Cysteine vs cystine
Amino acid is a simple molecule formed with C, H, O, N and may be S. It has the following general structure.
There are about 20 common amino acids. All the amino acids have a –COOH, -NH2 groups and a –H bonded to a carbon. The carbon is a chiral carbon, and alpha amino acids are the most important in the biological world. D-amino acids are not found in proteins and not a part of metabolism of higher organisms. However, several are important in the structure and metabolism of lower forms of life. In addition to common amino acids, there are a number of non-protein derived amino acids, many of which are either metabolic intermediates or parts of non-protein biomolecules (ornithine, citrulline). The R group differs from amino acid to amino acid. The simplest amino acid with R group being H is glycine. According to the R group, amino acids can be categorized into aliphatic, aromatic, non polar, polar, positively charged, negatively charged, or polar uncharged, etc. Amino acids present as zwitter ions in the physiological pH 7.4. Amino acids are the building blocks of proteins. When two amino acids join to form a dipeptide, the combination takes place in a -NH2 group of one amino acid with the –COOH group of another amino acid. A water molecule is removed, and the formed bond is known as a peptide bond. Thousands of amino acids can be condensed like these to form long peptides, which are then folded into make proteins.
Cysteine is an alpha-amino acid. It has the above general structure. R group of cysteine is –CH2SH which contains sulfur. The structure of cysteine is given below.
Cysteine is abbreviated as Cys. The thiol group (-SH) of cysteine makes that more polar. Therefore, this amino acid is soluble in water. Thiol group also participated in hydrogen bonding with water molecules. Cysteine can be synthesized within human bodies, so it is a semi- essential amino acid. However, sometimes it is necessary to take cysteine from dietary sources, which contain high protein. Chicken, eggs, milk, yoghurt, oats, broccoli are some of the sources that contain high levels of cysteine amino acid. Cysteine amino acid is important in biological systems, because it is a common amino acid found in the active sites of enzymes. The thiol group is nucleophilic; therefore, they participate in many enzymatic reactions. The disulfide bonds between thiol groups are crucial for protein folding and in determining the three dimensional structure of proteins.
When two cysteine amino acid residues form a disulfide bond, the resulting dimeric residue is known as cystine. This is in the solid form and is white in color. In proteins, cystine residues are found, and they help to keep the three dimensional shape of proteins. Cystine has the following structure.
What is the difference between Cysteine and Cystine?
• Cysteine is an amino acid, and when two amino acids join together via a disulfide bond a cystine is formed.
W• hen cystine is formed, two cysteine molecules are oxidized.
• Cystine is responsible for the tertiary structure of proteins.