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CYSTEINE
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Basic Info & Structure
 Nonessential, hydrophobic αamino acid
 Characterized as a non-polar
amino acid
 Has thiol group containing
sulfur
 Molecular formula is C3H7NO2S
 Cysteine along with glycine
and glutamic acid is found in all
human tissues
 Can form into Cystine when
exposed to air due to oxidation
 Stabilizes extracellular protein
Introduction to D & L Form
• Almost every amino acid has 2 forms that are isometric.
• It is because the central Carbon atom can have 2
enantiomers (optical isomers) around it.
• Enantiomers are chiral molecules. They are mirror images
of one another which makes them non-superimposable.
• No matter how much you rotate or arrange the molecule,
they can never be identical.
Optical Activity
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The structures of D and L amino acids are compared to Dglyceraldehyde and L-glyceraldehyde.
D and L are references to the optical activities of enantiomers (optical
isomers).
Optical activity is the interaction of optical isomers with planepolarized lights.
“When a plane-polarized light beam passes through a pure solution
of D-glyceraldehyde the emergent beam will be rotated the light
plane to the right, and hence is the enantiomer is considered to
be dextrorotatory ("dextra" is Latin for right) and is designated
the "D" enantiomer” (Amino Acid Stereoisomers, 2012).
“However, if beam of plane polarized light passes through a pure
solution of L-glyceraldehyde, the emergent light beam will be
rotated in the opposite direction to the left, and hence the
enantiomer is considered to be levorotatory (laevus is Latin for left)
and is designated the "L" enantiomer” (Amino Acid Stereoisomers,
2012).
D & L Isomers
• Only Levorotatory (L) isomers are used for protein
formation.
• Dextrorotatory (D) isomers are found in nature and on
cell walls of bacterial cells.
• Since your left and right hands display chiralty,
enantiomers are often classified as “left” and “righthanded.”
• L is the “left-handed” isomer and D is the “righthanded” isomer.
• D-amino acids have a hydrogen atom right below its
central carbon atom. The carboxyl, side group, and
amine group then follows in a clockwise direction
around the carbon.
• L-amino acids follow a counterclockwise order around
the central carbon atom.
Zwitterion
What does it mean?
Zwitterion is a neutral molecule. It has both a negative
and positive charge. Though it is neutral, a zwitterion
can contain parts with separate positive and negative
charges.
Amino Acids as Zwitterions
An amino acid has an acidic carboxylic acid group as well
as a basic amine group. This means the transfer of a
hydrogen leaves an ion with both a negative and
positive charge.
Amine & Carboxylic Acid
What are amines?
 Amines are compounds that has a single Nitrogen atom with lone
pairs (pair of valence electrons that are not shared between the
atoms).
 These compounds derive from ammonia by replacing one or more
hydrogen atoms with an organic group.
What is carboxylic acid?
 An acid containing a carbonyl (C=O) and a hydroxyl (O-H).
 The structure of a carboxylic acid is usually R-COOH
 The R represents a univalent atom which has the ability to form one
covalent bond.
Functions and Uses of Cysteine
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Stengthens the lining of the intestines and stomach
Acts as a powerful antioxidant and detoxifier
Can convert to glucose and used as a source of energy
Is believed to have the ability to protect the skin from
UV damage
Reduce effect of aging in skin
Prevent cataracts and cancer
L-Cysteine can aid in treatment for diabetes
Breakdown mucous in lungs
Also prevent organ damaged caused by acetaminophen
Deficiency & Food Sources
Deficiency
 Cysteine deficiency is uncommon.
 Seen in vegetarians with low intake of food containing cysteine
 Deficiency can reduce production of glutathione.
 Symptoms of cysteine deficiency include slowed growth in children,
loss of pigmentation in hair, liver damage, muscle loss, skin lesions,
weakness, fat loss (Cysteine, 2006).
Food Sources
 Found in high-protein food
 Animal Sources: Poultry, egg yolks, dairy, pork
 Plant Sources: Garlic, red peppers, broccoli, onions, brussels sprouts
Work Cited
"Amino Acid Stereoisomers." MCDB. Ed. Duane W. Sears and Holly Rich. N.p., 7 Aug. 2012.
Web. 27 Oct. 2013. <http://mcdb-webarchive.mcdb.ucsb.edu/sears/biochemistry/twamn/aas-stereo.htm>.
Clark, Jim. "The Acid-Base Behavior of Amino Acids." Chem Guide. N.p., 2004. Web. 26 Oct.
2013. <http://www.chemguide.co.uk/organicprops/aminoacids/acidbase.html>.
"Cysteine." Birkbeck. N.p., 2009. Web. 26 Oct. 2013.
<http://www.cryst.bbk.ac.uk/pps97/assignments/projects/leluk/project.htm#Fig1>.
"Cysteine." Vitamins & Health Supplements Guide. Vitamin Supplements Guide, 2005-2006. Web.
26 Oct. 2013. <http://www.vitamins-supplements.org/amino-acids/cysteine.php>.
"Cysteine." The World's Healthiest Foods. George Mateljan Foundation, 2001-2013. Web. 26 Oct.
2013. <http://whfoods.org/genpage.php?tname=nutrient&dbid=54>.
"Definition of Stereoisomers." Chemicool. Chemicool, 2013. Web. 27 Oct. 2013.
<http://www.chemicool.com/definition/stereoisomers.html>.
"L- and D-Amino Acids." PhSchool. Pearson Education, n.d. Web. 26 Oct. 2013.
<http://www.phschool.com/science/biology_place/biocoach/bioprop/landd.html>.
Leluk, Jacek. "WHY CYSTEINE IS SPECIAL?" Birkbeck. N.p., 2009. Web. 26 Oct. 2013.
<http://www.cryst.bbk.ac.uk/pps97/assignments/projects/leluk/project.htm#Fig1>.
Limbach, Alexander. "Molecule Cysteine L and D." Deposit Photos. Depositphotos, 2013. Web. 27
Oct. 2013. <http://depositphotos.com/9336135/stock-photo-Molecule-Cysteine-L-andD.html>.
"Optical Activity." ChemWiki. UCDavis, n.d. Web. 27 Oct. 2013.
<http://chemwiki.ucdavis.edu/Organic_Chemistry/Chirality/Optical_Activity>.
"Optical Isomers." Chemicool. Chemicool, 2013. Web. 27 Oct. 2013.
<http://www.chemicool.com/definition/stereoisomers.html>.
"Sulfur-Containing Amino Acids." The Biology Project. N.p., 25 Aug. 2003. Web. 26 Oct. 2013.
<http://www.biology.arizona.edu/biochemistry/problem_sets/aa/sulfur.html>.