Tyrosine

Tyrosine (abbreviated Tyr or Y; IUPAC name (2*S*)-2-amino-3-(4-hydroxyphenyl)propanoic acid; molecular formula C₉H₁₁NO₃; molecular mass 181.19 Da) is an amphipathic, aromatic proteinogenic α-amino acid encoded by the genetic code codons UAU and UAC. First isolated in 1846 by the German chemist Justus von Liebig from alkaline fusion digests of casein in cheese (from the Greek tyros, meaning "cheese"), tyrosine differs from phenylalanine by a single para-hydroxyl group (–OH) at position C4 of the phenyl ring, forming a 4-hydroxybenzyl (phenolic) side chain. Whereas bacteria, fungi, and plants synthesize tyrosine de novo from chorismate and prephenate via NAD⁺/NADP⁺-dependent prephenate (or arogenate) dehydrogenase, mammals synthesize L-tyrosine directly by tetrahydrobiopterin (BH₄)-dependent parahydroxylation of dietary L-phenylalanine via hepatic phenylalanine hydroxylase (PAH), making tyrosine nutritionally non-essential except in individuals with phenylketonuria (PKU).

The addition of the para-hydroxyl substituent onto the hydrophobic benzene ring endows tyrosine with three unique chemical properties that distinguish it from phenylalanine and tryptophan: (1) its phenolic hydroxyl group is weakly acidic (pK_aR ≈ 10.07 in aqueous solution; α-COOH pK_a1 = 2.20, α-NH₃⁺ pK_a2 = 9.11, isoelectric point pI = 5.66), allowing it to donate and accept hydrogen bonds while deprotonating at alkaline pH into a resonance-stabilized phenolate (tyrosinate) anion whose UV absorption maximum shifts bathochromically from 274.5 nm (ε = 1,490 M⁻¹ cm⁻¹) to 293 nm (ε = 2,400 M⁻¹ cm⁻¹); (2) its nucleophilic C4-hydroxyl oxygen serves as the target of reversible covalent O-phosphorylation by protein tyrosine kinases (PTKs) across metazoan growth-factor and immune signaling networks; and (3) its phenolic ring undergoes facile one-electron oxidation coupled to proton transfer (PCET) to form a neutral tyrosyl radical (Tyr–O•) in Photosystem II and ribonucleotide reductase, while serving as the metabolic precursor for catecholamines, thyroid hormones (T₃ and T₄), and melanin pigments.

Protein Tyrosine Kinases (RTKs & nRTKs), PTPs, and SH2/PTB Domain Recognition

Although phosphotyrosine (pTyr) accounts for less than 0.1% to 1% of total protein phosphorylation in resting eukaryotic cells (compared to ~88% phosphoserine and ~11% phosphothreonine), the evolutionary emergence of the triad of tyrosine kinases ("writers"), phosphotyrosine-binding domains ("readers"), and protein tyrosine phosphatases ("erasers") drove the transition from unicellular choanoflagellates to multicellular metazoan tissues:

1. Receptor and Non-Receptor Protein Tyrosine Kinases (PTKs): The human genome encodes 90 protein tyrosine kinases, divided into 58 receptor tyrosine kinases (RTKs—such as the epidermal growth factor receptor EGFR, insulin receptor INSR, vascular endothelial growth factor receptor VEGFR, and PDGFR) and 32 cytoplasmic non-receptor tyrosine kinases (nRTKs—such as Src, Abl, JAK1–3, and ZAP-70). Binding of an extracellular ligand (such as insulin or EGF) induces RTK dimerization and trans-autophosphorylation of specific tyrosine residues within the kinase activation loop (opening the catalytic cleft) and along the C-terminal cytoplasmic tail, transferring the γ-phosphate of ATP onto the phenolic O^η atom of tyrosine. Constitutively active oncogenic fusions or point mutations in tyrosine kinases (such as BCR–ABL1 in chronic myeloid leukemia or EGFR L858R in lung adenocarcinoma) drive unchecked proliferation and are targeted clinically by small-molecule kinase inhibitors (imatinib, osimertinib) designed via structure-based drug design.
2. Modular Recognition by SH2 and PTB Domains: Because the long, planar benzene ring of phosphotyrosine extends the dianionic phosphate group (~10 Å from the Cα backbone) far deeper than phosphoserine or phosphothreonine, pTyr motifs are selectively recognized (with K_d ≈ 50–500 nM) by two conserved modular protein folds that cannot bind pSer or pThr:

