Cut an apple and leave it on the plate, and within minutes the cut face starts to turn brown. That is an oxidation too, run by an enzyme in the apple.
Apple cells store molecules called phenols: benzene rings carrying -OH groups. Many apple phenols, such as chlorogenic acid and the catechins, have two -OH groups side by side on a ring. The figure shows catechol, the simplest molecule with this pair. The apple also makes an enzyme that oxidises them, called polyphenol oxidase. In a whole apple, each cell keeps its phenols and the enzyme in separate compartments. A knife breaks the cells open, the two mix, and oxygen from the air reaches them.
Each -OH loses its hydrogen and becomes a C=O: oxidation by losing hydrogen, as in the first section. Molecules like this, with two C=O groups on a ring, are called quinones, and they react easily. They join up with each other and with the apple's proteins into large molecules called melanins, and melanins are brown.
Lemon juice keeps it pale
Brush the cut face with lemon juice and it stays pale for much longer, for two reasons. Lemon juice is acidic, about pH 2, and the enzyme works slowly in acid. Lemon juice also holds vitamin C, which turns the quinones back into phenols before they can join up. The vitamin C is oxidised in their place, and the apple stays pale until the vitamin C is used up.
Bananas, avocados, potatoes and mushrooms turn brown in the same way. Black tea is made with this reaction on purpose: the fresh leaves are rolled to break their cells, and their own enzyme oxidises their phenols into the red-brown molecules that colour the drink. Green tea leaves are heated first, which stops the enzyme, so they stay green: heat unfolds enzymes, as you have seen.