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
|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|
|Glycine||Gly||G||so small it is amphiphilic (can exist in any surroundings)|
|Histidine||His||H||basic and hydrophilic|
|Lysine||Lys||K||strongly basic and hydrophilic|
|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|
The Essential Amino Acids
Humans must include adequate amounts of 9 amino acids in their diet.
- Methionine (and/or cysteine)
- Phenylalanine (and/or tyrosine)
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.