Cysteine: Significance, Functions and Applications

Cysteine is sulphur containing semi-essential amino acid which is significant for making protein, and for performing other metabolic functions. Its chemical formula is HO₂CCH(NH₂)CH₂SH. Cysteine is found in beta-keratin. It is the main protein in our nails, hair, and skin. Cysteine can be defined as a non-essential amino acid. 

Keyterms: Sulphur, Amino acid, Keratin, Metabolic functions, Protein, Collagen, Multivitamins, Food supplements

Also Read: Amino Acids

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Significance of Cysteine

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• It is crucial in the creation of collagen. 
• It affects skin elasticity and texture. 
• It has antioxidant properties.

Amino acids are available as single AAs or in AA combinations; they also come as part of proteins, multivitamins, and food supplements. The forms of amino acids are inclusive of powders, tablets, and fluids. 

Cysteine Structure

Foods with high protein content constitute cysteine. Some major sources of cysteine are poultry products, dairy products like cottage cheese, and meat like turkey, pork, duck, chicken, sausage, and lunch meats. Some plant sources of amino acids are onions, granola, lentils, red peppers, etc. Cysteine is ingested as a supplement in the form of N-acetyl-L-cysteine (NAC). From NAC and then it is converted into glutathione that acts as a potent antioxidant, the amino acid cysteine is converted. 

The antioxidants fight the free radicals present in our bodies. The DNA, cell wall, and cell membrane are injured by the radicals’ injurious compounds. Many problems have been associated, with radicals, such as cancer, breaking down of the mucus in our body, etc. cysteine, the L-enantiomer, significantly, is a precursor in personal care, food and pharmaceutical industries.

Cysteine is a residue in high protein foods. It may be important for preterm infants, elderly people, and people with certain metabolic conditions like mal-absorption syndromes, even though it is termed as a non or semi-essential amino acid. For its synthesis by the human body, a sufficient amount of methionine and the correct physiological conditions are required. 

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Biological functions

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The cysteine sulfhydryl group is nucleophilic and easily oxidized. The reactivity is enhanced when the thiol is ionized, and cysteine residues in proteins have pK, values close to neutrality, so are often in their reactive thiolate form in the cell. The sulfhydryl group of cysteine has numerous biological functions because of its high reactivity like:

• Precursor to antioxidant glutathione: Cysteine and cysteinyl residues have antioxidant properties due to the ability of thiols to undergo redox reactions. These properties are generally expressed in the tripeptide glutathione that takes place in humans and other living organisms. Glutathione should be biosynthesized from its constituent amino acids, cysteine, glycine, and glutamic acid due to its unavailability as a oral glutathione or GSH.

• As a precursor to iron-sulfur clusters: Cysteine acts as a crucial source of sulfide in our body for metabolism. From cysteine the sulfide in iron-sulfur clusters and in nitrogenase is extracted, which is then converted to alanine in the process.

• It has roles to play in the protein structure: cysteine is coded for by the UGU and UGC codons in the translation of messenger RNA molecules. cysteine side chain has been shown to stabilize hydrophobic interactions in micelles to a greater degree than the side chain in the nonpolar amino acid glycine and the polar amino acid serine. Free cysteine residues are bonded covalently to other cysteine residues to produce disulphide bonds that play a crucial role in stability and folding of some proteins, usually those that are secreted to the extracellular medium.

Biological functions

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Sources of Cysteine

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Cysteine can be endogenously synthesized from homocysteine issued from the transmethylation of methionine. It is present mainly in the liver. In our body, cysteine is produced from methionine, an essential amino acid. 

Some sources of foods that contain amino acids are:

Duck, granola, lunch meat, turkey, cottage cheese, pork, chicken, beef liver, yogurt, ricotta, oat flakes, sausage meat, wheat germ, somen noodles, etc.

