A comparison of glucose- and glucosamine-related inhibitors: Probing the interaction of the 2-hydroxy group with retaining beta-glucosidases

The inhibition of the beta-glucosidases from sweet almonds and Caldocellum saccharolyticum at varying pH values by the glucosamine-related inhibitors 1-7 has been compared to the inhibition by the known glucose analogues 8-14 . The amino derivatives 3, 4, 6, and 7 were prepared in one step from the k nown 15-18 (Scheme I), and the amino-1,2,3-triazole 5 by a variant of the s ynthesis leading to the glucose analogue 12 (Scheme 2). The key step Tor th e preparation of the aminoimidazole 1 and of the amino-1,2,4-triazole 2 is the regioselective cleavage of the benzyloxy group at C(2) of the gluconola ctam 35 and the mannonolactam 57 respectively, by BCl3 and B4NBr (Schemes 3 and 4, resp.). The pH optimum for the inhibition by the amines is lower th an their pK(HA) values, evidencing that they are bound as ammonium salts an d that H-bonding between C(2)-NH3+ and the cat. base B- contributes more st rongly to binding than any possible H-bond to the NH2-C(2) group. The influ ence of the ammonium group on the inhibitory strength correlates with the b asicity of the 'glycosidic heteroatom'. The strongest increase of the inhib itory strength is observed for the amines lacking a 'glycosidic heteroatom' (Delta Delta G(OH-->NH3+)=-1.5 to -2.9 kcal/mol). The increase is less; de rivatives 3-4, which possess a weakly basic 'glycosidic heteroatom' pronoun ced for the amino derivatives 3-4, which possess a weakly basic 'glycosidic heteroatom' (Delta Delta G(OH --> NH3+) = - 0.6 to - 1.1 kcal/mol); the am ino compounds 1 and 2, which possess a strongly basic 'glycosidic heteroato m', are weaker inhibitors than the corresponding hydroxy compounds, as expr essed by Delta Delta G(OH-->NH3+) between +4.3 and +4.7 kcal/mol for the am ino-imidazole 1, and between +2.3 and 2.8 kcal/mol for the amino-1,2,4-tria zole 2, denoting the dominant detrimental influence of a C(2) -NH3+ group o n the H-bond acceptor properties of a sufficiently basic 'glycosidic hetero atom'.