The other day I read an article about a genetically modified apple that doesn’t turn brown when you leave it out. At first glance, this sounds like they created an apple that doesn’t rot, which may sound scary. While it’s true that GMOs frequently sound scary to people, the point of this post is not to advocate for or against them, but to illuminate some of the science that is going on with this particular one.
When most fruits or vegetables are cut or bruised, the exposed bit will turn brown and that makes many people not want to eat it. The brown color is caused in part by one of a couple of enzymes — in the case of apples, it is “polyphenol oxidase.” Fruit will also turn brown when it rots, but that is not what is going on here. The spokesperson for the company that created these apples is quick to point out that the apples will still rot as normal, but will not discolor due to what is broadly called “enzymatic browning.”
An enzyme is a type of protein that functions a little bit like a piece of machinery in a factory. It is not a reactant in a chemical reaction, just as machinery is not a construction material, so it is not consumed by a chemical reaction. Also like some machines, each enzyme has only one function — the enzyme “catalase” can only break down hydrogen peroxide, and the enzyme “DNA helicase” can only separate the two strands of a DNA molecule.
Back to our apples.
The skin of a fruit is its first line of defense against bacteria. If the skin of a fruit is damaged, bacteria may get in and feed on the sugar present inside. To combat this, apples produce a chemical called “catechol” and the enzyme polyphenol oxidase. When exposed to oxygen (such as when the skin is damaged or removed) polyphenol oxidase allows catechol to convert into the chemical “benzoquine,” which is an antimicrobial agent, and usually looks brown on the fruit. Many plants have a system just like this, although the exact chemicals and enzymes involved may differ slightly. While most people don’t like to eat food that has discolored this way, it is still perfectly safe to eat.
Given the fairly simple process that causes fruit to turn brown when damaged, it’s not so hard to engineer one that doesn’t — you just have to disrupt the short series of events that leads it to happen. There are three obvious ways to interrupt this process via genetic engineering. First, you could disrupt the production of the enzyme so it can’t convert catechol into benzoquine. Second, you could disrupt the production of catechol so there is nothing for the enzyme to convert into benzoquine. The third way is through the production of an enzyme inhibitor. These are chemicals produced by the organism that help regulate the activity of the enzyme. To slow down a reaction, an inhibitor is produced that prevents the enzyme from working at all or just slows it down. To speed up the reaction again, the inhibitor is removed and the enzymes go about their business. If you modify a plant to produce more of a particular enzyme’s inhibiter, the enzyme will be present but will not function.
Of these three methods, the genetic engineers chose option 2. Paradoxically, they engineered the plants to produce more enzyme, rather than less, which causes the plant to “decide” on its own to shut down the production of the enzyme. Because I’m not a geneticist, I cannot tell you why this method is better than simply removing the gene that codes for polyphenol oxidase, but the people who ARE geneticists decided that this was the way to go. The end result is the same, and it creates an apple that does not produce benzoquine when the flesh is exposed to air.
These are the facts. How you feel about genetic engineering is still up to you.
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