cysteine

American  

[sis-tee-een, -in] / ˈsɪs tiˌin, -ɪn /

noun

Biochemistry.

1. a crystalline amino acid, C 3 H 7 O 2 NS, a component of nearly all proteins, obtained by the reduction of cystine. Cys; C

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cysteine British  

/ -ɪn, ˈsɪstɪˌiːn /

noun

1. a sulphur-containing amino acid, present in proteins, that oxidizes on exposure to air to form cystine. Formula: HSCH 2 CH(NH 2 )COOH

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cysteine Scientific  

/ sĭs′tə-ēn′ /

1. A nonessential amino acid. Chemical formula: C ₃ H ₇ NO ₂ S.

2. See more at amino acid

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Other Word Forms

• cysteinic
  adjective

Etymology

Origin of cysteine

First recorded in 1880–85; alteration of cystine

Compare meaning

How does cysteine compare to similar and commonly confused words? Explore the most common comparisons:

• cysteine vs. cystine

Example Sentences

They found that a mirror version of cysteine, a sulfur-containing amino acid, can significantly slow the growth of certain tumors while leaving healthy cells largely unaffected.

From Science Daily • Mar. 12, 2026

Over seven days, the researchers adjusted the levels of methionine and cysteine in the animals' diets.

From Science Daily • Feb. 27, 2026

The body can also make cysteine by converting another amino acid, methionine, in the liver.

From Science Daily • Oct. 23, 2025

When intestinal cells absorb cysteine from food, they convert it into CoA, a cofactor that moves into the mucosal lining of the intestine.

From Science Daily • Oct. 23, 2025

Beans have both lysine and tryptophan, but not the amino acids cysteine and methionine, which are provided by maize.

From "1491" by Charles C. Mann