= Tyrosine phosphorylation
\b[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.<Transforming gene product of Rous sarcoma virus phosphorylates tyrosine by Tony Hunter and others>[PNAS 77 (1980), 1311–1315: identification of phosphotyrosine in kinase products.]{ref}<JAK2 associates with the erythropoietin receptor and is tyrosine phosphorylated and activated following stimulation with erythropoietin by B. A. Witthuhn and others>[Abstract: EPO-dependent JAK2 tyrosine phosphorylation and kinase activation.]{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:
$$
\frac{dp}{dt}=k_p(1-p)-k_dp,\qquad p_* =\frac{k_p}{k_p+k_d}.
$$
For $k_p=4\,\mathrm{min^{-1}}$ and $k_d=1\,\mathrm{min^{-1}}$, the model gives $p_*=0.8$. 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.
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