The proteins in all living species, from bacteria to humans, are constructed from the same set of 20 amino acids, so called because each contains an amino group attached to a carboxylic acid. The amino acids in proteins are α-amino acids, which means the amino group is attached to the α-carbon of the carboxylic acid, the carbon next to the carbonyl carbon. Humans can synthesize only about half of the needed amino acids; the remainder must be obtained from the diet and are known as essential amino acids.

The amino acids are colorless, nonvolatile, crystalline solids, melting and decomposing at temperatures above 200°C. These melting temperatures are more like those of inorganic salts than those of amines or organic acids and indicate that the structures of the amino acids in the solid state and in neutral solution are best represented as having both a negatively charged group and a positively charged group. Such a species is known as a zwitterion.

Classification

In addition to the amino and carboxyl groups, amino acids have a side chain or R group attached to the α-carbon. Each amino acid has unique characteristics arising from the size, shape, solubility, and ionization properties of its R group. As a result, the side chains of amino acids exert a profound effect on the structure and biological activity of proteins. Although amino acids can be classified in various ways, one common approach is to classify them according to whether the functional group on the side chain at neutral pH is nonpolar, polar but uncharged, negatively charged, or positively charged. The structures and names of the 20 amino acids, their one- and three-letter abbreviations, and some of their distinctive features are given in Table 18.2 “Common Amino Acids Found in Proteins”.

Table 18.2 Common Amino Acids Found in Proteins

Common Name	Abbreviation	Structural Formula (at pH 6)	Molar Mass	Distinctive Feature
Amino acids with a nonpolar R group
glycine	gly (G)		75	the only amino acid lacking a chiral carbon
alanine	ala (A)		89	—
valine	val (V)		117	a branched-chain amino acid
leucine	leu (L)		131	a branched-chain amino acid
isoleucine	ile (I)		131	an essential amino acid because most animals cannot synthesize branched-chain amino acids
phenylalanine	phe (F)		165	also classified as an aromatic amino acid
tryptophan	trp (W)		204	also classified as an aromatic amino acid
methionine	met (M)		149	side chain functions as a methyl group donor
proline	pro (P)		115	contains a secondary amine group; referred to as an α-imino acid
Amino acids with a polar but neutral R group
serine	ser (S)		105	found at the active site of many enzymes
threonine	thr (T)		119	named for its similarity to the sugar threose
cysteine	cys (C)		121	oxidation of two cysteine molecules yields cystine
tyrosine	tyr (Y)		181	also classified as an aromatic amino acid
asparagine	asn (N)		132	the amide of aspartic acid
glutamine	gln (Q)		146	the amide of glutamic acid
Amino acids with a negatively charged R group
aspartic acid	asp (D)		132	carboxyl groups are ionized at physiological pH; also known as aspartate
glutamic acid	glu (E)		146	carboxyl groups are ionized at physiological pH; also known as glutamate
Amino acids with a positively charged R group
histidine	his (H)		155	the only amino acid whose R group has a pKa (6.0) near physiological pH
lysine	lys (K)		147	—
arginine	arg (R)		175	almost as strong a base as sodium hydroxide

The first amino acid to be isolated was asparagine in 1806. It was obtained from protein found in asparagus juice (hence the name). Glycine, the major amino acid found in gelatin, was named for its sweet taste (Greek glykys, meaning “sweet”). In some cases an amino acid found in a protein is actually a derivative of one of the common 20 amino acids (one such derivative is hydroxyproline). The modification occurs after the amino acid has been assembled into a protein.

Configuration

Notice in Table 18.2 “Common Amino Acids Found in Proteins” that glycine is the only amino acid whose α-carbon is not chiral. Therefore, with the exception of glycine, the amino acids could theoretically exist in either the D- or the L-enantiomeric form and rotate plane-polarized light. As with sugars, chemists use glyceraldehyde as the reference compound for the assignment of configuration to amino acids. (For more information about stereoisomers and configuration, see Chapter 16 “Carbohydrates”, Section 16.2 “Classes of Monosaccharides”.) Its structure closely resembles an amino acid structure except that in the latter, an amino group takes the place of the OH group on the chiral carbon of the sugar.

We learned in Chapter 16 “Carbohydrates” that all naturally occurring sugars belong to the D series. It is interesting, therefore, that nearly all known plant and animal proteins are composed entirely of L-amino acids. However, certain bacteria contain D-amino acids in their cell walls, and several antibiotics (e.g., actinomycin D and the gramicidins) contain varying amounts of D-leucine, D-phenylalanine, and D-valine.

Concept Review Exercises

1. What is the general structure of an α-amino acid?

2. Identify the amino acid that fits each description.

   1. also known as aspartate
   2. almost as strong a base as sodium hydroxide
   3. does not have a chiral carbon

Show Answer

1.

2. a. aspartic acid

b. arginine

c. glycine

Key Takeaways

• Amino acids can be classified based on the characteristics of their distinctive side chains as nonpolar, polar but uncharged, negatively charged, or positively charged.
• The amino acids found in proteins are L-amino acids.

Exercises

1. Write the side chain of each amino acid.

   1. serine
   2. arginine
   3. phenylalanine

2. Write the side chain of each amino acid.

   1. aspartic acid
   2. methionine
   3. valine

3. Draw the structure for each amino acid.

   1. alanine
   2. cysteine
   3. histidine

4. Draw the structure for each amino acid.

   1. threonine
   2. glutamic acid
   3. leucine

5. Identify an amino acid whose side chain contains a(n)

   1. amide functional group.
   2. aromatic ring.
   3. carboxyl group.

6. Identify an amino acid whose side chain contains a(n)

   1. OH group
   2. branched chain
   3. amino group

Show Answer

1. a. CH₂OH⁻

b. 

c. 

3. a. 

b. 

c. 

5.

a. asparagine or glutamine

b. phenylalanine, tyrosine, or tryptophan

c. aspartic acid or glutamic acid