Cysteine

Cysteine (abbreviated Cys or C; systematic IUPAC name (2R)-2-amino-3-sulfanylpropanoic acid, chemical formula C₃H₇NO₂S, molecular mass 121.16 Da) is a polar, sulfur-containing α-amino acid incorporated into proteins and biochemical macromolecules during translation, specified in the standard genetic code by the two mRNA codons UGU and UGC. Distinguished by its β-methylene thiol (sulfhydryl or mercapto, –CH₂–SH) side chain, L-cysteine is the most redox-active of the twenty standard proteinogenic amino acids. In the oxidizing lumen of the endoplasmic reticulum (assisted by chaperone proteins and protein disulfide isomerase), pairs of cysteine thiol groups undergo two-electron oxidative coupling to form covalent disulfide bonds (–S–S–) within a cystine residue, stabilizing secreted hormones and antibodies. In the reducing intracellular compartments, the cysteine thiol (pK_a3 ≈ 8.18–8.37) ionizes readily into a potent thiolate anion (–S⁻), which functions as the catalytic nucleophile in cysteine proteases, coordinates structural Zn²⁺ ions in zinc-finger transcription factors and catalytic [2Fe-2S] / [4Fe-4S] iron-sulfur clusters in the respiratory electron transport chain, undergoes reversible oxidation by reactive oxygen species in cellular redox signaling, and supplies the rate-limiting thiol moiety of the cellular antioxidant glutathione (GSH).

Why L-Cysteine Has the (2R) Cahn–Ingold–Prelog Configuration

Although all nineteen chiral proteinogenic amino acids share the same spatial L-configuration at the α-carbon (C_α, matching L-glyceraldehyde in a Fischer projection), 18 of those 19 L-amino acids have an (2S) Cahn–Ingold–Prelog (CIP) stereochemical designation, whereas L-cysteine is the only standard L-amino acid with the (2R) configuration (alongside its selenium analog, L-selenocysteine). This apparent reversal is an artifact of the atomic-number priority rules of the CIP system:

• Around the α-carbon (C2), Priority 1 is always the α-amino group (–NH₂, atomic number Z = 7) and Priority 4 is always the α-hydrogen (–H, Z = 1).
• In all other standard L-amino acids, the α-carboxyl carbon (–COOH, bonded to (O,O,O)) outranks the β-carbon (C3, bonded to C, N, or O), receiving Priority 2 (S counterclockwise).
• In L-cysteine, however, the β-carbon (–CH₂–SH) is bonded directly to a sulfur atom (Z = 16), which has a higher atomic number than oxygen (Z = 8). Consequently, the –CH₂SH side chain (bonded to (S,H,H)) outranks the –COOH carboxyl group (bonded to (O,O,O)), reversing Priorities 2 and 3 (1 → 2 → 3 is clockwise = (2R)) without any change in 3D geometry.

Thiolate (–S⁻) Nucleophilicity and Catalytic Dyads/Triads

In aqueous solution at 25 °C, free L-cysteine exhibits pK_a1 = 1.96 (α-COOH), pK_a2 = 10.28 (α-NH₃⁺), and a side-chain thiol dissociation constant of pK_a3 ≈ 8.18–8.37 (isoelectric point pI = 5.07). Because sulfur sits in Period 3 of the periodic table (3p valence orbitals), its atomic radius is larger and its electron cloud is more polarizable ("softer" in Pearson hard–soft acid–base theory) than the 2p oxygen of serine (–CH₂OH, pK_a ≈ 13.6). Even at physiological pH 7.4, approximately 10–15% of an unperturbed cysteine side chain exists as the deprotonated thiolate anion (–CH₂–S⁻), which is orders of magnitude more nucleophilic than a neutral alcohol or amine.

Inside the active sites of cysteine proteases, protein tyrosine phosphatases (PTPs), peroxiredoxins, and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), electrostatic interaction with an adjacent histidine imidazolium ring (His⁺) and the positive end of an α-helix macrodipole depresses the catalytic cysteine pK_a to 3.5–6.0, creating a preformed Cys-S⁻ / His-ImH⁺ ion pair at neutral pH:

