High Quality Content by WIKIPEDIA articles! Tyrosine phosphorylation is the addition of a phosphate (PO43-) group to the amino acid tyrosine on the protein. This transfer of phosphate group from ATP to the amino acid tyrosine on the protein is made possible through the enzyme tyrosine kinase. Tyrosine phosphorylation is considered to be one of the key steps in signal transduction and regulation of enzymatic activity.