Xiaofeng, 

The nonessential aromatic amino acid, L-tyrosine (L-phenylalanine and L-tryptophan are the other aromatic amino acids), is biotranformed from L-phenylalanine, an essential amino acid for humans (the body cannot make it), by mediation of the enzyme, phenylalanine hydroxylase. The enzyme, tyrosine hydroxylase, catalyses the hydroxylation of L-tyrosine to L-DOPA (L-3,4-dihydroxyphenylalanine), the rate-limiting step in the biosynthesis of (dopamine) catecholamines.  Since you have mentioned cell signaling I am presuming that you have L-tyrosine phosphorylation (and tyrosine kinases) in mind.

The family of receptor tyrosine kinases (RTKs) are ligand-binding cell surface receptors that are essential for regulating cell-cell signaling and communication. There are receptor tyrosine kinases and non-receptor tyrosine kinases. RTKs  play critical roles during development and throughout life. They regulate a broad spectrum of crucial cellular events, including cell growth and cellular proliferation, differentiation, cell-cycle progression, cell survival and apoptosis. Deregulation of RTKs and of associated signaling pathways are implicated in several disorders such as diabetes, coronary artery disease, immunodeficiency, psoriasis and cancer.

Chithan，

I am very grateful for your kind answer. Actually i am seeking some tyrosine mimics or derivatives that cannot be incorporated into polypeptides. In such case, I wish to rescue the phenotype of my mutant with those tyrosine mimics or derivatives. Thank you very much.

Xiaofeng,

Thank you!

Para-tyrosine is the physiological isomer of L-tyrosine. The other isomers of L-tyrosine, meta- and ortho-tyrosine, can, however,  be incorporated into cellular proteins in cells lines. Phosphotyrosine mimetic agents have been identified. However, I have no idea about L-tyrosine mimetics. The antiplatelet medicine, tirofiban hydrochloride, is a non-peptide L-tyrosine derivative.