• SH2 (Src Homology 2) Domains (~110 domains in humans): A central antiparallel β-sheet flanked by two α-helices containing an invariant arginine within a buried FLVR motif that forms a bidentate salt bridge with the pTyr phosphate group, while a specificity pocket recognizes residues C-terminal to pTyr (positions +1 to +3, such as pY–E–E–I for Src SH2 or pY–X–N–X for Grb2 SH2).
• PTB (Phosphotyrosine-Binding) Domains: Recognize β-turn motifs N-terminal to the phosphorylated tyrosine (specifically N–P–X–pY sequences in the insulin receptor substrate IRS-1 and EGFR).

1. Protein Tyrosine Phosphatases (PTPs): Dephosphorylation is catalyzed by ~107 human PTPs (such as PTP1B and CD45), which utilize a low-pK_a catalytic cysteine nucleophile (pK_a ≈ 4.7–5.4) within a signature H–C–X5–R P-loop to form a transient cysteinyl-phosphate intermediate without requiring metal cofactors. Additionally, inside the trans-Golgi network, tyrosylprotein sulfotransferases (TPST1/2) transfer a sulfate group from PAPS onto tyrosine residues of secreted proteins and chemokine receptors (such as CCR5) to form sulfotyrosine (sTyr), which regulates leukocyte rolling and HIV-1 gp120 co-receptor entry.

Proton-Coupled Electron Transfer (PCET): Photosystem II Tyr_Z and Ribonucleotide Reductase

Because oxidizing neutral phenol (Tyr–OH) by one electron without removing its proton generates a highly acidic cation radical (Tyr–OH^•+, pK_a ≈ −2.0) at a prohibitive redox potential (+1.4 V), biological enzymes couple electron removal from the tyrosine ring to concerted transfer of the phenolic proton to an immediately adjacent hydrogen-bonded histidine or carboxylate base (Proton-Coupled Electron Transfer, PCET, lowering the redox potential at pH 7 to E°′ ≈ +0.93 to +1.22 V and generating a neutral tyrosyl radical, Tyr–O•):

• Photosystem II Tyr_Z (D1-Tyr161) in Oxygenic Photosynthesis: In cyanobacteria, algae, and green plants, absorption of a photon by the P₆₈₀ chlorophyll special pair ejects an electron to pheophytin and plastoquinone Q_A, generating the strongest biological oxidant known (P₆₈₀^•+, E_m ≈ +1.25 V). Within 20 to 250 nanoseconds, Tyr_Z (residue Tyr161 of the D1 reaction-center subunit) transfers an electron to reduce P₆₈₀^•+ back to P₆₈₀ while simultaneously transferring its phenolic proton across a short hydrogen bond to D1-His190, forming the neutral Tyr_Z–O• radical. Tyr_Z–O• then extracts electrons and protons step-by-step across the S₀ → S₄ Kok cycle from the adjacent Mn₄CaO₅ oxygen-evolving complex (OEC) to split 2 H₂O → O₂ + 4 H⁺ + 4 e⁻, producing the molecular oxygen of Earth's atmosphere.
• Class I Ribonucleotide Reductase (RNR): To reduce ribonucleoside diphosphates (NDPs) into the 2′-deoxyribonucleoside diphosphates (dNDPs) required for all DNA synthesis, the β₂ subunit of E. coli Class Ia RNR generates a stable, buried tyrosyl radical at Tyr122 (Tyr122–O•) stabilized adjacent to a diferric (Fe^III–O–Fe^III) cluster. Upon substrate binding, the unpaired electron tunnels reversibly across a ~35-Å proton-coupled electron-transfer pathway spanned by three conserved aromatic tyrosine residues (β-Tyr122• ⇌ β-Tyr356 ⇌ α-Tyr731 ⇌ α-Tyr730) to oxidize α-Cys439 into a thiyl radical in the catalytic α₂ subunit.

