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Protein is one of the most important bio molecules composed of a number of monomer units called as amino acids which play an important roles as an intermediates in metabolism. There are total twenty amino acids found within proteins having a vastarray of chemical versatility. The content and sequence of precise amino acid in a specific protein is determined by the sequence of the bases in the gene which encodes that particular protein.

Proteins control almost all cellular processes and the biological activity of protein is determined by the chemical properties of amino acids. On the basis of need of amino acids in living system, they can be classified in two types.

  • Essential amino acids
  • Non-essential amino acids
Non-essential amino acids can produce in human body while essential amino acids must be supplied in the food. The deficiency of any one of the essential amino acid results the degradation of proteins and effects metabolism processes. Since amino acids cannot be stores in body like fat and starch, therefore must be supplied in every day food. Plants can synthesize all the amino acids, human body does not have all the enzymes to bio synthesized all the amino acids, therefore must be supplied from diet.


Isoleucine Structure

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Isoleucine amino acid has to provide by food, therefore acts as an essential amino acid. Like leucine and valine, Isoleucine is also a branched amino acid with an aliphatic side chain which is non-polar and hydrophobic in nature. The side chain of Isoleucine along is chiral in nature and forms four steroisomers of isoleucine with two diastereomers. It is coded by AUU, AUA and AUC codons.

Since the side chain of acid is not so reactive, hence cannot bonded covalently on enzyme active centers but important for ligand binding to proteins. The common name of Isoleucine is α-Amino-β-methylvaleric acid and IUPAC name is 2-Amino-3-methylpentanoic acid. The molecular formula of Isoleucine is C6H13NO2 with molar mass is 131.17 g/mol.

Isoleucine Structure

Six forms of Isoleucine are listed in given table with their common name.

Common names
(2R,3R) - Isoleucine (2S,3S) - Isoleucine ($R^{\ast}, R^{\ast}$) - Isoleucine alloisoleucine

The isoelectric point of Isoleucine is 6.05 with pK1=2.4 and pK2=9.7.

Isoelectric Point of Isoleucine

Isoleucine was first isolated from fibrin in 1904 and found in most common proteins. The most common sources of Isoleucine are Soy products, meats and fish, dairy Products eggs, legumes and nuts. Some other sources are as follows.

Food source Amount in per 10gm of substance
Egg white, dried 4.581g
Soya protein
Egg whole, dried 2.583g
Soy flour, defatted
Fish, Whitefish
Chorizo, pork and beaf
Cheese, parmesan 2.202g
Soy meal, defatted 2.180g
Seeds, sesame flour 2.157g
Milk, buttermilk 2.075g
Soybeans, mature seeds, raw 1.971g
Cheese, parmesan, grated
Egg mix 1.938g
Cheese, parmesan 1.894g
Peanut flour 1.836g
Egg yolk 1.734g
Cheese, romano 1.732g
Soy flour 1.685g
Lupins, mature seeds 1.675g
Cheese, gruyere 1.615g
Cheese, caraway 1.612g
Cheese, cheddar 1.563g
Spices, parsley, dried 1.546g
Cheese, swiss 1.546g
Winged beans, mature seeds, raw 1.537g
Snacks, pork skins, plain 1.468g
Game meat, goat, cooked, roasted 1.382g
Beaf, cured, breakfast strips, cooked 1.371g
Seeds, sesame flour, high fat 1.352g

Since its an essential amino acid, so cannot synthesized in human body, but in plants and some microorganism, it formed through multistep process where pyruvic acid and alpha-ketoglutarate act as precursor of process and process catalyzed by various enzymes like Acetolactate synthase , acetohydroxy acid isomeroreductase, Dihydroxyacid dehydratase, Valine aminotransferase.

Essential Amino Acid


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Except some of the amino acids like taurine, GABA and glycine, amino acids exist in two forms; D or L form which are mirror reverse images of each other. The L form is more common and found in living plants and animal tissues as it is more compatible to human biochemistry than the D form.

The L- form can rotate the plane polarized light to the left i.e. anti-clockwise as well as the amino group is on the L-eft side in its Fisher projection of the compound. The L-form of Isoleucine is as follows.


The L-form can also be shown by R, S system, in which molecule placed in such a way that alpha carbon atom placed in front and H in back side. Than observe the orientation of other groups from Carboxy to amino through alkyl group.

For example, in L-form of Isoleucine at 2nd and 3rd chiral carbon atoms of molecule the CORN circle will form through anti-clockwise movement, hence show S-form and also denotes as (2S,3S) Isoleucine, where 2 and 3 shows the number of carbon atoms in parent chain and S denotes the configuration.


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The D and L notation is given on the basis of glyceraldehyde structure which has three chiral carbon atoms. The enantiomers of amino acid which can be synthesized from the D-form of glyceraldehyde are labelled D, while those from the L-form are labelled as L. The D and L notation can be determined on the basis of alpha carbon atom of molecule. Place alpha carbon atom in such a way that the hydrogen atom is directly behind it. This should place the three other groups of amino acids in a circle, than follow from COOH to R to NH2, or CORN. If this sequence is in an anti-clockwise direction, the amino acid is in the L-isomer and clockwise direction show a D-isomer.

The R, S system is also used for assign the D and L form. The R-isomer is L-form and S-isomer is D-form of amino acid and assigned by using same method. For example, in D-form of Isoleucine at 2nd and 3rd chiral carbon atoms of molecule the CORN circle will form through clockwise movement, hence show R-form and also denotes as (2R,3R) Isoleucine, where 2 and 3 shows the number of carbon atoms in parent chain and R denotes the configuration.
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