Amino acids are a type of organic acid that contain both a carboxyl group (COOH) and an
amino group (NH2). The general formula for an amino acid is given below. Although the
neutrally-charged structure is commonly written, it is inaccurate because the acidic COOH
and basic NH2 groups react with one another to form an internal salt called a zwitterion. The
zwitterion has no net charge; there is one negative (COO-) and one positive (NH3+) charge.
There are 20 amino acids derived from proteins. While there are several methods of
categorizing them, one of the most common is to group them according to the nature of their
side chains.
Nonpolar Side Chains
There are eight amino acids with nonpolar side chains. Glycine, alanine, and proline have
small, nonpolar side chains and are all weakly hydrophobic. Phenylalanine, valine, leucine,
isoleucine, and methionine have larger side chains and are more strongly hydrophobic.
Polar, Uncharged Side Chains
There are also eight amino acids with polar, uncharged side chains. Serine and threonine have
hydroxyl groups. Asparagine and glutamine have amide groups. Histidine and tryptophan
have heterocyclic aromatic amine side chains. Cysteine has a sulfhydryl group. Tyrosine has
a phenolic side chain. The sulfhydryl group of cysteine, phenolic hydroxyl group of tyrosine,
and imidazole group of histidine all show some degree of pH-dependent ionization.
Charged Side Chains
There are four amino acids with charged side chains. Aspartic acid and glutamic acid have
carboxyl groups on their side chains. Each acid is fully ionized at pH 7.4. Arginine and lysine
have side chains with amino groups. Their side chains are fully protonated at pH 7.4.
This table shows amino acid names, three- and one-letter standard abbreviations, and linear
structures (atomis in bold text are bonded to each other). Click on the amino acid name for its
Fischer projection formula.
Table of the Amino Acids
Name
Abbreviation Linear Structure
Alanine
ala A
CH3-CH(NH2)-COOH
Arginine
arg R
HN=C(NH2)-NH-(CH2)3-CH(NH2)-COOH
Asparagine
asn N
H2N-CO-CH2-CH(NH2)-COOH
Aspartic Acid asp D
HOOC-CH2-CH(NH2)-COOH
Cysteine
HS-CH2-CH(NH2)-COOH
cys C
Glutamic Acid glu E
HOOC-(CH2)2-CH(NH2)-COOH
Glutamine
gln Q
H2N-CO-(CH2)2-CH(NH2)-COOH
Glycine
gly G
NH2-CH2-COOH
Histidine
his H
NH-CH=N-CH=C-CH2-CH(NH2)-COOH
Isoleucine
ile I
CH3-CH2-CH(CH3)-CH(NH2)-COOH
Leucine
leu L
(CH3)2-CH-CH2-CH(NH2)-COOH
Lysine
lys K
H2N-(CH2)4-CH(NH2)-COOH
Methionine
met M
CH3-S-(CH2)2-CH(NH2)-COOH
Phenylalanine phe F
Ph-CH2-CH(NH2)-COOH
Proline
pro P
NH-(CH2)3-CH-COOH
Serine
ser S
HO-CH2-CH(NH2)-COOH
Threonine
thr T
CH3-CH(OH)-CH(NH2)-COOH
Tryptophan
trp W
Ph-NH-CH=C-CH2-CH(NH2)-COOH
Tyrosine
tyr Y
HO-Ph-CH2-CH(NH2)-COOH
Valine
val V
(CH3)2-CH-CH(NH2)-COOH