CYCLIC GUANIDINO-SUGARS WITH LOW PK(A) AS TRANSITION-STATE ANALOG INHIBITORS OF GLYCOSIDASES - NEUTRAL INSTEAD OF CHARGED SPECIES ARE THE ACTIVE FORMS

Cyclic guanidino-sugars with different pK(a) values are designed and s ynthesized as transition-state analog inhibitors of galactosidases. Ch aracterization of these structures (7, 10, 12) reveals that 7 and 10 a re in a pH-dependent equilibrium between a furanose form and a mixture of neutral and protonated tetrahydropyrimidine forms. In contrast, th e O-linked guanidino-sugar 12 exists as the tetrahydropyrimidine forms above pH 5. The furanose-tetrahydropyrimidine equilibrium can thus be modulated with the appropriate N-substituent which affects the guanid ino-sugar pK(a) value. Enzymatic inhibition by 7, 10, and 12 is also p H-dependent, indicating that the enzymes recognize the tetrahydropyrim idine form. Evidence is presented to support a dominant role for the u ncharged form of the six-membered cyclic guanidino-sugar in the inhibi tion of galactosidases. Though the inhibition potency is moderate (K-i range 4-50 mu M), the use of cyclic guanidino-sugars in the study pro vides new insights into the mechanism of inhibition of glycosidases.