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Cysteine

Proteins are one of the most important bio molecules for living organisms. They mainly act as structural unit for different organelles and effectively involve in metabolism.

Proteins formed by condensation polymerization of various amino acids which bonded through peptide linkage. Therefore amino acids are building blocks of proteins. Amino acids contain two functional groups, amino group and carboxyl groups.

  1. Only α-amino acids are involved in protein synthesis in which both functional groups of molecule bonded on same carbon atom.
  2. There are twenty amino acids involve in protein synthesis.
  3. The precise amino acid sequence in proteins is determined by the sequence of the bases in the gene that encodes that protein.
  4. The chemical properties and sequence of the amino acids of proteins determine the biological activity of the protein.
  5. Proteins have ability to catalyze most of the biochemical reactions in living bodies and control virtually all cellular process.
  6. Hence the amino acid structure and properties and their sequence is to be able to understand protein structure and properties.
Twenty amino acids can be classified in two types.

Essential amino acids cannot be synthesized in human bodies and must be supplied in the food.
There are 10 essential amino acids; tryptophan, arginine, leucine, phenylalanine, lysine, methionine, threonine, valine, histidine and isoleucine. However rest of ten amino acids are non-essential amino acids as they can be synthesized within the human body.

Ten non- essential amino acids are as follow; Asparagine, cysteine, glutamine, aspartic acid, glycine, proline, tyrosine, serine, glutamic acid and alanine.

 

Amino Acid Cysteine

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There are two sulfur-containing amino acids, Cysteine and methionine.
  1. Cysteine is a non-essential amino acid while methionine is an essential.
  2. Both non-polar and hydrophobic amino acid as zwitterion is neutral due to –COO- and –NH3+ ion in molecules.
  3. Cysteine exists as white crystalline powder with molecular mass 121.61 g/mol.
  4. It is quite soluble in water and sparingly soluble in alcohol (1.5g/100g ethanol).
  5. There are two codons for Cysteine amino acid; UGC and UGU.

Because of hydrophobic nature of Cysteine, it generally found on the interior of proteins. Some times Cysteine can be ionized to form the thiolate anion but very less found on surface of proteins.

Amino Acid Cysteine

Other reasons for the occurrence of Cysteine inside the proteins are.
  1. Sulfur has a low tendency to form hydrogen bond.
  2. The thiol group (-SH) of Cysteine can react with other thiol groups in an oxidation reaction and form a disulfide bond.
The presence of thiol group make the molecule to susceptible for various enzymatic reactions in which thiol group act as nucleophile and involve in disulphide bond.

Disulfide Bridge Reaction

Compare to serine which has a hydroxyl group (-OH) instead of thiol group (-SH) in Cysteine show quite different physical and chemical properties. As the oxygen atom of hydroxyl group can involve in hydrogen bonding and make the molecule less acidic. Due to the same reason the thiol group of Cysteine is much more reactive compare to hydroxyl group of serine.

Cysteine and Serine
  • The white color six sided crystals of cysteine can be found in animal and human urine.
  • The presence of cysteine crystals in animal urine indicates the reabsorption defect of amino acid.
  • The presence of Cysteine in human urine indicates a rare genetic disease cystinosis.
Although Cysteine is a non-essential amino acid, but it can be essential during some malabsorption syndromes. There are many dietary and industrial sources for this amino acid.
Various sources of Cysteine
Animal sources Plant sources Industrial sources
1 Pork and sausage meat Red papper By hydrolysis of human hair
2 Chicken, Turkey, duck and luncheon meat Garlic and sprouted lentils By fermentation of a mutant of Escherichia coli
3 Eggs and milk Onions and broccoli By hydrolysis of racemic 2-amino-thiazoline-4-carboxylic acid with Pseudomonas thiazolinophilum
4 Whey proteins Brussels sprout
5 Ricotta, cottage Oats
6 Cheese and yoghurt Granola and wheat germ


Because of the presence of thiol group, the chemical reactivity of Cysteine is quite unique compare to other alpha- amino acids. It plays a key role in stabilizing extracellular proteins as it can undergoes oxidation-reduction reaction due to thiol group and form a dimer through a disulfide bond by the removal of two hydrogen atoms.

The presence of disulphide bond in dimer is important in the identification of tertiary structure of proteins. The reduction of dimer involves the cleavage of disulfide bond and again forms two separate Cysteine molecules by the addition of hydrogen. Since living cells have strong reducing environment inside the cell, therefore Cysteine exists in Free State within cell but in the form of dimer in extracellular environment.

Some important biological functions of Cysteine are as follows.
  • Because of the redox properties of thiol group, Cysteine acts as good antioxidant.
  • The presence of disulphide bond provides stability and folding to protein structure in extracellular medium.
  • Due to the presence of sulfur in molecule, it acts as a precursor of iron-sulfur clusters in iron-sulfur proteins. It also involves in bonding with other metal ions like zinc in zinc fingers and alcohol dehydrogenase, nickel in the [NiFe]-hydrogenases, iron in cytochrome P-450 and copper in the blue copper proteins.
  • Another key role of Cysteine amino acid is in post translational modifications which is due to the nucleophilic nature of thiol group, allows it to bond with other groups.

Cysteine Structure

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In general, an amino acid is composed of four different component bonded on same carbon atom one amino group, one carboxyl group, one alkyl group and one hydrogen. This type of amino acid is called as α-amino acids. In case of Cysteine, that alkyl group is one methyl group substituted with thiol group (-CH2SH).
Cysteine Structure
  • All amino acid exist in the form of zwitterion in which two opposite charges are present in the same molecule.
  • As there is one acidic carboxyl group which can donate proton and convert in carboxylate ion (-COO-).
  • Similarly the basic amino group accept that proton to form ammonium ion (-NH3+).
  • The overall molecule is neutral and non-polar in nature.
  • During electrolysis, the movement of amino acid molecule depends upon the pH of medium.
  • The pH at which there is no net movement of molecules in solution, hence no current will passed through the system is called as isoelectric point of respective amino acid.
  • The isoelectric point of Cysteine is 5.02.
Amino Acid Cysteine

The molecular formula for Cysteine is C3H7NO2S with symbol ‘cys’ and abbreviated as ‘c’. There are three functional in amino acid; carboxyl group, thiol group and amino group with pKa value, 1.71, 8.33 (thiol), 10.78 respectively.
Because of the presence of chiral carbon atom in molecule, Cysteine is also optically active and can exist in dextro-rotatory and levo-rotatory form. Generally amino acids in levo-rotatory forms are more common and found naturally.

Cysteine Hydrochloride

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Although Cysteine is a non-essential amino acid as it can easily synthesize by methionine through the trans-sulphuration pathway. But generally infants lack the enzyme, cystathionase which is necessary for this conversion. Hence Cysteine the need of Cysteine get covered by injection its hydrochloride salt that is Cysteine Hydrochloride.

Cysteine Hydrochloride exist in mono hydrate form and is chemically designated as C3H7NO2S• HCl• H2O with formula weight 175.63. It is a white crystalline hygroscopic tasteless powder which is soluble in water (650 gm /l) and sparingly soluble in alcohols. Cysteine hydrochloride is quite stable at room temperature with melting point 446K temperature.
Cysteine Hydrochloride

It is one of the most important derivatives of Cysteine amino acid which is widely used in various industries like chemical, food and medical industries. Cysteine Hydrochloride mono hydrate is also used as antiseptics, biochemical reagent, antioxidant and foodstuff additive etc.

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