Definitions

1. noun

a nonessential amino acid, C3H5NO2S, occurring in proteins and being a precursor of methionine and other amino acids

“Cysteine is an amino acid found in many proteins and is an important component in the structure and function of enzymes, antibodies, and other biological molecules.”

2. chemistry

an amino acid with the chemical formula C3H7NO2S

“Cysteine is a naturally occurring amino acid with the chemical formula C3H7NO2S.”