The key difference between cysteine and cystine is that cysteine is an amino acid while cystine forms when two amino acids join together via a disulfide bond.
An amino acid is a simple molecule formed with C, H, O, N and maybe S. There are about 20 common amino acids. All 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. Furthermore, the 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 a “peptide bond”.
CONTENTS
1. Overview and Key Difference
2. What is Cysteine
3. What is Cystine
4. Side by Side Comparison – Cysteine vs cystine in Tabular Form
5. Summary
What is Cysteine?
Cysteine is an alpha-amino acid. R group of cysteine is –CH2SH, which contains sulfur. The structure of cysteine is given below.
Figure 01: Structure of Cysteine Molecule
We can abbreviate it as Cys. The thiol group (-SH) of cysteine makes the compound more polar. Therefore, this amino acid is soluble in water. Moreover, the thiol group participates in hydrogen bonding with water molecules. We can synthesize cysteine within our body, so it is a semi-essential amino acid. However, sometimes it is necessary to obtain cysteine from dietary sources which contain high protein. Chicken, eggs, milk, yoghurt, oats, and broccoli are some of the sources that contain high levels of this amino acid.
Cysteine amino acid is important in biological systems because it is a common amino acid we can find 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.
What is Cystine?
When two cysteine amino acid residues form a disulfide bond, the resulting dimeric residue is known as cystine. It is in solid form and is white in colour. The structure of this molecule is as follows:
Figure 02: Structure of Cystine
Furthermore, we can find the cystine residues in proteins, and they help to keep the three-dimensional shape of the protein.
What is the Difference Between Cysteine and Cystine?
The two terms Cysteine and cystine are often confusing because the spellings of the two terms and pronunciation are similar to each other. However, the key difference between cysteine and cystine is that cysteine is an amino acid, while a cystine forms when two amino acids join together via a disulfide bond. When considering the importance of these two compounds, Cysteine is important for protein synthesis, detoxification and many other metabolic functions while cystine is important to keep the tertiary structure of a protein. So, this also a significant difference between cysteine and cystine.
Summary – Cysteine vs cysteine
Cysteine and cystine are important biological components in our body. In summary, the key difference between cysteine and cystine is that cysteine is an amino acid, while a cystine forms when two amino acids join together via a disulfide bond.
Reference:
1. “Cysteine.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine, Available here.
Image Courtesy:
1. “Cystine-skeletal” By Benjah-bmm27 assumed (based on copyright claims). – No machine-readable source provided. Own work assumed (based on copyright claims) (Public Domain) via Commons Wikimedia
2. “gordo Cysteine” By bigblue0092 (CC BY 2.0) via Flickr