C-4 AND C-5 ADDUCTS OF COFACTOR PQQ (PYRROLOQUINOLINEQUINONE) - MODEL STUDIES DIRECTED TOWARD THE ACTION OF QUINOPROTEIN METHANOL DEHYDROGENASE

Methanol addition to the trimethyl ester of cofactor PQQ (PQQTME) was investigated in detail to obtain information on the action of quinoprotein methanol dehydrogenase. The hemiacetal-type adduct was easily isolated from a methanol solution of PQQTME. The crystal structure of the adduct was determined by X-ray diffraction for the first time, showing that methanol addition occurred at-the 5-position (C-5) of the quinone as in the case of the acetone adduct formation. On the other hand, treatment of PQQTME in methanol under acidic conditions gave the dimethyl acetal derivative as a major product for which the addition position of methanol was determined to be C-4 by X-ray crystallographic analysis. Studies of the adduct formation reactions with methanol using a series of PQQ model compounds and the molecular orbital calculations provided a clear-cut explanation for the difference in positions between the hemiacetal formation and the acetal formation. Because the C-5 hemiacetal was not very stable, it readily reverted to the quinone in solution, while the C-4 acetal was reduced to the quinol derivative when treated with base. The spectral characteristics and biological significance (particularly in the enzymatic alcohol oxidation mechanism) of the C-4 and C-5 adducts of cofactor PQQ are discussed.