Protein folding
Working out the three-dimensional shape a protein takes, which determines what it does in the body.
Proteins start as a long chain of chemical building blocks, and then fold themselves into a specific three-dimensional shape. That shape is what matters: it determines whether a protein carries oxygen, fights an infection or fails in a way that causes disease. Two proteins with similar chains can do completely different jobs depending on how they fold.
Predicting the shape from the chain resisted scientists for about fifty years, and working it out experimentally could take months per protein. AlphaFold largely solved the prediction problem and released results for essentially every known protein, which is why it is the standard example of AI doing real scientific work rather than just answering questions.