Cysteine
Properties
| State | Solid (white crystalline powder) |
| Color | White |
| Solubility | Highly soluble in water (280 g/L at 25°C); slightly soluble in ethanol |
| Melting Point | 240°C (decomposes) |
| Boiling Point | Decomposes before boiling |
About Cysteine
Cysteine is the only proteinogenic amino acid that gets actively oxidized inside proteins as part of its job. The thiol side chain has a pKa around 8.3 — slightly above physiological pH — so a small but significant fraction sits as the much more nucleophilic thiolate (RS⁻) at any cellular pH, and that's what drives essentially all of cysteine's reactive chemistry. Two cysteine thiolates can be oxidized to a disulfide (–S–S–, the cystine motif) at a redox potential around -250 mV vs SHE, which means disulfides form spontaneously in the oxidizing environment of the endoplasmic reticulum and extracellular space but stay reduced in the cytoplasm where the GSH/GSSG ratio is around 100:1. This redox switch is why secreted proteins like insulin, antibody chains, and lysozyme are riddled with disulfides while cytoplasmic proteins almost never have them — the cell uses oxidation state to mark inside vs outside. Cysteine is also the active-site residue in the cysteine protease family (papain in pineapple, cathepsins in lysosomes, caspases in apoptosis), where the thiolate attacks the substrate carbonyl in a covalent intermediate mechanism analogous to serine proteases but with sulfur as the nucleophile. The tripeptide glutathione (γ-Glu-Cys-Gly) at 1–10 mM in most cells is the cellular redox buffer; cysteine is the rate-limiting precursor in its synthesis. N-acetylcysteine (NAC) is the classical antidote for acetaminophen overdose — paracetamol's toxic metabolite NAPQI depletes hepatic glutathione, and NAC replenishes it by feeding cysteine into glutathione synthesis. In food chemistry, L-cysteine added to bread dough at ~50 ppm cleaves gluten disulfides, weakening the dough so it stretches faster — a standard tool in industrial baking.
Where you'll encounter it
If you've ever taken NAC to clear mucus from a cold or seen it administered in an ER for an acetaminophen overdose, that's cysteine chemistry. In a biochemistry lab you reduce protein disulfides with DTT or β-mercaptoethanol and re-oxidize them with O₂ or glutathione redox buffer — that's all just controlling cysteine oxidation state. In commercial bread (especially fast-proofed sandwich loaves), the slight sulfurous smell when you tear open a fresh loaf comes partly from L-cysteine added as a dough conditioner; historically it came from hydrolyzed hair and feathers, now mostly from microbial fermentation.
Common Uses
- N-acetylcysteine (NAC) IV antidote for acetaminophen overdose at 150 mg/kg loading dose
- Mucolytic agent (NAC inhalation) for cystic fibrosis and chronic bronchitis at 10–20% solution
- Dough conditioner in industrial bread baking at 30–90 ppm to cleave gluten disulfides
- Flavor precursor for meat/savory aroma development in Maillard reactions with reducing sugars
- Reducing agent in protein chemistry buffers (alongside DTT and β-mercaptoethanol)
- Glutathione synthesis precursor for nutritional supplementation in oxidative-stress conditions
- Antioxidant in cosmetic permanent-wave formulations for breaking and reforming hair disulfides
- Precursor for taurine biosynthesis via cysteine dioxygenase and cysteine sulfinate decarboxylase
Safety Information
Generally Recognized As Safe (GRAS) by FDA at typical dietary and food-additive levels. Pure L-cysteine ingestion above ~7 g/day can cause GI upset and elevated homocysteine. NAC at therapeutic doses (>5 g IV) occasionally produces anaphylactoid reactions — flushing, hypotension — that are usually managed by slowing the infusion. The thiol gives cysteine a faintly sulfurous smell at higher concentrations and reacts with heavy metals (Hg²⁺, Pb²⁺, Cu²⁺) to form RS-metal complexes — useful for chelation but means cysteine solutions slowly oxidize in air to cystine and lose activity. Store buffered solutions cold under N₂ for redox-sensitive work.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.