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# Serine

The chains of amino acids form protein molecule. Including two functional groups(amino and carboxyl group), amino acids also have one side chain which composed of different functional group and the nature of side chains influenced the topography of the protein.

1. The various bonds between amino acid side chains forms different levels of complex protein structure; primary, secondary, tertiary, and quaternary.
2. The sequence of amino acids in proteins is known as primary structure of a protein.
3. Amino acids are bonded with peptide bonds between both functional groups of amino acids.
4. The regular arrangement of amino acid chains due to the formation of hydrogen bonds between side chains of amino acids is called as secondary structure of proteins.

There are three possible secondary structures of proteins, α-helix, the β-pleated sheet, and the triple helix. Further bonding in secondary structure of proteins between side chains either within the protein or around the protein generates the tertiary structure of proteins. The orientation of proteins depends upon the polarity of molecules.

The polar amino acids move to the outside while the non-polar amino acids move to the inside in a polar solution. Other than peptide bonds di-sulfide bonds, ionic bonds, hydrogen bonds can be formed in tertiary structure of proteins. The bonding between two or more polypeptide chain results the formation of quaternary structure.

## Serine Definition

1. Serine is a proteinogenic, non-essential amino acid which becomes essential in certain cases. It is abbreviated as Ser or S with the formula HO2CCH(NH2)CH2OH.
2. Serine exists in the form of white crystal or powder and molecular mass is 105.09 gmolâˆ’1. There are six codons for serine; AGU, AGC, UCC, UCU, UCA and UCG.
3. Serine is a non-aromatic polar uncharged amino acid with hydroxyl group. The melting point of serine is 519 K and density 1.603 g/cm3 at 295 K. This amino acid was first isolated from silk protein in 1865 and named serine after silk.
4. Since it is a non-essential amino acid, it can be easily bio synthesized in body by various biochemical reactions.
5. L - serine is synthesized by using a glycolytic intermediate, 3-phosphoglycerate (3-PG) in an enzymatic reaction which catalyzed by three enzymes, phosphoserine phosphatase (PSP), 3-phosphoglycerate dehydrogenase (3-PGDH) and phosphoserine aminotransferase (PSAT).

Biosynthesis of serine starts from the oxidation of 3-phosphoglycerate and forms 3-phosphohydroxy pyruvate and NADH. A transamination reaction forms 3-phosphoserine which forms serine by removal of the phosphate group. The removal of methyl group generated glycine in the presence of serine hydroxymethyl transferase enzyme.

Parallel method of synthesis involves a reversible reaction from glycine by degradation of protein and phospholipids or by dietary intake. Industrially, L-serine can be produced by fermentation in the laboratory. Methyl acrylate is the precursor for this fermentation and produce racemic serine.

L-Serine plays an important role in cell growth and development. The conversion of L-serine to glycine by serine hydroxy methyl transferase results in the formation necessary components like pyrimidine nucleotide, deoxythymidine mono phosphate and purine bases, adenine and guanine which involve in formation of DNA, RNA, ATP and GTP. It also acts a precursor for the neurotransmitters like D-serine, Taurine and glycine.

L-Serine plays a central role in the production of myelin which acts as lipid messenger molecules and phosphatidylserine, an important messenger for apoptosis. Some common food sources of serine amino acid are as follow.

## Serine Structure

Like other amino acids, serine also contains one amino group (-NH2 group), one carboxylic group (-COOH) with one side chain which consist of â€“CH2OH group with the IUPAC name 2-amino-3-hydroxypropanoic acid.
• Because of the presence of polar hydroxyl group, overall serine molecule is polar and uncharged.
• Because of the presence of hydroxyl group, serine has the ability to form hydrogen bond and hydrophilic in nature.
• It is quite similar to Alanine only differ as one of the methylenic hydrogens is replaced by a hydroxyl group.
• Different orientations of amino group and carboxyl group in amino acid molecule, it can be exist in D and L configurations.

Amino acids can exist in the form of dipolar ion also known as zwitterion. Each zwitterion has two opposite charges on same molecule. As amino group is a basic group so accept proton in acidic medium to form ammonium ion (-NH4+).

Similarly carboxyl group is an acidic group, hence lose proton to form carboxylate ion (-COO-). In acidic medium, amino acid exist as positive charged ion and move towards cathode during electrolysis while in basic medium, it exists in the form of negative ion which move towards anode during electrolysis.

At a certain pH, these two ions exist in equilibrium and show no net movement of ions. That certain pH is called as isoelectric point at which the amino acid does not migrate in an electric field, hence at isoelectric point the amino acid is neutral and the zwitterion form is dominant.

If the pKa1 is taken for $\alpha$-carboxylic group and pKa2 for $\alpha$-ammonium ion of amino acid, the isoelectric point will be equals to 1/2 (pKa1 + pKa2).

For serine, the pKa1 = 2.21, pKa2 = 9.15,
hence the isoelectric point for serine is 5.68

which is almost equivalent to glycine and Alanine.
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