The PLP cofactor: Lessons from studies on model reactions

Experimental probes of the acidity of weak carbon acids have been developed and used to determine the carbon acid pK _as of glycine, glycine derivatives and iminium ion adducts of glycine to the carbonyl group, including 5′-deoxypyridoxal (DPL). The high reactivity of the DPL-stabilized glycyl carbanion towards nucleophilic addition to both DPL and the glycine-DPL iminium ion favors the formation of Claisen condensation products at enzyme active sites. The formation of the iminium ion between glycine and DPL is accompanied by a 12-unit decrease in the pK _a of 29 for glycine. The complicated effects of formation of glycine iminium ions to DPL and other aromatic and aliphatic aldehydes and ketones on carbon acid pK _a are discussed. These data provide insight into the contribution of the individual pyridine ring substituents to the catalytic efficiency of DPL. It is suggested that the 5′-phosphodianion group of PLP may play an important role in enzymatic catalysis of carbon deprotonation by providing up to 12 kcal/mol of binding energy that is utilized to stabilize the transition state for the enzymatic reaction. This article is part of a Special Issue entitled: Pyridoxal Phospate Enzymology.