Chris is right, in that almost anything can be used to make protein. When the body digests food, it pulls out amino acids and recombines them to make protein.
However, one thing that's good to break a fast with in spirulina. It's very high in protein, but more importantly, it's very easy to digest. And it has lots of vitamins, trace minerals, chlorophyll, and other good stuff. I highly recommend using it after ending a fast whether you need extra protein or not. It's very easy on the digestive system and has lots to offer. It's also one of the few vegetarian sources of vitamin B12.