Catechol oxidase

It is present in a variety of species of plants and fungi including Ipomoea batatas (sweet potato)[1] and Camellia sinensis (Indian tea leaf).

Thus, tissue damage facilitates the interaction of catechol oxidase with its substrate to produce o-benzoquinone, which can polymerize non-enzymatically to yield melanins that form an insoluble barrier for wound protection.

[7] Initially transcribed as a pro-enzyme, the catechol oxidase precursor undergoes two rounds of proteolytic processing and transport before it enters the thylakoid lumen.

Utilizing a [35S] methionine-labeled precursor protein, Sommer et al. elucidated a proteolytic processing pathway common to a variety of plants including pea (Pisum sativum), tomato (Lycopersicon esculentum), and maize (Zea mays).

[9] Based on analysis of the precursor and mature catechol oxidase purified from Ipomoea batatas, proteolytic processing removes both the N-terminal transit peptide as well as a C-terminal domain that covers the enzyme active site.

[14] This cysteine-histidine cross-linking may further restrain the enzyme active site from assuming the bidentate coordination complex readily formed in tyrosinase.

One mechanism proposed by Eicken et al. is based on the crystal structure of catechol oxidase purified from Ipomoea batatas.

[15] This proposed catalytic cycle is supported by the experimental observation that stoichiometric amounts of o-quinone form after catechol addition to the enzyme, even when dioxygen is absent.

[15][16] However, certain intermediates in the proposed cycle are not consistent with experimental findings such as that stoichiometric amounts of o-quinone can form after catechol addition in the absence of oxygen.

Over half of fruit losses are estimated to occur as a result of enzymatic browning, and tropical produce are particularly vulnerable to this reaction.

[6] The loss of nutrients can occur due to the interaction of quinones, produced by the oxidation of diphenols, with the side chains of essential amino acids derived from plant proteins.

In particular, thiol and amine functional groups on the side chains of amino acids are highly susceptible to quinone binding and alkylation.