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Amino acid

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, that is, those amino acids in which the amino and carboxylate functionalities are attached to the same carbon. Some molecules like proline don't contain an amino group and chemically are not amino acids (technically proline is an imino acid), but are also classified as such because of functional similarity to real amino acids in living cells.

There are 20 amino acide which are directly expressed in the genetic code. If a protein cotains a different amino acide it was modified after building the protein. These modifications are often esential for the function of the protein. Over 500 amino acides have been found in nature.

The generalized structure of alpha amino acids is:

    COOH
    |
  H-C-R
    |
    NH₂

Where "R" represents a side chain specific to each particular 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. Proteins are created by polymerization of amino acids by peptide bonds in a process called translation:

1. Amino acid; 2, zwitterion structure; 3, two amino acids forming a peptide bond.

Twenty amino acids are represented in the genetic code:

The chemical properties of the side chains are:

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.

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 isn't 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

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

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