Development of the active site model for calcium-containing quinoprotein alcohol dehydrogenases

A new PQQ model compound [dimethyl 7-(1,4,7,10-tetraoxa-13-azacyclopentadec-13-yl)carbonyl-4,5-dihydro-4,5-dioxo-1H-pyrrolo[2,3-f]quinoline-2,9-dicarboxylate, 1], in which a 1-aza-15-crown-5 group is attached through an amide linkage at the 7-position, has been synthesized in order to develop an efficient model system of calcium-containing quinoprotein alcohol dehydrogenases. It has been found that Ca²⁺ binds to the quinone most strongly among the alkaline earth metal ions examined (Ca²⁺+>Sr²⁺+>>Ba²⁺+>>Mg²⁺) and the binding constant (K(M)) for Ca²⁺ is as large as 2.1x10⁵ M^-1. Formation of the C-5 hemiacetal derivatives with ethanol is also investigated spectrophotometrically to show that the alcohol-addition to the quinone is enhanced in the presence of the metal ions. In this case, Ca²⁺ and Sr²⁺ show a similar efficiency that is several times larger than that of Ba²⁺. Addition of a strong base such as DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) into a MeCN solution containing the metal ion complex of 1 and ethanol leads to redox reactions to give the Ca²⁺ complex of 1H₂ (quinol form) and acetaldehyde. Kinetic studies on the redox reactions have been performed to gain insight into the mechanism of the alcohol-oxidation reaction catalyzed by the metal complexes of coenzyme PQQ. Copyright (C) 2000 Elsevier Science B.V.