Protein Phosphorylation

Phosphorylation may occur on serine, threonine, tyrosine, and histidine residues in eukaryotic organisms. Among these four types of phosphorylation, tyrosine phosphorylation is most rare, but very crucial. The most famous tyrosine kinase receptor is in the very upstream of the MAPK pathway, and regulates Ras and other kinase, initiating the phosphorylation cascade.