Amino Acids

Amino acids are the building blocks (monomers) of proteins. 20 different amino acids are used to synthesize proteins. The shape and other properties of each protein is dictated by the precise sequence of amino acids in it.

Each amino acid consists of an alpha carbon atom to which is attached

  • a hydrogen atom
  • an amino group (hence "amino" acid)
  • a carboxyl group (-COOH). This gives up a proton and is thus an acid (hence amino "acid")
  • one of 20 different "R" groups. It is the structure of the R group that determines which of the 20 it is and its special properties. The amino acid shown here is Alanine.

The Amino Acids
(For each amino acid, both the three-letter and single-letter codes are given. Click the name to see the structural formula)
AlanineAlaA hydrophobic
ArginineArgR free amino group makes it basic and hydrophilic
AsparagineAsnN carbohydrate can be covalently linked ("N-linked) to its -NH
Aspartic acidAspD free carboxyl group makes it acidic and hydrophilic
CysteineCysC oxidation of their sulfhydryl (-SH) groups link 2 Cys (S-S)
Glutamic acidGluE free carboxyl group makes it acidic and hydrophilic
GlutamineGlnQ moderately hydrophilic
GlycineGlyG so small it is amphiphilic (can exist in any surroundings)
HistidineHisH basic and hydrophilic
IsoleucineIleI hydrophobic
LeucineLeuL hydrophobic
LysineLysK strongly basic and hydrophilic
MethionineMetM hydrophobic
PhenylalaninePheF very hydrophobic
ProlineProP causes kinks in the chain
SerineSerS carbohydrate can be covalently linked ("O-linked") to its -OH
ThreonineThrT carbohydrate can be covalently linked ("O-linked") to its -OH
TryptophanTrpW scarce in most plant proteins
TyrosineTyrY a phosphate or sulfate group can be covalently attached to its -OH
ValineValV hydrophobic

Humans must include adequate amounts of 9 amino acids in their diet. These "essential" amino acids cannot be synthesized from other precursors. However, cysteine can partially meet the need for methionine (they both contain sulfur), and tyrosine can partially substitute for phenylalanine.

The Essential Amino Acids
Methionine (and/or cysteine)
Phenylalanine (and/or tyrosine)

Two of the essential amino acids, lysine and tryptophan, are poorly represented in most plant proteins. Thus strict vegetarians should ensure that their diet contains sufficient amounts of these two amino acids.

19 of the 20 amino acids listed above can exist in two forms in three dimensions. Link to a discussion.

21 December 2010