CHEMICAL MODIFICATION AND AMINO-ACID-SEQUENCE OF ACTIVE-SITE IN SUGAR-BEET ALPHA-GLUCOSIDASE

The modification of amino acid residues in sugar beet alpha-glucosidas e with conduritol B epoxide (CBE), an affinity labeling reagent, inact ivated the enzyme. The inactivation followed pseudo-first-order kineti cs. The enzyme was protected from inactivation by a competitive inhibi tor, Tris, and the partially inactivated enzymes showed only the decre ase of V values and no change in K-m value. An H-3-CBE labeled peptide isolated from the digest of the inactivated enzyme with Lys-C proteas e was sequenced. The -COO- group of Asp was found to be specifically l abeled, implicating that it is a catalytic group of the enzyme. The se quence around the essential Asp was determined to be-DGIWIDMNE-, which showed a high homology with those of other alpha-glucosidases.