Overview
Cysteine is a sulfur-containing amino acid whose thiol (-SH) group makes it the primary metal-binding residue in biology. It is the rate-limiting precursor for glutathione (the master intracellular antioxidant), a key residue in metallothioneins (metal-binding proteins), and a precursor for hydrogen sulfide (via CBS/CSE enzymes). In the WikiBiome framework, cysteine sits at the intersection of metal detoxification, antioxidant defense, and microbial sulfur metabolism.
Metal Binding
- Cadmium, mercury, lead, arsenic all bind cysteine thiol groups with high affinity, depleting free cysteine and glutathione pools → oxidative stress [1] [2].
- Zinc-finger proteins use cysteine (and histidine) residues to coordinate zinc — metal displacement at these sites is the basis of mis metallation.
- Iron-sulfur clusters use cysteine ligands — heavy metal disruption of Fe-S clusters is a primary toxicity mechanism [3].
- Nickel: Bacterial nickel-binding proteins (Hpn, HypB) use histidine-rich motifs, but cysteine residues are critical in nickel efflux and storage proteins [4].
Microbial Context
- Gut bacteria metabolize dietary cysteine via desulfhydrase enzymes → H₂S production (hydrogen sulfide).
- Microbial cysteine metabolism affects the gut sulfur pool, influencing both host detoxification capacity and desulfovibrio ecology.
Cross-References
- glutathione — cysteine is the rate-limiting precursor
- hydrogen sulfide — cysteine as H₂S precursor
- mis metallation — metal displacement at cysteine-coordinated sites
- iron — iron-sulfur cluster cysteine ligands
- cadmium — high-affinity cysteine thiol binding
- oxidative stress — cysteine depletion reduces antioxidant capacity