Hold on...

My textbook is so cute when it tries to build a sense of drama...

Protein kinases catalyze the transfer of the gamma-phosphate from ATP (or rarely, GTP) to amino acid side chains of proteins. Most vertebrate protein kinases phosphorylate either serine or threonine. Other phosphorylate tyrosine. A few phosphorylate serine, threonine, and tyrosine.

Ooooh! And tyrosine! Some little kinase is working overtime...