For something to be used in PoW, it needs to be both hard to solve and easy to verify. Protein folding checks the first box but not the second. As an example, prime number calculations check the both boxes and there's Primecoin which bases its proof algorithm on prime chains.

This only applies if you prefer to be trustless though. If you are willing shed a bit, you can trust Folding@Home's scoreboard data (and whoever supplies the data to you) to use it as a pseudo-"PoW" and run a proper proof algorithm beneath. (which is also done by a number of coins like Banano, Curecoin, Gridcoin. You can see their people on the leaderboard with weird names like x_ALL_x, x_GRC_x or just bunch of non-sensical alphanumeric characters.)