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# Threonine

Amino acids are bonded through peptide linkage to form di- and tripeptides molecules. The pure and powdered free-form amino acids are absorbed from the small intestine into the bloodstream and are available to the tissues very quickly. So pure di- and tripeptides are efficient in their ability to be absorbed into the bloodstream while pure free-form amino acids are equal or superior for bodybuilders and other athletes and more important, are as close as your nearest health food store.

• Proteins are polypeptides made up of more than thousand amino acids bonded with peptide linkage.
• Proteins are structural components of living organism and balance the nitrogen level in body.
• Proteins hydrolyzed in living system to release amino acids which are promptly re-incorporated into fresh proteins.
Most of the protein is oxidized to provide energy and the excess of nitrogen is excreted in the form of urea and little amount of ammonia. In mammals, protein digestion is mainly completed in the small intestine at a slightly alkaline pH. The digestion of protein involves many enzymes like trypsin, chymotrypsin and elastase etc.

Amino acids are final products of hydrolysis of amino acids and can be two types, essential and non-essential amino acids. Nonessential amino acids are synthesized as intermediates of carbohydrate metabolism, for example, alanine is formed from pyruvate and aspartate from oxaloacetate and asparagine is formed from aspartate.
Similarly glutamate is formed from a-ketoglutarate and glutamine from glutamate and glutamate acts as precursor of proline and arginine. Cysteine is found from 3-phosphoglycerate and during this process, serine formed as an intermediate compound which further gives rise to glycine. ß-aspartate acts as precursor for some non-essential amino acids like Threonine, methionine and lysine. Similarly phosphoenol pyruvate and erythrose-4-phosphate are used to form Phenyl alanine, tyrosine and Tryptophan through the intermediate compound called chorismate.

## Threonine Structure

1. Threonine is an essential, Non-aromatic hydroxyl, Polar (uncharged) $\alpha$-amino acid, which is hydrophilic in nature.
2. It is abbreviated as Thr or T is an amino acid with the chemical formula HO2CCH(NH2)CH(OH)CH3 and molecular mass is 119.12 g $mol^{-1}$.
3. Since threonine is an essential amino acid, therefore must be supplied through food sources like nuts, seed, beans, lentils, dairy products and meat, fish and poultry.
Some common threonine rich food sources are as follows.

There are four codons for threonine, ACA, ACU, ACG and ACC. Just like serine, threonine is also a proteinogenic amino acids with an alcohol group. The side chain of threonine amino acid is chiral in nature like Isoleucine amino acid. There is a chance to get post translational modifications with threonine residue like the hydroxy present on side chain of molecule can undergo O-linked glycosylation and threonine residues can undergo phosphorylation in the presence of threonine kinase. The phosphorylated form of threonine is known as phosphothreonine.

The different orientations of all three functional groups on molecule; hydroxy group, carboxy group and amino group, it exhibits D and L-configuration.

There are two chiral carbon atoms in threonine molecule, first is alpha carbon atom bonded with amino group, carboxy group and a side chain, while second chiral carbon atom is located in side chain. Out of D and L- configuration, L-threonine is found in nature with 2S, 3R as the absolute configuration. The mirror image of this compound that is called as enantiomer is D-threonine with 2R, 3S configuration.
The diastereomer with 2S, 3S configuration is known as L-allo-threonine and D-allo-threonine (2R, 3R). Since the amino group and carboxy group present in molecule are of opposite nature, amino group is basic in nature with acidic carboxy group. Both of these functional groups are bonded on same carbon atom in $\alpha$-amino acid. Hence it exists in the form of dipolar ion which is also called as zwitterion. In acidic medium, it exists in the form of a positively charged ion which move towards cathode, while in basic medium, it converts in carboxylate ion (negative ion) which move towards anode.

At a certain pH of solution, there will be no net movement of ion is observed due to equilibrium concentration of both ions. This pH is known as isoelectric point. The magnitude of isoelectric point depends upon the pKa of both functional groups. For example, in threonine, the isoelectric point is equals to the average of the sum of both pKa's. The pKa1 for $\alpha$-carboxylate ion is 2.1 and pKa2 (for $\alpha$-amino group) is 9.6, hence the isoelectric point(pL) for threonine is around 5.85 which is very similar to serine and valine as they are differ only in methyl group and hydroxy group on side chain of molecule.

Like other amino acids, threonine also involves in many biochemical process which promotes the normal growth by maintaining the proper protein balance in the body. it also supports central nervous, cardiovascular, immune system function and liver. Some other functions are as follows.
1. It acts as precursor for two amino acids, glycine and serine which are necessary for the production of muscle tissue and connecting tissues like elastin, collagen etc.
2. Threonine helps to keep connective tissues and muscles strong and elastic and build strong bones and tooth enamel.
3. It can combine with aspartic acid and methione amino acids to help in lipotropic function of liver and digestion of fats and fatty acids.
4. Threonine involves in the production of antibodies and supports the immune system and may be helpful in treating depression.
5. Threonine amino acid is useful for treatment of Lou Gherigs Disease which increases glycine levels in the central nervous system.

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