Re: Can proteins be converted into ketones by the liver? Or is it just fats?

Yes, some amino acids can be converted to ketone bodies by the liver.  
These amino acids are referred to as "ketogenic amino acids", meaning 
that at least part of their carbon skeletons can be converted to acetyl 
CoA or acetoacetate.  Acetoacetate is one of the ketone bodies and acetyl 
CoA can be converted to acetoacetate in the liver.

The amino acids that are at least partially ketogenic are leucine, 
isoleucine, lysine, threonine and tryptophan.

For more information, you might refer to "Advanced Nutrition and Human 
Metabolism" by S. Gropper, J. Smith and J. Groff.  It was published in 
2005 by Thomson and Wadsworth.