Cysteine vs. Cystine

What's the Difference?

Cysteine and cystine are two closely related amino acids that contain a sulfur atom. The main difference between them lies in their chemical structure. Cysteine is a nonpolar amino acid with a thiol group (-SH) on its side chain, making it highly reactive and capable of forming disulfide bonds with other cysteine residues. On the other hand, cystine is a disulfide-bonded dimer of two cysteine molecules, formed when two cysteine residues oxidize and link together. This disulfide bond gives cystine a more stable structure compared to cysteine. Both cysteine and cystine play crucial roles in protein structure and function, as well as in various biological processes.


Chemical FormulaC3H7NO2SC6H12N2O4S2
Functional GroupThiol (-SH)Disulfide (-S-S-)
FormationDerived from methionineFormed by oxidation of cysteine
Role in Protein StructureForms disulfide bondsStabilizes protein structure
Redox PotentialLowHigh

Further Detail


Cysteine and cystine are two important amino acids that play crucial roles in various biological processes. While they share some similarities, they also possess distinct attributes that set them apart. In this article, we will explore the characteristics of cysteine and cystine, their structures, functions, and their significance in human health.


Cysteine and cystine are both sulfur-containing amino acids. Cysteine is a non-essential amino acid, meaning it can be synthesized by the human body, while cystine is a derivative of cysteine formed by the oxidation of two cysteine molecules. The primary difference between the two lies in their chemical structure. Cysteine consists of a thiol group (-SH) attached to an alpha carbon, while cystine is formed by the oxidation of two cysteine molecules, resulting in a disulfide bond (-S-S-) between the sulfur atoms.


Cysteine and cystine have various functions in the body. Cysteine is a key component in the synthesis of proteins and plays a vital role in the formation of disulfide bonds, which contribute to the structural stability of proteins. It is also involved in the detoxification of harmful substances, acting as a precursor for the antioxidant glutathione. Additionally, cysteine is essential for the synthesis of taurine, an amino acid important for cardiovascular health and the development of the central nervous system.

Cystine, on the other hand, is primarily involved in the structural stability of proteins. The disulfide bonds formed by cystine contribute to the three-dimensional folding of proteins, which is crucial for their proper function. These bonds help maintain the structural integrity of proteins, ensuring their stability under various physiological conditions. Cystine is particularly abundant in structural proteins such as keratin, which is responsible for the strength and resilience of hair, skin, and nails.

Food Sources

Cysteine and cystine can be obtained from various dietary sources. Cysteine-rich foods include poultry, eggs, dairy products, legumes, and whole grains. These sources provide the body with the necessary building blocks to synthesize cysteine. Cystine, on the other hand, is found in lower quantities in food as it is formed by the oxidation of cysteine. However, some dietary sources of cystine include meat, fish, eggs, and dairy products.

It is worth noting that the bioavailability of cysteine and cystine from food sources can be influenced by factors such as cooking methods and food processing. For example, prolonged heating or processing of foods can lead to the degradation of cysteine and cystine, reducing their availability for absorption in the body.

Health Benefits

Both cysteine and cystine offer several health benefits. Cysteine's role as a precursor for glutathione, a powerful antioxidant, makes it important for maintaining cellular health and protecting against oxidative stress. Glutathione helps neutralize harmful free radicals, supports the immune system, and aids in detoxification processes. Cysteine also plays a role in the synthesis of collagen, a protein essential for the health of connective tissues, skin, and joints.

Cystine's contribution to protein structure and stability is crucial for the proper functioning of various tissues and organs. It helps maintain the integrity of structural proteins, ensuring their resilience and strength. Additionally, cystine is involved in the formation of hair and nails, contributing to their growth and overall health.

Deficiency and Supplementation

Deficiencies of cysteine and cystine are rare since the body can synthesize cysteine from other amino acids. However, certain conditions or dietary restrictions may lead to reduced cysteine levels. In such cases, supplementation with cysteine or cystine may be beneficial. Cysteine supplements are available in various forms, including N-acetylcysteine (NAC), which is commonly used to support respiratory health and as an antioxidant. Cystine supplements are less common but may be used in specific cases where cystine deficiency is identified.


Cysteine and cystine are sulfur-containing amino acids with distinct attributes and functions. While cysteine is involved in protein synthesis, detoxification, and the synthesis of taurine, cystine primarily contributes to protein stability and structural integrity. Both amino acids offer health benefits and can be obtained from various dietary sources. Understanding the roles and characteristics of cysteine and cystine is essential for maintaining optimal health and ensuring the proper functioning of various biological processes in the body.

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