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Proteins are bio molecules, mainly involve in structural units and are used to repair muscles, bones, organs, glands, hair, and connective tissue. They are formed by the polymerization of α-amino acids which consists of a basic amino group (-NH2), an acidic carboxyl group (-COOH) with an organic R group as side chain. The condensation polymerization of two or more amino acids formed peptide which plays a vital role in formation of various proteins and also involves in metabolism.

Peptide chains combined in three dimensional patterns to form protein structure. The monomer units of proteins, amino acids are cell signaling molecules and also involve in regulation of gene expression and in the protein phosphorylation cascade as well as they also act as precursors for many hormones and low-molecular weight nitrogenous substances.
There are mainly twenty α-amino acids involve in protein formation which can be further classified in three types.

    1. Essential amino acids: These amino acids cannot be synthesized by the body, therefore must come from food.
    2. Nonessential amino acids: These amino acids produce in body, so not necessary to take in diet.
    3. Conditional amino acids: Some amino acids are not essential but become essential during the time of illness and stress.


    Methionine Oxidation

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    Methionine is sulfur containing non-polar, hydrophobic, essential amino acid which also abbreviated as ‘Met’ or ‘M’. The codon of methionine is AUG which is also known as the initiation codon as it indicates mRNA's coding region in translation process. It is a white crystalline powder with C5H11NO2S and molecular mass 149.21 g/mol. Methionine acts as precursor for other amino acids also like cystine and creatine. It helps to increase the level of antioxidant like glutathione and also involve in reduction of blood cholesterol levels. It helps to remove toxic wastes from the liver and also involves in the regeneration of liver and kidney tissue.

    Because of hydrophobic side chain in amino acid, it is usually found buried within proteins. The nucleophilicity of Methionine is less than cysteine which is also a sulfur containing amino acid. Still it will react with some electrophilic sites but not a participant in the covalent chemistry at active sites of enzymes. The sulfur atom of thiol ether of methionine is prone to oxidation which is essential for activity of proteins. The oxidation of methionine first formed methionine sulphoxide which can be reversed by standard thiol containing reducing agents. In second step, methionine sulphoxide converts into methionine sulphone through an irreversible process.

    Methionine Structure

    The formation of methionine sulphoxide is a reversible process. The forward reaction leads to the formation of both R- and S-stereoisomers of product. The backward reaction is catalyzed by methionine sulfoxide reductases and back to methionine residues.

    Methionine Structure

    Methionine sulphoxide affects many mechanisms like an increase in the rate of ROS generation, a decrease in the antioxidant capacity, a decrease in proteolytic activities which preferentially degrade oxidized proteins, a decrease in the ability to convert Methionine sulphoxide residues back to amino acid.

    The overall oxidation process can be shown as given below.

    Methionine Oxidation


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    The property of compound to rotate the plane polarized light is called as optical activity. A compound can be either rotate light clockwise or anti-clockwise the plane polarized light. A compound which can rotate the plane polarized light clockwise is known as dextrorotatory compound, designated as‘d’ or ‘+’. Similarly a compound which can rotate light counterclockwise is termed as levorotatory, represented by ‘l’ or –ve sign.
    The three dimensional structure of molecules can be determined by X-ray Crystallography. The absolute configuration of molecule is assigned with respect to Glyceraldehyde molecule and represented by ‘D’ and ‘L’ symbols.

    x-ray Crystallography of Glyceraldehyde

    The absolute configuration is not related to optically activity of molecule and ‘D’ and ‘L’ symbols are quite differ from ‘d’ and ‘l’ symbols. In case of amino acids; except glycine all amino acids contain chiral carbon atom, hence show optical isomerism. ‘d’ and ‘l’-amino acids are non-superimposable mirror of each other and known as Enantiomers.


    In case of methionine amino acid; there are dour different groups bonded with carbon atom that is; amino group (-NH2 group), carboxylic group (-COOH), side chain (-CH2-CH2-S-CH3) and hydrogen.

    In absolute configuration, if the –OH group of chiral carbon atom positions at left side, it assigned as ‘L-configuration’ and –OH group at right side assigned as D-configuration’. Similarly amino acids are also assigned by ‘D’ and ‘L’ configurations with respect to the position of amino group and side chain of molecule. For example, the L-configuration of methionine is as follows.

    L-Configuration of Methionine


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    The non-superimposable mirror image of ‘l-enantiomer’ is known as‘d-enantiomer’. Both Enantiomers show same chemical and physical properties but differ in optical properties. ‘d-methionine’ rotates the plane polarized light clockwise while l-methionine rotates the plane polarized light counter clock wise.
    Just like L-configuration, 'D-configuration’ of methionine is also show one of the three dimensional configuration.

    D-Configuration of Methionine

    Methionine Benefits

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    1. Methionine is an essential amino acid, therefore must be obtained through diet or supplements as the body cannot synthesize the compound on its own. The main food sources of Methionine are sesame seeds, brazil nuts, soy nuts, wheat germ, oats, peanuts, chickpeas, yellow corn, almonds, pinto beans and brown rice. There are many supplements available to overcome from deficiency of methionine.
    2. The methionine amino acid is involved in cleavage of fats and to prevent build up in arteries.
    3. It may also be used to regulate digestion and aid in the removal of heavy metals from the body. It also acts as a precursor for other amino acids like cycteine which further produces gluthione, involves in liver detoxification.
    4. Methionine amino acid is also a powerful antioxidant which combats free radicals in the body. Methionine also involves in management of depression, arthritis pain, chronic liver disease and memory problems.
    5. A common dietary supplement ingredient S-adenosyl methionine or SAMe, is used to improves the quality of mood, promoting liver health, in relieving symptoms of osteoarthritis and in relieving symptoms of fibromyalgia. It is a naturally occurring compound found in nearly every cell in the human body. The deficiency of Vitamin B12 and Vitamin B6 affects the levels of S-adenosyl methionine.
    6. S-adenosyl methionine is also effective in NSAIDS for reducing pain and improving functional limitation in patients with osteoarthritis without the adverse effects often associated with NSAID therapies. It plays a vital role in the liver as a protective agent for oxidative stress.
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