Free radicals can damage cells by altering protein structures.

“Free radicals. What are free radicals? Free radicals can damage cells. They don't always damage cells, but they can damage cells because they are essentially free electrons. They are a charge that's sitting out there not bound to anything and therefore can interact with the molecular structure of certain proteins and change those structures by binding to them or interfering with the normal binding processes of those proteins to water or to other things. And in that way cause damage to those proteins and potentially damage to cells.”