Catecholamine Neurotransmitters, Thyroid Hormones (T₃/T₄), and Melanin

In vertebrate neuroendocrinology and dermatology, L-tyrosine is the direct biosynthetic precursor for three major hormone and pigment families:

1. Catecholamines (Dopamine, Norepinephrine, and Epinephrine): Inside dopaminergic neurons of the substantia nigra and ventral tegmental area, noradrenergic locus coeruleus neurons, and chromaffin cells of the adrenal medulla, the four-step Blaschko pathway proceeds sequentially:

• Tyrosine Hydroxylase (TH): The rate-limiting enzyme, a BH₄- and non-heme Fe²⁺-dependent monooxygenase that hydroxylates the C3 (meta) position of L-tyrosine to form L-DOPA (L-3,4-dihydroxyphenylalanine) (administered clinically with carbidopa to cross the blood-brain barrier in Parkinson's disease).
• Aromatic L-Amino-Acid Decarboxylase (AADC / DOPA decarboxylase): Uses PLP to decarboxylate L-DOPA (releasing CO₂) into dopamine.
• Dopamine β-Hydroxylase (DBH): A vesicular Cu²⁺- and ascorbate (vitamin C)-dependent monooxygenase that hydroxylates the β-carbon of dopamine using O₂ to yield norepinephrine (noradrenaline).
• Phenylethanolamine N-Methyltransferase (PNMT): Induced by high local cortisol from the adrenal cortex, PNMT transfers a methyl group from SAM to the amine nitrogen of norepinephrine to yield epinephrine (adrenaline).

1. Thyroid Hormones (Triiodothyronine T₃ and Thyroxine T₄): Within the follicular colloid of the thyroid gland, thyroid peroxidase (TPO) uses H₂O₂ to oxidize iodide (I⁻) and iodinate specific tyrosine residues embedded within the 660-kDa glycoprotein dimer thyroglobulin, forming 3-monoiodotyrosine (MIT) and 3,5-diiodotyrosine (DIT). TPO then catalyzes an intramolecular free-radical phenolic coupling reaction that transfers the iodinated hydroxyphenyl ring of a donor DIT (or MIT) onto the phenolic oxygen of an acceptor DIT (leaving a dehydroalanine scar on the donor backbone), generating thyroxine (3,5,3′,5′-tetraiodothyronine, T₄) and triiodothyronine (3,5,3′-T₃), which are liberated into the bloodstream upon lysosomal proteolysis of thyroglobulin.
2. Eumelanin and Pheomelanin Pigments: In neural-crest-derived melanocytes (within melanosomes), the binuclear copper (Cu_A–Cu_B) enzyme tyrosinase (TYR) catalyzes both the ortho-hydroxylation of L-tyrosine to L-DOPA and its subsequent two-electron oxidation to DOPAquinone. In the absence of thiols, DOPAquinone cyclizes via dopachrome into dark brown/black eumelanin polymers that absorb UV radiation and scavenge free radicals; in the presence of cysteine or glutathione, DOPAquinone conjugates to form yellow/reddish-brown sulfur-containing pheomelanin. Loss-of-function mutations in TYR cause oculocutaneous albinism type 1 (OCA1).

