Difference Between Cystine and Cysteine

Main Difference – Cystine vs Cysteine

Amino acids are the structural units of proteins that are essential for the functional metabolisms of the body. It is vital to understand the structure and properties of amino acids in order to determine the protein structure and function. The human body needs 20 different amino acids. The body can produce only 10 amino acids, and the rest must be taken from food sources. The amino acids which are produced in the body are named as non-essential amino acids, and those that cannot be produced within the human body are known as essential amino acids. Cystine and cysteine are two such amino acids needed for the body.  These two types are interconvertible in the body. Both these amino acids contain sulfur. Despite their similarities, each of this amino acid is responsible for different tasks in the body. Cystine is usually regarded as a semi-essential amino acid whereas Cysteine is a non-essential amino acid. This is the main difference between cystine and cysteine. In this article, the difference between cystine and cysteine is discussed.Difference Between Cystine and Cysteine - infographic

What is Cystine

Cystine is a non-essential amino acid that can be produced in the human body. Cystine is formed by the oxidation of two cysteine molecules. This amino acid is considered as a semi-essential amino acid.  It is abundant in the skeleton, connective tissues and in hair. Cystine is least soluble and more stable than cysteine. Cystine is required to form glutathione, a natural antioxidant, which removes toxins in the liver.

Main Difference - Cystine vs Cysteine

Cystine Structure

What is Cysteine

Cysteine is formed from the two amino acids: methionine and serine. Methionine provides the sulfur atom while serine provides the carbon skeleton for cysteine. Cysteine is then converted to cystine by oxidation with the presence of cystine reductase enzyme. Cysteine is important in many ways. It is unique from the rest of the amino acids due to the presence of thiol group. This group can undergo redox reaction (oxidation/reduction). Thus, cysteine shows antioxidant properties. The conversion of cysteine to pyruvate results in the formation of glucose. It is the most vital dietary source of sulfur for the body. Moreover, the sulfur-containing compounds including insulin, coenzyme-A, glutathione and vasopressin are derived from cysteine. Although it is classified as a non-essential amino acid, it may be essential for infants and adults who show malabsorption syndromes.

Difference Between Cystine and Cysteine

Cysteine Structure

Difference Between Cystine and Cysteine


Cystine is less soluble than cysteine.

Cysteine is more soluble than Cystine.


Cystine is more stable than cysteine.

Cysteine is less stable than Cystine.


Cystine is formed by the oxidation of cysteine.

Cysteine is formed from the two amino acids: methionine and serine.


Cystine is usually regarded as a semi-essential (maybe essential) amino acid.

Cysteine is a non-essential amino acid.


Cystine is not as easily absorbed by the body as Cysteine.

Cysteine is more easily absorbed by the body than cystine.


Naik, P. (2015). Biochemistry. JP Medical.

Llamas, E. (2011). Biologia para el examen de admision. S.l.: Lulu Com.

Cystine | amino acid. (n.d.). Retrieved June 29, 2016, from here 

Image Courtesy:

“Cystine-3D-balls” By Ben Mills – Own work (Public Domain) via Commons Wikimedia

“L-cysteine-3D-balls2” By Photohound – Derivative work of User:Benjah-bmm27 (Public Domain) via Commons Wikimedia 



About the Author: Yashoda

Yashoda has been a freelance writer in the field of biology for about four years. He is an expert in conducting research related to polymer chemistry and nano-technology. He holds a B.Sc. (Hons) degree in Applied Science and a Master of Science degree in Industrial Chemistry.