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Enzymes in search of a function, for polyphenol oxidases (PPOs), described as such, this situation has changed recently. A tyrosinase is involved in betacyanin biosynthesis in common portulaca (see picture) and red beet, and a chalcone-specific PPO is responsible for the formation of aurones in yellow snapdragon flowers.
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Strack et al. (2001) studied this question.
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