• Papain, Cathepsins (B, K, L, S), and Deubiquitinases (DUBs): Utilize a Cys–His–Asn catalytic triad (Cys25–His159–Asn175 in papain) in which the Cys thiolate attacks the scissile peptide carbonyl carbon to form a covalent acyl-enzyme thioester intermediate, followed by His-assisted hydrolysis.
• Caspases (Caspase-1 to -14) and SARS-CoV-2 Main Protease (M^pro / 3CL^pro): Utilize a Cys–His catalytic dyad (Cys145–His41 in SARS-CoV-2 M^pro, the covalent target of the antiviral drug nirmatrelvir in Paxlovid; Cys285–His237 in caspase-1, cleaving after aspartate residues during apoptosis and pyroptosis).
• Protein Tyrosine Phosphatases (PTP1B, SHP2, PTEN): Utilize a signature HCX₅R motif in which the low-pK_a (~4.7) cysteine thiolate attacks the phosphorus atom of phosphotyrosine (or PIP₃ in PTEN) to form a transient cysteinyl-phosphate (E-Cys-S-PO₃²⁻) intermediate.

Sulfide Assimilation in Plants/Bacteria and Mammalian Transsulfuration

Bacteria, archaea, fungi, and plants are sulfur autotrophs that assimilate inorganic sulfate (SO₄²⁻ → APS → PAPS → sulfite SO₃²⁻ → sulfide H₂S / HS⁻) directly into L-cysteine in two steps starting from L-serine: serine acetyltransferase (CysE) converts L-serine and acetyl-CoA into O-acetyl-L-serine (OAS), and the PLP-dependent enzyme O-acetylserine (thiol)-lyase (OASS / CysK) condenses OAS with H₂S (eliminating acetate) to synthesize L-cysteine.

Mammals lack O-acetylserine sulfhydrylase and instead synthesize L-cysteine via the transsulfuration pathway (active in liver, kidney, and pancreas), where the sulfur atom comes from essential dietary L-methionine and the three-carbon backbone and nitrogen atom come from L-serine (making L-cysteine conditionally essential in preterm infants and liver disease):

1. Dietary L-methionine is converted via S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH) into the four-carbon thiol L-homocysteine.
2. Cystathionine β-Synthase (CBS, EC 4.2.1.22): A PLP- and heme-dependent enzyme (allosterically activated by SAM) condenses L-homocysteine with L-serine to form the thioether L-cystathionine (mutations in human CBS cause classical homocystinuria).
3. Cystathionine γ-Lyase (CSE / CTH, EC 4.4.1.1): A second PLP-dependent enzyme cleaves L-cystathionine at the C_γ–S bond to release L-cysteine, α-ketobutyrate, and NH₄⁺. When CBS and CSE act on L-cysteine and L-homocysteine via alternative β-replacement reactions, they generate endogenous hydrogen sulfide (H₂S), a physiological gasotransmitter.

Iron–Sulfur ([Fe-S]) Clusters, Zinc Fingers, Glutathione, and Ferroptosis

• Iron–Sulfur ([2Fe-2S], [4Fe-4S]) Clusters and Zinc Fingers: In mitochondria, the PLP-dependent cysteine desulfurase NFS1 (IscS in bacteria) abstracts sulfur from free L-cysteine (forming L-alanine) to build [2Fe-2S] and [4Fe-4S] clusters coordinated by four cysteine thiolates in Complex I, II, and III of the electron transport chain and aconitase. Similarly, Cys2His2 and Cys4 zinc-finger transcription factors and metallothioneins use cysteine thiolates to coordinate Zn²⁺ and Cu⁺.
• Glutathione (GSH), System x_c⁻, and Ferroptosis: Cells store 1–10 mM cysteine in the tripeptide glutathione (GSH, γ-L-glutamyl-L-cysteinyl-glycine), synthesized by rate-limiting glutamate–cysteine ligase (GCL) and glutathione synthetase (GSS). Proliferating cells import extracellular L-cystine (Cys–S–S–Cys) via the plasma membrane cystine/glutamate antiporter System x_c⁻ (SLC7A11 / xCT) and reduce it to L-cysteine to fuel GSH synthesis for Glutathione Peroxidase 4 (GPX4); inhibiting SLC7A11 (with erastin) or GPX4 (with RSL3) triggers iron-dependent lipid peroxidation and ferroptosis. Cysteine also donates the thiol arm of Coenzyme A and is oxidized via cysteine dioxygenase (CDO1) to taurine.
• Oxidative Post-Translational Modifications (OxiPTMs): Cellular reactive oxygen species (H₂O₂) oxidize regulatory thiolates into reversible sulfenic acid (R–SOH), disulfides, or S-glutathionylation (R–S–SG); higher H₂O₂ flux yields sulfinic acid (R–SO₂H, reduced by sulfiredoxin) and irreversible sulfonic acid (R–SO₃H), alongside nitric oxide S-nitrosylation (R–SNO), H₂S S-persulfidation (R–SSH), and lipid S-palmitoylation.

