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Researcher in protein phosphorylation. Hunter and collaborators showed that the transforming product of Rous sarcoma virus phosphorylates tyrosine, connecting a viral oncogene with a previously underappreciated mode of cellular regulation.^[ref]

Researcher in targeted cancer therapy. Druker and collaborators investigated selective inhibition of the Abl tyrosine kinase in BCR-ABL-positive cells, connecting a molecular driver of disease with an experimentally targeted drug mechanism.^[ref]

c-src is the normal cellular counterpart of viral src, encoding a regulated protein tyrosine kinase. Conservation in normal cells distinguishes a proto-oncogene from an exclusively virus-specific genetic invention.

A sorghum P450 catalysing conversion of tyrosine toward p-hydroxyphenylacetaldoxime in dhurrin biosynthesis. It has served as a model for photosynthesis-powered heterologous P450 activity.

A receptor tyrosine kinase gene also called HER2, expressed at different levels in some tumours and normal tissues. Target abundance alone does not establish a safe therapeutic window.

FGFR2 is a receptor tyrosine kinase in the fibroblast growth factor receptor family. Ligand-dependent activation connects an extracellular signal to intracellular phosphorylation pathways that influence development and cellular behaviour. Alternative receptor isoforms differ in ligand recognition; FGF10 is a ligand for the epithelial FGFR2b isoform. Thus expression of the generic receptor name alone does not specify every ligand response. ^[ref]

For hydrogen-bond acceptance, the three strongest groups are the terminal carboxylate, the Cys–CO–N(Pro) tertiary-amide carbonyl, and the Pro–CO–NH(Tyr) secondary-amide carbonyl, in that qualitative order. Negative charge makes carboxylate a particularly strong acceptor. Both amide oxygens accept strongly; greater nitrogen alkyl donation generally makes a tertiary amide oxygen more basic than the comparable secondary-amide oxygen. This last ordering is a functional-group estimate, not a precise calculation of a folded tripeptide's site parameters.

For donation, the order is terminal ammonium $>$ tyrosine phenolic OH $>$ the peptide NH between Pro and Tyr. The thiol is a weaker donor. Protonated ammonium has no available lone pair to accept; the proline amide nitrogen has no N–H bond to donate, and amide nitrogen lone pairs are delocalized into carbonyl groups. The carboxylate has no O–H donor in the drawn protonation state.

Y91F implicates the Tyr hydroxyl, while E57Q implicates the Glu carboxylic-acid chemistry. Replacing tyrosine by phenylalanine retains an aromatic side chain but removes its hydroxyl, supporting a role for Tyr91 as a hydrogen bond donor that polarizes the steroid C3 carbonyl and stabilizes an enolate-like intermediate or transition state. Its hydroxyl could participate in a proton relay, but inactivity alone does not establish that it supplies the final proton.

Replacing a glutamate residue by glutamine approximately preserves side-chain size while replacing a titratable carboxylic acid by an amide. Loss of activity, combined with the acidic activity range, supports protonated Glu57 as a carbonyl-activating acid and possible proton donor. The two residues can cooperate to stabilize oxygen charge during reduction. The inhibitor-binding structure supports local contacts; it is not a direct picture of a testosterone transition state, and an N–H contact of the aza inhibitor need not occur in testosterone.

To distinguish these chemical roles from indirect effects, compare mutant and wild-type abundance, membrane insertion and folding, cofactor binding, and steroid binding. Measure initial rates with matched enzyme amounts and include no-enzyme and inhibitor-treated wild-type controls. If binding survives but turnover fails, the evidence for a catalytic defect is stronger. The published study combines structure, mutagenesis and modeled testosterone placement, so the proposed residue roles should be distinguished from direct observation of proton trajectories.^[ref]

Tyrosine has a phenolic side chain. Its hydroxyl can participate in hydrogen bonding and, when the environment is suitable, acid/base chemistry.

v-src is the transforming src determinant carried by Rous sarcoma virus. Its product is a deregulated protein tyrosine kinase related to the normal cellular c-src product.