https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4714867/
https://pubmed.ncbi.nlm.nih.gov/8747436/
A protein kinase is a kinase which selectively modifies other proteins by covalently adding phosphates to them (phosphorylation) as opposed to kinases which modify lipids, carbohydrates, or other molecules. 1