Trehalase: Stereocomplementary Hydrolytic and Glucosyl Transfer Reactions with α- and β-D-Glucosyl Fluoride

A new understanding has been obtained of the catalytic capabilities of trehalase, an enzyme heretofore held to be strictly specific for hydrolyzing α,α -trehalose and devoid of transglycosylative ability. Highly purified rabbit renal cortical trehalase and a partly purified Candida tropicalis yeast trehalase were found to utilize both α- and β-D-glucosyl fluoride as substrates. In each case, the reactions were competitively inhibited by α,α -trehalose. Both enzymes catalyzed rapid hydrolysis of α-D-glucosyl fluoride to form β-D-glucose (also, of α,α -trehalose to form equimolar a- and β-D-glucose). In addition, digests of β-D-glucosyl fluoride plus α-D-[14C]-glucopyranose with either trehalase (but not controls of enzyme with -D- [14C] glucopyranose alone) yielded small amounts of radioactive trehalose (α-D-glucopyranosyl a-D-[14C]glucopyranoside) which does not accumulate since it is rapidly hydrolyzed. Trehalase thus catalyzes two stereocomplementary types of glycosylation reactions: (I) -D-glucosyl fluoride (or α,α -trehalose) + H2O→ β-D-glucose + HF (or a-Dglucose); (II) β-D-glucosyl fluoride + a-D-glucopyranose -α,α -trehalose + HF. Such behavior shows that the catalytic groups of trehalase, as recently found for other glycosylases, are functionally flexible. The results illustrate the inadequacy of conventional views of carbohydrase specificity and the rigor, as a basic guiding principle, of the concept that glycoside hydrolases and glycosyltransferases form a class of glycosylases effecting glycosyl/proton interchange.