Tyrosine phosphorylation

Tyrosine phosphorylation is addition of a phosphate group to the hydroxyl group of a tyrosine residue in a protein, producing a phosphotyrosine residue. A protein kinase commonly transfers the phosphate from ATP; a protein tyrosine phosphatase can remove it. This modifies a side chain without changing the underlying order of amino acid residues.

The modification can change binding interactions or enzyme behavior. It is not intrinsically an “on” instruction: the consequence depends on which tyrosine residue is modified and what molecular partners recognize that state. Nor does detecting phosphate somewhere in a protein identify tyrosine as the modified amino acid.

The discovery was sharpened by experiments on the transforming Src protein of Rous sarcoma virus. Analysis of phosphorylated reaction products identified phosphotyrosine, establishing a phosphorylation target beyond the familiar serine/threonine cases. Later work on JAK2 and the erythropoietin receptor connected stimulus-dependent tyrosine phosphorylation with a receptor-associated signaling enzyme.^[ref][ref]

A simple occupancy model distinguishes modification from total protein amount. If $p$ is the fraction carrying one specified phosphate site, assume fixed first-order effective rates $k_p$ for modification and $k_d$ for removal:For $k_p=4\mathrm{mi}{\mathrm{n}}^{-1}$ and $k_d=1\mathrm{mi}{\mathrm{n}}^{-1}$, the model gives $p_{\ast }=0.8$. The protein population can remain constant while phosphate occupancy changes. Real signaling may require additional sites, enzyme saturation and interactions beyond this deliberately simple example.