ęDavid B. Fankhauser, Ph.D.,
Professor of Biology and Chemistry
University of Cincinnati Clermont College,
Batavia OH 45103
Tyrosine is a polar amino acid
because of the alcohol group carried
on the phenyl ring.

This page has been accessed  Countertimes since 30 August 2013. 
Created 30 August 2013

Two cysteine amino acids are joined
together to form a cystine.


There are twenty naturally occurring amino acids.  I ask my sophomore Genetics/Cell Biology to learn these eight representative amino acids.

Class of Amino Acid Image
Side Chain

(They all contain hydrocarbon side chains.)
Glycine Glycine
The side chain is a single hydrogen

This is the simplest amino acid.  The name "glycine" means "sweet nitrogen containing".
Alanine Alanine
The side chain is a methyl group.
Phenylalanine Phenylalanine
The side chain is a phenyl group attached by a linking -CH2-.

All of these amino acids contain side chains capable of hydrogen bonding, enhancing their solubility in water.
Serine Serine
the -CH2-linker carries an alcohol (hydroxyl) moiety.

All of these amino acids contain side chains capable of hydrogen bonding, enhancing their solubility in water.
Cysteine Cysteine
Cysteine resembles serine but the oxygen in the alcohol is replaced with a sulfur, creating a "thiol."

See below for an explanation how two cysteines can form a disulfide linkage, having a profound effect on the tertiary level of protein structure.
Tyrosine Tyrosine
Tyrosine resembles phenylalanine except it has an an alcohol added to its phenyl ring, making it polar.

These amino acids are capable of ionization, depending on the pH of the solution in which they are dissolved.
Aspartic Acid Glutamic Acid
The side chain in glutamic acid is a carboxylic acid moeity attached by a CH2 linker to the amino acid back bone.  Because the hydrogen in the carboxylic acid can ionize to form negatively charged carboxylate, this side chain can form an ionic bond with an cation such as an ionized amino group (see the next amino acid).

Lysine Lysine
Lysine has a four carbon linkage to the back bone, with an amino group at the end.  The nitrogen possesses an unshared pair of electrons that can accept a proton, forming a cation, or positively charged, group.  This group will be strongly attracted to anions as in the previous glutamic acid.


   At pH 7, carboxylic acids ionize (releasing a H+) to form carbolylate, which is an anion, negatively charged.  At the same time, the amino group absorbs a hydrogen ion into its unshared electron pair, becoming a cation, positively charged.  Because amino acids are therefore doubly charged, they are called "zwitterions". Here is glycine before and after the ionization process:

Glycine in the un-ionized state. 
Note that nitrogen carries two hydrogens, and the carboxyl contains a single hydrogen
Glycine as a "zwitterion": 
The carboxyl moiety has lost a proton (hydrogen ion) and the amino moeity has gained a hydrogen ion.
Glycine unionized The process of ionization includes the loss of a proton from the carboxyl (becoming a carboxylate)and the gaining of a proton by the amino moeity
ionized glycine

The thiol moieties of two cysteine can be oxidized, resulting in a disulfide bridge.  The resulting molecule is named cystine.  Because the disulfide bond is a covalent bond, it is the strongest determinant of protein tertiary structure and is often critical to the proper folding of a protein for its function.  Heavy metals are particularly poisonous to many enzymes because the break the disulfide bond, forming the metal sulfide bond.  Lead, arsenic, chromium, mercury all are poisonous for this reason.

The sulfur containing side chains are aligned for two cysteine molecules.  The hydrogens attached to the sulfurs will be removed during oxidation,
The two sulfurs are joined to form a disulfide bridge, enforcing a loop in the polypeptide chain.
                  cysteines lined up The hydrogens from the two thiol groups are removed (remember that oxidation is the removal of an electron or hydrogen atom from an atom or molecule).

The reverse reaction (reduction) takes place when hydrogens are added to each of the joined sulfurs.  This splits the disulfide bond and restores the two cysteines
.Cystine has the cysteines joined by a disulfide

Here is a dipeptide used as an artificial sweetener.  It is aspartame.  See if you can spot the peptide bond (a special amide bond: a nitrogen attached to a carbonyl) and these two amino acids in its structure aspartic acid and phenylalanine.  Besides the two amino acids, phenylalanine has a methoxy group linked by an ester linkage to its carbonyl (in the upper right of the image):
The phenylalanine contained in aspartame posses a special risk to those carrying the genetic condition called phenylketonuria (PKU).  These individuals cannot metabolize phenylalanine completely, and the breakdown product, phenylketone, accumulates and is excreted in the urine.  Persons with PKU who consume phenylalanine suffer mental retardation, seizures, etc.  These effects are prevented by maintaining a strict diet to minimize phenylalanine.

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