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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.
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.
There are twenty amino acids commonly found in nature:
Abbrev. Full Name Side chain
A Ala Alanine -CH3
C Cys Cysteine -CH2SH
D Asp Aspartic acid -CH2C(O)OH
E Glu Glutamic Acid -CH2CH2C(O)OH
F Phe Phenylalanine -CH2C6H5
G Gly Glycine -H
H His Histidine -CH2-C=CH-NH-CH=N \_________/
I Ile Isoleucine -CH(CH3)-CH2-CH3
K Lys Lysine -(CH2)4-NH2
L Leu Leucine -CH2-CH(CH3)2
M Met Methionine -CH2CH2SCH3
N Asn Asparagine -CH2C(O)NH2
P Pro Proline -NH-(CH2)3-CH \_______/
Q Gln Glutamine -CH2CH2C(O)NH2
R Arg Arginine -(CH2)3-NH-C(NH2)=NH
S Ser Serine -CH2OH
T Thr Threonine -CH(OH)-CH3
V Val Valine -CH(CH3)-CH3
W Trp Tryptophan -CH2-C=CH-NH-Phenyl \______/
Y Tyr Tyrosine -C6H4OH
Note that in the case of Proline the side chain links to the amino acid's amino group nitrogen atom as well as the central carbon.