Which amino acids can be phosphorylated?
There's a Wikinome page called Phosphorylation of unusual amino acids describing phosphorylation of histidine, aspartate, cysteine, lysine, and arginine. Lysine and arginine are apparently phosphorylated on the nitrogens. I was a little surprised to see histidine and aspartate on the list, as histidine kinases seem to come up often in my area of research.
I'm also aware of at least one non-proteinogenic amino acid, creatine, which is phosphorylated by a dedicated creatine kinase. It functions as a sort of phosphate shuttle for rapid regeneration of cellular ATP from ADP in situations of high metabolic demand.
References:
Phosphorylation on unusual amino acids. (http://kinase.com/wiki/index.php/Phosphorylation_on_unusual_amino_acids)
PubMed Abstracts: https://pubmed.ncbi.nlm.nih.gov/?term=18558627%2C10772951%2C10698688%2C1654322%2C9553104%2C8577821%2C19498169%5Buid%5D
Guimarães-Ferreira L. Role of the phosphocreatine system on energetic homeostasis in skeletal and cardiac muscles. Einstein (Sao Paulo). 2014;12(1):126-131. doi:10.1590/s1679-45082014rb2741