Why does potato extract turn brown when catechol oxidase is added?

Answer 1

It is because the enzyme facilitates a reaction between catechol and oxidase. In the presence of oxygen, the compound catechol is oxidized by the removal of two hydrogen atoms. Catechol is thus converted to benzoquinone, and oxygen is reduced by the addition of two hydrogen atoms to form water. Benzoquinone molecules then link together to form long, branched chains. These chains are the structural backbones fo the read and brown melanoid pigments that cause the darkening. The intensity of the colour depends on the amount of the coloured product formed.

Answer 2

Potato extract contains catechol oxidase enzyme which oxidizes catechol in the presence of oxygen, resulting in the formation of a brown pigment called benzoquinone. This reaction is similar to how cut apples or potatoes turn brown when exposed to air, due to the enzyme catalyzing the oxidation of phenolic compounds.