Why is selenocysteine ("the 21st amino acid") superior to cysteine in antioxidant selenoenzymes like GPX4?

L-Selenocysteine (Sec, U)—inserted co-translationally at in-frame UGA codons directed by a 3'-UTR Selenocysteine Insertion Sequence (SECIS) stem-loop and tRNA^[Ser]Sec—differs from L-cysteine by a single atom: selenium (Se, Z = 34) replaces sulfur (S, Z = 16). This substitution confers two chemical advantages to human glutathione peroxidases (GPX1–4) and thioredoxin reductases (TXNRD1–3): first, the selenol group (–SeH) has a lower pK_a of 5.2 (compared to 8.3 for –SH), so >99% of selenocysteine exists as the nucleophilic selenolate anion (–Se⁻) at physiological pH 7.4; second, whereas a cysteine sulfenic acid (–SOH) exposed to hydroperoxide flux is easily irreversibly overoxidized to sulfonic acid (–SO₃H), selenocysteine resists permanent overoxidation and is rapidly reduced back to –Se⁻ by GSH or NADPH.

Why is N-acetylcysteine (NAC) the clinical antidote for acetaminophen (paracetamol) overdose?

At therapeutic doses, ~90% of acetaminophen is conjugated in the liver to glucuronide and sulfate, while ~5–10% is oxidized by cytochrome P450 2E1 (CYP2E1) into the hepatotoxic electrophile N-acetyl-p-benzoquinone imine (NAPQI), which is detoxified by conjugation with the cysteine thiol of hepatic glutathione (GSH). In an acetaminophen overdose, elevated NAPQI production depletes hepatic GSH stores below 20% of normal, allowing unbound NAPQI to arylate mitochondrial proteins and trigger hepatic necrosis. Clinicians administer intravenous or oral N-acetyl-L-cysteine (NAC), which enters hepatocytes and is deacetylated by cytosolic acylase I into free L-cysteine—restoring the rate-limiting substrate for glutamate–cysteine ligase (GCL) to replenish hepatic GSH and scavenge NAPQI.

What causes the genetic kidney stone disorder cystinuria, and why does it involve cystine rather than cysteine?

Cystinuria (~1 in 7,000 births) is an autosomal recessive defect in the renal proximal tubule apical heterodimeric amino acid transporter rBAT / b^0,+AT (encoded by SLC3A1 and SLC7A9), which reabsorbs filtered L-cystine alongside the dibasic amino acids ornithine, lysine, and arginine ("COLA"). The disease involves the disulfide dimer cystine (Cys–S–S–Cys) rather than monomeric cysteine (Cys–SH) because free cysteine in oxidizing blood plasma and glomerular filtrate is oxidized into cystine. While ornithine, lysine, and arginine are soluble in urine, L-cystine is the least water-soluble of all naturally occurring amino acids (solubility ~0.1–0.2 g/L at acidic urinary pH 5.0–6.5), causing un-reabsorbed cystine to precipitate as characteristic flat hexagonal kidney stones.

1. Poole, L. B. (2015). "The basics of thiols and cysteines in redox biology and chemistry." Free Radical Biology and Medicine, 80, 148–157. https://doi.org/10.1016/j.freeradbiomed.2014.11.013
2. Stipanuk, M. H. (2004). "Sulfur amino acid metabolism: pathways for production and removal of homocysteine and cysteine." Annual Review of Nutrition, 24, 539–577. https://doi.org/10.1146/annurev.nutr.24.012003.132418
3. Paul, B. D., & Snyder, S. H. (2012). "H₂S signalling through protein sulfhydration and beyond." Nature Reviews Molecular Cell Biology, 13(8), 499–507. https://doi.org/10.1038/nrm3391
4. Dixon, S. J., Lemberg, K. M., Lamprecht, M. R., Skouta, R., Zaitsev, E. M., Gleason, C. E., ... & Stockwell, B. R. (2012). "Ferroptosis: an iron-dependent form of nonapoptotic cell death." Cell, 149(5), 1060–1072. https://doi.org/10.1016/j.cell.2012.03.042