Amino Acids

Amino acids are organic compounds containing amino (-NH2) and carboxyl (-COOH) functional groups along with a side chain (R group) specific to each amino acid. These are amphoteric electrolytes (also called as ampholytes) and can act as both acid (proton donor) and base (proton acceptor).

All amino acids have five basic parts:

  • a central carbon atom (α- carbon)
  • a hydrogen atom
  • an amino group – consisting of a nitrogen atom and two hydrogen atoms
  • a carboxyl group – consisting of a carbon atom, two oxygen atoms, and one hydrogen atom
  • an R-group or side chain – consisting of varying atoms


General structure of Amino Acids


R-group (side chain) makes each amino acid unique. Due to variation in R group more than 300 different amino acids have been described in nature but only 20 are commonly found as constituents of mammalian proteins (amino acids that are coded for by DNA in the cell), each of which has different side chain structure. Side chains contain mainly hydrogen, carbon, and oxygen atoms. Some amino acids have sulfur or nitrogen atoms in their R-groups. Different amino acids with their side chain and abbreviations are tabulated in table below.


Amino AcidAbbreviationR-group Linear Formula
1.AlanineAla (A)CH3
2.ArginineArg (R)HN=C(NH2)-NH-(CH2)3
3.AsparagineAsn N)H2N-CO-CH2
4.Aspartic acidAsp D)HOOC-CH2
5.CysteineCys (C)HS-CH2
6.Glutamic acidGlu E)HOOC-(CH2)2
7.GlutamineGln Q)H2N-CO-(CH2)2
8.GlycineGly (G)H
9.HistidineHis (H)NH-CH=N-CH=C-CH2
11.LeucineLeu (L)(CH3)2-CH-CH2
12.LysineLys (K)H2N-(CH2)4
13.MethionineMet (M)CH3-S-(CH2)2
14.PhenylalaninePhe (F)C6H5-CH2
15.ProlinePro (P)(CH2)3
16.SerineSer (S)HO-CH2
17.ThreonineThr (T)CH3-CH(OH)
18.TryptophanTrp (W)C6H4-NH-CH=C-CH2
19.TyrosineTyr (Y)HO-C6H4-CH2
20.ValineVal (V)(CH3)2-CH


R groups vary in structure, size, and electric charge etc. which influence their solubility in water.


They are joined together by condensation reaction between amino group of one acid and carboxyl group of another. Bond formed between two amino acids is known as peptide bond. Amino acids combine through peptide bond formation to form proteins, building blocks of life. Proteins are polymers of amino acids, with each amino acid residue joined to its neighbor by a specific type of covalent bond (peptide bond).

When carbon atom is attached to four different groups, it is asymmetric and therefore exhibits optical isomerism. Amino acids (except glycine) possess four distinct groups (R, H, -COOH, -NH2) held by α-carbon. Thus all of them  (except glycine where R = H) have optical isomers (L- and D- isomers). Proteins are composed of L-amino acids.


Isomers of amino acids(Alanine)


They possess two functional groups (-COOH) and (-NH2).  COOH is weak acid and can lose proton to form negative ion (-COO) while NH2 can gain a proton to form positive ion (NH3+). Due to this amino acids can have negative or positive charge. However, they mostly occur in zwitterion form (Has one positive and one negative charge), which is electrically neutral.

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