Read More: NCERT Solutions for Class 11 Biology Biomolecules

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Applications of Cysteine

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Cysteine has the following uses:

• It is a precursor in the pharmaceutical, food, and personal-care industries in the form of L-enantiomer. 
• One of the largest applications of cysteine is the production of flavours. In the Maillard reaction, cysteine reacts with sugars to yield meaty flavours.
• It is used as a processing aid for baking is being used with L-Cysteine.
• Cysteine is one of the very few amino acids that contain sulfur. Cysteine maintains the structure of proteins in our body. 
• It is a component of the antioxidant glutathione.
• Another amino acid cysteine is being used to produce taurine
• Cigarettes also contain cysteine.
• It is used to treat schizophrenia.
• Cysteine acts as an antidote for liver damage and hangovers as it reduces the toxic effects of alcohol. It counteracts the dangerous and toxic effects of acetaldehyde. 
• It supports the transformation of acetaldehyde into acetic acid in the metabolism process. 
• Cysteine is also being used by the Asian countries for personal care applications predominantly for [permanent-wave applications. It is useful in breaking up the disulphide bonds in hair’s keratin.
• N-Acetylcysteine, a derivative of cysteine, is used as a dietary supplement and as an antidote to acetaminophen overdose. 
• It is needed by sheep for the production of wool.
• It helps in controlling blood sugar levels in the body and thus is used by patients with type 2 diabetes. 
• Asthma, cystic fibrosis, and Emphysema have been greatly responsive to cysteine. 
• It has also helped in renal stone formation with side effects like nausea.
• L-cysteine has anti-inflammatory properties.
• It is widely used for site-directed labelling experiments by scientists to investigate biomolecular structure and dynamics.
• It is extensively used for site-directed spin labelling for EPR or paramagnetic relaxation-enhanced NMR.
• N-acetyl cysteine might help to prevent cancer. It is an antioxidant. It is also used for flu, dry eye, cough, lung, and other medical conditions. 

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Points to remember

1. It is one of the very few amino acids that contain sulphur.
2. Cysteine exists as a zwitterion like other amino acids.
3. Cysteine is a semi-essential amino acid, thus, that humans can make it. The thiol oxidized to give the disulphide derivative cystine, which is important in many protein structures.
4. It is essential for preterm infants. It is an important precursor for the synthesis of proteins such as glutathione, inorganic sulphate, taurine, and coenzyme A. 
5. L-cysteine is significant against acetaminophen overdose. hepatic glutathione is essential for the liver's role in detoxification, however during acetaminophen overdose, hepatic glutathione is depleted which is fatal. L-cysteine via N-acetylcysteine is the antidote as it helps to restore hepatic glutathione, and hence, to prevent liver damage.
6. Cysteine is different from cystine as the former is a non-essential amino acid while the latter is formed from two cysteine molecules bonded together. Cystine is more stable than cysteine but may not be absorbed well. Cystine, unlike cysteine, is rarely used as a dietary supplement. 

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Previous Years’ Questions

1. The segment of DNA which acts as the instrumental… (NEET 2009)
2. Which one of the following gives positive Fehling's solution… (NEET 2001)
3. Enzymes take part in a reaction and… (NEET 1993)
4. Glycolysis is… (NEET 2003)
5. The hormone that helps in the conversion of glucose to glycogen… (NEET 2004)
6. Phospholipids are esters of glycerol with… (NEET 2003)
7. Which of the following does not exhibit the phenomena of… (NEET 2010)
8. Which of the following biomolecules is common to… (NEET 2016)
9. Metabolism can be best defined as…
10. Insulin regulates the metabolism of… (KCET 2006)
11. Metabolism refers to the sum of chemical reactions that occur…
12. One mole of glucose on metabolism liberates how many…
13. Which one of the following is a peptide hormone… (NEET 2006)
14. During the process of digestion, the proteins present in food… (NEET 2006)
15. Which of the following is not a fat-soluble vitamin… (NEET 2011)