Some fruits and vegetables turn brown when peeled because they contain the enzyme catecholase, which catalyzes a reaction between oxygen and a colorless compound, catechol. The product of this reaction is benzoquinone, which forms the red and brown pigments responsible for the browning. When preparing fruit salads, some cooks sprinkle the sliced fruits with lemon juice to prevent discoloration. How might lemon juice act to prevent the browning of fruit?

Catecholase is an enzyme, an enzyme is a biological catalyst which is required for speeding up the reactions that take place in the body.
Every enzyme needs to have an optimum temperature and pH to effectively function. if the enzyme is not provided its optimum pH or temperature then this can lead to its inactivity.
Catechlase needs copper ions to function as copper ions act as its cofactor. However for this cofactor to be able to bind to the enzyme catecholase the pH must be optimum.
Lemon juice being acidic lowers the pH and hence causes the copper ions to dissociate from the enzyme and thus catecholase is unable to function and browning of the fruits and vegetables is prevented.