MONOPHENOLASE AND DIPHENOLASE REACTION-MECHANISMS OF APPLE AND PEAR POLYPHENOL OXIDASES

This paper reports a quantitative study of the effect of ring substitu ents on the rate of monophenol hydroxylation and o-diphenol oxidation catalyzed by apple and pear polyphenol oxidases. A good correlation be tween the electronic density at the carbon atom in the 4-position of t he aromatic ring for each monophenol and the V-max values was found. H owever, this correlation was not so good in the 3- and 4-positions for the o-diphenols assayed. NMR studies on the monophenols demonstrated the higher reactivity of 4-hydroxyanisole compared with the other mono phenols assayed. Catechol was the best o-diphenolic substrate assayed because of the absence of a ring substituent, All of these data confir med the proposed enzyme's reaction mechanism and indicate that the rat e-limiting step in the monophenolase reaction mechanism could be the n ucleophilic attack of the oxygen atom belonging to the hydroxyl group at the carbon atom in the 4-position on the copper atoms of the enzyme 's active site. However, in the diphenolase reaction mechanism, the ra te-limiting step could be related to both the nucleophilic attack of t he oxygen atom from the hydroxyl group at the carbon atom in the 3-pos ition on the copper atoms of the enzyme's active site and the molecula r size of the substituent side chain.