Substrate Specificity

Whereas PPOs from animal tissues are relatively specific for tyrosine and dopa, those from fungi and higher plants act on a wide range of monophenol and o-diphenols. There are individual differences among PPOs from different sources. The rate of oxidation of o-diphenols by PPO increases with increasing electron withdrawing power of substituents in the para position. o-Diphenol substitution (-CH3) at one of the positions adjacent to the -OH groups prevents oxidation. These positions should remain free for oxidation to take place. As mentioned earlier, some PPO preparations from some plants lack cresolase activity, which may be due to changes in the structure of the protein during preparation. Some arguments regarding the physical relationships between the cresolase and catechol oxidase functions suggested that both functions are catalyzed by a single site, whereas others imply the participation of two sites, either on the same enzyme molecule or different ones. Although PPO could oxidize a wide range of pheno-lics, the individual enzymes tend to prefer a particular substrate or certain type of phenolic substrates. The affinity of plant PPOs of the phenolic substrates is relatively low. The Km is high, usually around 1 mM, which is higher than most of fungi and bacteria, 0.1 mM. The affinity to oxygen is also relatively low, ranging from 0.1 to 0.5 mM. Monophenols and o-diphenols have been considered as the exclusive substrates of PPO for a long time, but a recent study showed that aromatic amines and o-aminophenols also undergo the same catalytic reactions (ortho hydroxy-lation and oxidation), as documented by product analysis and kinetic studies (44).

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