Essential Information & explanations, latest texts & monographs on
Amino_acid.
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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: those amino acids in which the amino and carboxylate functionalities are attached to the same carbon.
Table of contents showTocToggle("show","hide")
1 Overview
2 Uses of amino acids
3 General structure of an amino acid
3.1 Isomerism
4 Reactions
5 List of amino acids
5.1 Structures
5.2 Chemical properties
Overview
Amino acids are biochemical building blocks. They form long chemical chains called proteins (see below), and shorter chains called polypeptides or peptides.
Twenty amino acids are encoded by the standard genetic code and are called proteinogenic. Rarer, more complicated ones are "made to order" by the body. 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.
Uses of amino acids
Monosodium glutamate is a food additive to enhance flavor.
L-DOPA (dihydroxyphenylalanine) is a drug used to treat Parkinsonism.
5-HTP (5-hydroxytryptophan) has been used to treat neurological problems associated with PKU (phenylketonuria).
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).
Isomerism
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.
Reactions
Proteins are created by polymerization of amino acids by peptide bonds in a process called translation.
Peptide bond formation1. Amino acid; 2, zwitterion structure; 3, two amino acids forming a peptide bond. (See also bond.)
List of amino acids
Structures
Following is a chart displaying the structures and symbols of the 20 amino acids represented in the genetic code.
Chemical properties
Following is a table listing the one letter symbols, the three letter symbols, and the chemical properties of the side chains of the amino acids. The one letter symbol for an undetermined amino acid is X. The three letter symbol asx means the amino acid is either asparagine or aspartic acid.
Abbrev. Full Name Side chain type Mass pI pK1(&alpha -COOH) pK2(&alpha -+NH3) pKr (R) Remarks
A
Ala
Alanine
hydrophobic
89.09
6.11
2.35
9.87
C
Cys
Cysteine
hydrophilic
121.16
5.05
1.92
10.70
8.37
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
1.99
9.90
3.90
E
Glu
Glutamic acid
acidic
147.13
3.15
2.10
9.47
4.07
F
Phe
Phenylalanine
hydrophobic
165.19
5.49
2.20
9.31
G
Gly
Glycine
hydrophilic
75.07
6.06
2.35
9.78
Because of the two hydrogen atoms at the α carbon, glycine is not optically active.
H
His
Histidine
basic
155.16
7.60
1.80
9.33
6.04
I
Ile
Isoleucine
hydrophobic
131.17
6.05
2.32
9.76
K
Lys
Lysine
basic
146.19
9.60
2.16
9.06
10.54
L
Leu
Leucine
hydrophobic
131.17
6.01
2.33
9.74
M
Met
Methionine
hydrophobic
149.21
5.74
2.13
9.28
Always the first amino acid to be incorporated into a protein; sometimes removed after translation.
N
Asn
Asparagine
hydrophilic
132.12
5.41
2.14
8.72
P
Pro
Proline
hydrophobic
115.13
6.30
1.95
10.64
Can disrupt protein folding structures like α helix or β sheet.
Q
Gln
Glutamine
hydrophilic
146.15
5.65
2.17
9.13
R
Arg
Arginine
basic
174.20
10.76
1.82
8.99
12.48
S
Ser
Serine
hydrophilic
105.09
5.68
2.19
9.21
T
Thr
Threonine
hydrophilic
119.12
5.60
2.09
9.10
V
Val
Valine
hydrophobic
117.15
6.00
2.39
9.74
W
Trp
Tryptophan
hydrophobic
204.23
5.89
2.46
9.41
Y
Tyr
Tyrosine
hydrophilic
181.19
5.64
2.20
9.21
10.46
AminoAcidhydrophobic
positivenegative
polarcharged
smalltiny
aromatic aliphaticvan 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 -
- - -
- - -
X124
Leu X -
- - -
- - -
X124
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 - -
X105
Trp
X
-
-
X
-
-
-
X
-163
Tyr
X
-
-
X
-
-
-
X
-141
The above article is adapted from from Wikipedia All Wikipedia article text is available under the terms of the GNU Free Documentation License
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