INHIBITION OF THE ALPHA-L-ARABINOFURANOSIDASE-III OF MONILINIA-FRUCTIGENA BY 1,4-DIDEOXY-1,4-IMINO-L-THREITOL AND 1,4-DIDEOXY-1,4-IMINO-L-ARABINITOL

1,4-Dideoxy-1,4-imino-L-threitol was synthesized and the synthesis of 1,4-dideoxy-1,4-imino-L-arabinitol was improved. Both compounds are competitive inhibitors of Monilinia fructigena α-L-arabinofuranosidase III, the addition hydroxymethyl group in the arabinitol contributing about 17.8kJ/mol (4.25 kcal/mol) to the Gibbs free energy of binding. The affinities (1/K(i)) of both compounds vary with pH in a classical bell-shaped way, the pK(a) value being that of the acid-catalytic group on the enzyme [5.9; Selwood and Sinnott (1988) Biochem. J. 254, 899-901] and the pK(b) values being those of the free inhibitors, 7.6 and 7.8 respectively. On the basis of these and literature data we suggest that efficient inhibition of a glycosidase at its pH optimum by an appropriate iminoalditol will be found when the pK(a) of the iminoalditol is below that of the acid-catalytic group of the target enzyme.