Amino Acids

• Amino acids are the "building blocks" of proteins.

• There are 20 amino acids which are genetically encoded into proteins (see below).

• Other amino acids are obtained following the hydrolysis of certain proteins. However, these amino acids are derived from certain amino acids genetically encoded into proteins (e.g.: hydroxyproline from proline).

• Amino acids naturally occuring in proteins are of the "L" isomeric form.  Due to the chiral carbon atom (assymetric carbon; the carbon atom to which the carboxyl [-COOH] and amino [-NH₃] groups are attached), both "D" and "L" stereoisomeric forms are possible (due to its lack of a chiral carbon atom, the amino acid glycine does not have stereoisomers).  This "D" versus "L" nomenclature is based upon that which resembles the patterns of either D-glyceraldehyde or L-glyceraldehyde.  Chemical (non-biological) synthesis of amino acids results in equal amounts of both the D- and L- isomeric forms of the amino acids.  However, for biological activity (utilization in protein synthesis), only the L-forms are useful.

• Animals & humans can synthesize many of these amino acids (dispensable amino acids). However, certain amino acids CANNOT be synthesized in sufficient quantities to meet their physiological needs by animals & humans. Thus, some quantities of these amino acids MUST be supplied by the diet (dietary essential or indispensable amino acids).

Click here for a page of graphic images of the structures of all 20 of these amino acids.

The 20 Genetically Coded Amino Acids in Proteins

 Name           Abbr.   Linear structural formula
 ======================================================

Alanine         ala    H3-CH(NH2)-COOH


Arginine **     arg    HN=C(NH2)-NH-(CH2)3-CH(NH2)-COOH


Asparagine      asn    H2N-CO-CH2-CH(NH2)-COOH


Aspartic acid   asp    HOOC-CH2-CH(NH2)-COOH


Cysteine        cys    HS-CH2-CH(NH2)-COOH


Glutamine       gln    H2N-CO-(CH2)2-CH(NH2)-COOH


Glutamic acid   glu    HOOC-(CH2)2-CH(NH2)-COOH


Glycine         gly    NH2-CH2-COOH


Histidine **    his    NH-CH=N-CH=C-CH2-CH(NH2)-COOH
                       |__________|


Isoleucine *    ile    CH3-CH2-CH(CH3)-CH(NH2)-COOH


Leucine *       leu    (CH3)2-CH-CH2-CH(NH2)-COOH


Lysine *        lys    H2N-(CH2)4-CH(NH2)-COOH


Methionine *    met    CH3-S-(CH2)2-CH(NH2)-COOH


Phenylalanine * phe    Ph-CH2-CH(NH2)-COOH


Proline         pro    NH-(CH2)3-CH-COOH
                       |_________|


Serine          ser    HO-CH2-CH(NH2)-COOH


Threonine *     thr    CH3-CH(OH)-CH(NH2)-COOH


Tryptophan *    trp    Ph-NH-CH=C-CH2-CH(NH2)-COOH
                        |_______|


Tyrosine        tyr    HO-p-Ph-CH2-CH(NH2)-COOH


Valine *        val    (CH3)2-CH-CH(NH2)-COOH

Ph in the linear structural formulas given above represents a phenyl ring.

* Indispensable (dietary essential) amino acids for animals & humans.

** Conditionally indispensable (dietary essential) amino acids for animals & humans (dependent upon species and physiological state [e.g.: growth, maintenance, reproduction]).

This page updated by G. R. Frank.