Glucose-fructose oxidoreductase from the bacterium Zymomonas mobilis catalyzes a transhydrogenation reaction in which D-fructose reduction to D-sorbitol is coupled to the oxidation of D-glucose or other aldoses to the corresponding aldonolactones. Tightly protein-bound NADP(H) serves as the cofactor. We found that the interaction of glucose-fructose oxidoreductase with its aldonolactone product triggered a sequential process that affects the protein structure conformationally and chemically and, ultimately, results in an irreversible loss of enzyme activity. (1) Probably as a mechanistic requirement during the catalytic cycle, conformational realignments in glucose-fructose oxidoreductase are induced by binding of the lactone and are manifested by a 1.7-fold increased accessibility to iodide quenching of the fluorescence of the active-site-bound NADPH, the exposure of one reactive cysteine (likely Cys127) and strongly red-shifted tryptophan fluorescence. (2) As a fast subsequent reaction in vitro, the cysteine residue is deactivated, thus leading to a local, structural destabilization of glucose-fructose oxidoreductase that, without affecting enzyme activity, leads to twofold tryptophan fluorescence as well as the exposure of three further cysteine residues. (3) The completed deactivation of these cysteines is accompanied by a twofold increase in hydrophobic surface and thus aggregation of the glucose-fructose oxidoreductase tetramer. Aggregation, but not release of the tightly bound NADP(H), ultimately leads to the loss of activity and completes the inactivation of glucose-fructose oxidoreductase. Apparently small conformational changes at the NADP(H)-binding site of glucose-fructose oxidoreductase trigger high-order protein associations and seem to be thus responsible for an incorrect oligomerization of the enzyme.