Evidence of Diradicals Involved in the Yeast Transketolase Catalyzed Keto-Transferring Reactions.

Transketolase (TK) catalyzes a reversible transfer of a two-carbon (C2 ) unit between phosphoketose donors and phosphoaldose acceptors, for which the group-transfer reaction that follows a one- or two-electron mechanism and the force that breaks the C2"-C3" bond of the ketose donors remain unresolved. Herein, we report ultrahigh-resolution crystal structures of a TK (TKps) from Pichia stipitis in previously undiscovered intermediate states and support a diradical mechanism for a reversible group-transfer reaction. In conjunction with MS, NMR spectroscopy, EPR and computational analyses, it is concluded that the enzyme-catalyzed non-Kekulé diradical cofactor brings about the C2"-C3" bond cleavage/formation for the C2 -unit transfer reaction, for which suppression of activation energy and activation and destabilization of enzymatic intermediates are facilitated.