Wikipedia: Amino acid

Amino acid
From Wikipedia, the free encyclopedia.

In chemistry, an amino acid is any molecule that contains both amino and carboxylic acid functional groups. In biochemistry, this shorter and more general term is frequently used to refer to alpha amino acids: those amino acids in which the amino and carboxylate functionalities are attached to the same carbon.

Overview

Amino acids are a biochemical building block. They form long chemical chains called proteins (see below), and shorter chains called peptides.

Twenty amino acids are encoded by the standard genetic code and are called proteinogenic. Proline is the only cyclic proteinogenic amino acid. Other amino acids contained in proteins are usually formed by modification after translation (protein synthesis). These modifications are often essential for the function of the protein. At least two amino acids other than the standard 20 are sometimes incorporated into proteins during translation:

Selenocysteine is incorporated into some proteins at a UGA codon, which is normally a stop codon.
Pyrrolysine is used by some methanogens in enzymes that they use to produce methane. It is coded for similarly to selenocysteine but with the codon UAG instead.

Over 500 amino acids have been found in nature. Some of them have also been found in meteoritic material. Microorganisms and plants often produce very uncommon amino acids, which can be found in peptidic antibiotics (for example nisin or alamethicin). Lanthionine is a sulfide bridged alanine dimer which is found together with unsaturated amino acids in lantibiotics (antibiotic peptides from microbial origin). 1-Aminocycloproane-1-carboxylic acid ACC is a small disubstituted cyclic amino acid and a key intermediate in the production of the herbal hormone ethylene.

In addition to amino acids for protein synthesis, there are other biologically important amino acids, such as the neurotransmitter GABA, carnitine (used in lipid transport within a cell), ornithine, citrulline, homocysteine, hydroxyproline, hydroxylysine, and sarcosine.

Some of the 20 amino acids in the genetic code are called essential amino acids, because they cannot be synthesized by the body from other compounds through chemical reactions, but instead must be taken in with food. In humans, the essential amino acids are lysine, leucine, isoleucine, methionine, phenylalanine, threonine, tryptophan, valine, and (in children) histidine and arginine.

General structure of an amino acid

The general structure of proteinogenic alpha amino acids is:

   COOH
   |
 H-C-R
   |
   NH₂

Where "R" represents a side chain specific to each amino acid. Amino acids are usually classified by properties of the side chain into four groups: acidic, basic, hydrophilic (polar), and hydrophobic (nonpolar).

Except for glycine, where R = H, amino acids occur in two possible optical isomers, called D and L. L amino acids represent the vast majority of amino acids found in proteins. D amino acids are found in some proteins produced by exotic sea-dwelling organisms, such as cone snails. They are also abundant components of the cell walls of bacteria. Proteins are created by polymerization of amino acids by peptide bonds in a process called translation.

Peptide bond formation
1. Amino acid; 2, zwitterion structure; 3, two amino acids forming a peptide bond. (See also bond.)

Abbrev. Full Name Side chain type Mass Isoelectric point Remarks

A Ala Alanine hydrophobic 89.09 6.11

C Cys Cysteine hydrophilic 121.16 5.05 Two cysteines can form a disulfide bond. This enforces tertiary structure, and such proteins as insulin have disulfide bonds.

D Asp Aspartic acid acidic 133.10 2.85

E Glu Glutamic acid acidic 147.13 3.15

F Phe Phenylalanine hydrophobic 165.19 5.49

G Gly Glycine hydrophilic 75.07 6.06 Because of the two hydrogen atoms at the α carbon, glycine is not optically active.

H His Histidine basic 155.16 7.60

I Ile Isoleucine hydrophobic 131.17 6.05

K Lys Lysine basic 146.19 9.60

L Leu Leucine hydrophobic 131.17 6.01

M Met Methionine hydrophobic 149.21 5.74 Always the first amino acid to be incorporated into a protein; sometimes removed after translation.

N Asn Asparagine hydrophilic 132.12 5.41

P Pro Proline hydrophobic 115.13 6.30 Can disrupt protein folding structures like α helix or β sheet.

Q Gln Glutamine hydrophilic 146.15 5.65

R Arg Arginine basic 174.20 10.76

S Ser Serine hydrophilic 105.09 5.68

T Thr Threonine hydrophilic 119.12 5.60

V Val Valine hydrophobic 117.15 6.00

W Trp Tryptophan hydrophobic 204.23 5.89

Y Tyr Tyrosine hydrophilic 181.19 5.64

Amino
Acid hydrophobic positive negative polar charged small tiny aromatic aliphatic van der Waals volume

Ala X - - - - X X - - 67

Cys X - - - - X - - - 86

Asp - - X X X X - - - 91

Glu - - X X X - - - - 109

Phe X - - - - - - X - 135

Gly X - - - - X X - - 48

His X X - X X - - X - 118

Lys X X - X X - - - - 135

Ile X - - - - - - - X 124

Leu X - - - - - - - X 124

Met X - - - - - - - - 124

Asn - - - X - X - - - 96

Pro - - - - - X - - - 90

Gln - - - X - - - - - 114

Arg - X - X X - - - - 148

Ser - - - X - X X - - 73

Thr X - - X - X - - - 93

Val X - - - - X - - X 105

Trp X - - X - - - X - 163

Tyr X - - X - - - X - 141

Overview

General structure of an amino acid

List of amino acids

Structure

Chemical properties