[sis-tee-een, -in]

noun Biochemistry.

a crystalline amino acid, C3H7O2NS, a component of nearly all proteins, obtained by the reduction of cystine. Abbreviation: Cys; Symbol: C

Origin of cysteine

First recorded in 1880–85; alteration of cystine
Related formscys·te·in·ic, adjective Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

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British Dictionary definitions for cysteine



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

Word Origin for cysteine

C19: variant of cystine
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

cysteine in Medicine


[sĭstē-ēn′, -ĭn, sĭ-stēĭn]


An alpha-amino acid found in most proteins and especially abundant in keratin.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

cysteine in Science



A nonessential amino acid. Chemical formula: C3H7NO2S. See more at amino acid.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.