Fumarate/Acetoacetate Catabolism, Alkaptonuria, and Tyrosinemia Type I

In the liver, excess dietary tyrosine is degraded via a five-step oxidative pathway into both a glucogenic intermediate (fumarate) and a ketogenic intermediate (acetoacetate), making tyrosine (like phenylalanine, isoleucine, and tryptophan) both glucogenic and ketogenic:

1. Tyrosine Aminotransferase (TAT): Transaminates tyrosine with α-ketoglutarate to form 4-hydroxyphenylpyruvate and glutamate (deficiency causes Tyrosinemia Type II, with corneal crystals and palmoplantar hyperkeratosis).
2. 4-Hydroxyphenylpyruvate Dioxygenase (HPPD): An Fe²⁺- and ascorbate-dependent dioxygenase that decarboxylates, hydroxylates, and migrates the side chain (NIH shift) of 4-hydroxyphenylpyruvate to form homogentisate (2,5-dihydroxyphenylacetate).
3. Homogentisate 1,2-Dioxygenase (HGD): Uses Fe²⁺ and O₂ to cleave the aromatic benzene ring of homogentisate into 4-maleylacetoacetate. Autosomal recessive deficiency of HGD causes alkaptonuria—historically recognized in 1902 by Sir Archibald Garrod as the first human disease shown to obey Mendelian recessive inheritance ("inborn errors of metabolism")—in which excreted homogentisate oxidizes in air to dark alkapton polymers (turning standing urine black) and deposits ochronotic pigment in cartilage and intervertebral discs.
4. Maleylacetoacetate Isomerase (MAAI) and Fumarylacetoacetate Hydrolase (FAH): Glutathione-dependent MAAI isomerizes 4-maleylacetoacetate to 4-fumarylacetoacetate, which FAH hydrolyzes into fumarate and acetoacetate. Deficiency of FAH causes severe hepatorenal Tyrosinemia Type I, in which accumulated fumarylacetoacetate dehydrates into the potent alkylating toxin succinylacetone. Clinical administration of nitisinone (NTBC)—a potent competitive inhibitor of upstream HPPD—blocks homogentisate and succinylacetone formation, rescuing patients from fatal liver failure when paired with a phenylalanine/tyrosine-restricted diet.

Why is tyrosine wildly overrepresented in the antigen-binding CDR loops of human antibodies?

Across human immunoglobulin heavy- and light-chain complementarity-determining regions (especially CDR-H3), tyrosine accounts for up to 20% to 25% of all contact residues—roughly 7-fold higher than its proteome-wide average (~3.2%). Because tyrosine combines a large, rigid nonpolar benzene plane (capable of hydrophobic packing and π–π/cation–π stacking) with a terminal polar phenolic –OH group (capable of donating and accepting directional hydrogen bonds) and only two rotatable side-chain dihedral angles (χ₁, χ₂, minimizing conformational entropy loss upon binding), it can form high-affinity contacts with both polar and nonpolar epitopes.

Why can patients with Parkinson's disease be treated with oral L-DOPA instead of oral dopamine or L-tyrosine?

Dopamine itself is charged at physiological pH and cannot cross the blood-brain barrier (BBB), while feeding excess oral L-tyrosine fails to boost brain dopamine because the first enzyme of the pathway—tyrosine hydroxylase (TH)—is already saturated at normal physiological brain tyrosine concentrations (K_m ≈ 25–50 μM). By administering L-DOPA (the immediate product downstream of the rate-limiting TH step), L-DOPA crosses the blood-brain barrier via the LAT1 (SLC7A5) neutral amino-acid transporter and is rapidly decarboxylated by neuronal AADC into dopamine (co-administered with carbidopa, which cannot cross the BBB and blocks peripheral DOPA decarboxylation).

How does the herbicide-derived drug nitisinone (NTBC) cure fatal liver failure in Tyrosinemia Type I?

In Tyrosinemia Type I, loss of the final enzyme fumarylacetoacetate hydrolase (FAH) causes the upstream metabolites fumarylacetoacetate and succinylacetone to accumulate inside hepatocytes and proximal renal tubules, inducing DNA cross-linking, apoptosis, hepatocellular carcinoma, and porphobilinogen synthase inhibition. Nitisinone (NTBC) acts via "substrate reduction therapy": it potently inhibits the second enzyme of the pathway (4-hydroxyphenylpyruvate dioxygenase, HPPD), converting severe Tyrosinemia Type I into a biochemically benign upstream block managed easily with dietary Phe/Tyr restriction.

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