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Biology LibreTexts

2.7: 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
Table 2.7.1: Types of Amino Acids. For each amino acid both three-letter and single letter codes are given
Alanine Ala A hydrophobic
Arginine Arg R free amino group makes it basic and hydrophilic
Asparagine Asn N carbohydrate can be covalently linked ("N-linked) to its -NH
Aspartic acid Asp D free carboxyl group makes it acidic and hydrophilic
Cysteine Cys C oxidation of their sulfhydryl (-SH) groups link 2 Cys (S-S)
Glutamic acid Glu E free carboxyl group makes it acidic and hydrophilic
Glutamine Gln Q moderately hydrophilic
Glycine Gly G so small it is amphiphilic (can exist in any surroundings)
Histidine His H basic and hydrophilic
Isoleucine Ile I hydrophobic
Leucine Leu L hydrophobic
Lysine Lys K strongly basic and hydrophilic
Methionine Met M hydrophobic
Phenylalanine Phe F very hydrophobic
Proline Pro P causes kinks in the chain
Serine Ser S carbohydrate can be covalently linked ("O-linked") to its -OH
Threonine Thr T carbohydrate can be covalently linked ("O-linked") to its -OH
Tryptophan Trp W scarce in most plant proteins
Tyrosine Tyr Y a phosphate or sulfate group can be covalently attached to its -OH
Valine Val V hydrophobic

The Essential Amino Acids

Humans must include adequate amounts of 9 amino acids in their diet.

  • Histidine
  • Isoleucine
  • Leucine
  • Lysine
  • Methionine (and/or cysteine)
  • Phenylalanine (and/or tyrosine)
  • Threonine
  • Tryptophan
  • Valine

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. 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.