Reference
Cysteine
**Cysteine** is a conditionally essential sulfur-containing amino acid that the body can synthesize from methionine and serine, but which becomes dietarily essential when methionine supply is limited or metabolic demand is high. It is the rate-limiting precursor for glutathione, the body's principal intracellular antioxidant, and contributes the thiol group that forms disulfide bonds stabilizing protein structure.
Why it is “conditionally essential”
Most amino acids are classified as either essential (must come from diet) or non-essential (synthesized internally). Cysteine sits between the two. Its carbon skeleton and sulfur atom derive from methionine through the transsulfuration pathway, so adequate methionine intake usually covers cysteine needs. When methionine is scarce — or during periods of rapid growth, illness, or oxidative stress — endogenous production falls short and dietary cysteine becomes necessary. For this reason, nutritionists frequently report methionine and cysteine together as the “sulfur amino acids” when scoring protein quality.
The methionine-sparing relationship
Because methionine is the metabolic source of cysteine, dietary cysteine reduces the amount of methionine the body must divert to make it — an effect described as methionine sparing. A diet supplying ample cysteine allows more methionine to remain available for its other roles, including initiating protein translation and serving as a methyl donor. This interdependence is why amino-acid scoring methods such as PDCAAS and DIAAS evaluate the sulfur amino acids as a combined pool rather than individually.
Relevance to plant proteins and potato protein
The sulfur amino acids are the most common limiting factor in legume proteins. In an analysis of new pea genotypes, the sum of methionine plus cysteine was the limiting amino acid, averaging only 2.6 g per 100 g of protein — a chemical score of roughly 46% (Molecules, 2024, PMID 39519674). Collagen is similarly poor in sulfur amino acids, which is one reason such proteins receive low quality ratings despite their other functional roles. These shortfalls help explain why a protein’s sulfur-amino-acid content, rather than its lysine or leucine, often determines its overall quality rating.
Potato protein isolate is a complete protein whose amino-acid profile supports high digestibility-corrected scores; for background on how it is produced and rated, see what is potato protein. Evaluating any single amino acid in isolation is less informative than reading the full profile, since the limiting amino acid — whichever it happens to be — sets the ceiling on usable protein quality.
Related terms
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