicate that the dispositions in space of OH-3 of maltose relative to OH-4' and OH-6' in the preferred conformation for the maltoside (1) is energetically more readily achieved by methyl 6R-C-methyl-alpha-isomaltoside (3), than for its 6-S-isomer (4). A kinetic evaluation of the hydrolysis in fact has shown that the R-compound is more strongly bound by the enzyme (Km = 0.9 mM) than the parent isomaltoside (Km = 24.5 mM), whereas the S-compound has the weakest enzyme binding (Km = 90 mM). Since the kcat values were all within the range 0.85 +/- 0.20 s-1, it is evident that the relative rates of hydrolysis are related to the relative ease for the compounds to achieve an interaction of a hydroxyl group in the aglycon of an alpha-D-glucopyranoside with the enzyme for the formation of the enzyme-substrate complex. The relative rates of hydrolysis of the alpha-glucosides of the 1,3-dihydroxy-trans-decalins, 5 and 6, provide further support for this highly desirable but not necessary recognition for the orientation of the reducing glucose unit in the active site.