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.