Mechanistic comparison of saccharide depolymerization catalyzed by dicarboxylic acids and glycosidases

Abstract

Dicarboxylic acids have been identified as promising catalysts for the depolymerization of cellulose and other polysaccharides. It has been suggested that they might act in “biomimetic” analogy to the active site of glycosidases, where also two carboxylic groups are present, and it is assumed that one residue acts as proton donor and the other one as proton acceptor. In the present paper, a series of structurally distinct carboxylic acids were experimentally assessed as catalysts in the hydrolysis of cellobiose under fully identical acidic conditions. The results clearly show a pH-dependent activity profile in bulk aqueous solutions without any evidence for a cooperative mechanism. In contrast, the protein environment at the active site in glycosidases was found to be essential for the cooperative action of the two carboxylic acid groups. A detailed computational chemistry study is presented, focusing on the protein electric potential, as well as on a reduced dielectric constant (ε) within the active site, resulting from the limited presence of water. These two aspects alter the pK_a of the carboxylic acid groups dramatically, providing the necessary local environment for a cooperative proton donor–acceptor mechanism, which cannot be mimicked by simple diacids in bulk aqueous phase.