What is the Difference Between Cysteine and Selenocysteine

The key difference between cysteine and selenocysteine is that cysteine is a proteinogenic amino acid that has sulphur in its structure, while selenocysteine is a proteinogenic amino acid that has selenium in its structure.

Cysteine and selenocysteine are two proteinogenic amino acids. Proteinogenic amino acids are amino acids incorporated into proteins during the process of translation. They are very important for the biosynthesis of functional proteins. There are 22 genetically encoded proteinogenic amino acids throughout the known life forms. Twenty amino acids are in the standard genetic code, and two can be incorporated by a special translational mechanism. Among these proteinogenic amino acids, nine amino acids are essential, and the rest of the amino acids are non-essential.

CONTENTS

1. Overview and Key Difference
2. What is Cysteine 
3. What is Selenocysteine
4. Similarities – Cysteine and Selenocysteine
5. Cysteine vs Selenocysteine in Tabular Form
6. Summary – Cysteine vs Selenocysteine

What is Cysteine?

Cysteine is a semi-essential proteinogenic amino acid that has sulphur in its structure. It has the chemical formula of HOOC-CH-(NH₂)-CH₂-SH. The thiol side chain of cysteine frequently participates in biochemical enzymatic reactions as a nucleophile. The thiol group can undergo redox reactions. Therefore, cysteine has antioxidant properties. Cysteine’s antioxidant property is typically expressed in tripeptide glutathione, the master antioxidant in the human body. Cysteine is encoded by the genetic codons UGU and UGC. When used as a food additive, it has the E number of E920.

Cysteine exists as zwitterion-like other amino acids. Cysteine is a common residue in high protein foods. It may be essential for infants, elders, and other individuals who have malabsorption syndromes. Cysteine can be normally synthesized by the human body if a sufficient amount of methionine is available. Cysteine is industrially obtained through the hydrolysis of animal materials such as poultry feathers or hog hair. Furthermore, cysteine has numerous biological functions like precursor to antioxidant glutathione, precursor to iron-sulfur clusters, metal iron-binding property, vital roles in protein structure, etc.

What is Selenocysteine?

Selenocysteine is a proteinogenic amino acid that has selenium in its structure. It is structurally very similar to cysteine. Selenocysteine is the 21^st amino acid. Both eukaryotes and prokaryotes can incorporate selenocysteine into their proteins through a nucleotide sequence known as “SECIS element”. This directs the cells to translate a nearby UGA codon (usually stop codon) as selenocysteine. This proteinogenic amino acid is present in several enzymes, including glutathione peroxidase, tetraiodothyronine 5’ deiodinases, thioredoxin reductases, formate dehydrogenases, glycine reductases, selenophosphate synthetase 2, methionine R sulfoxide reductase B1, and some hydrogenases. Moreover, selenocysteine was first discovered by the biochemist Thressa Stadtman at the National Institute of Health.

Currently, 136 human proteins are known to contain selenocysteine. Furthermore, it is biotechnologically applied in ⁷³Se-lablled Sec in position emission tomography (PET) studies, ⁷⁵ Se-labelled Sec in specific radiolabelling and ⁷⁷Se isotope in high-resolution NMR.

What are the Similarities Between Cysteine and Selenocysteine?

• Cysteine and selenocysteine are two proteinogenic amino acids.
• Both have similar structures.
• These are acidic amino acids.
• They are produced by the human body.
• They are found in both prokaryotes and eukaryotes.

What is the Difference Between Cysteine and Selenocysteine?

Cysteine is a proteinogenic amino acid that has sulphur in its structure, while selenocysteine is a proteinogenic amino acid that has selenium in its structure. So, this is the key difference between cysteine and selenocysteine. Moreover, cysteine is coded by the genetic codons UGU and UGC, while selenocysteine is coded by the genetic codon UGA.

The below infographic lists more differences between cysteine and selenocysteine in tabular form for side by side comparison.

Summary – Cysteine vs Selenocysteine

Cysteine and selenocysteine are two proteinogenic amino acids. They are incorporated biosynthetically into proteins during translation. Cysteinehas sulphur in its structure, while selenocysteine has selenium in its structure. Thus, this summarizes the difference between cysteine and selenocysteine.

Reference:

1. “Cysteine.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine.
2. “Selenocysteine.” An Overview | ScienceDirect Topics.

Image Courtesy:

1. “Cysteine w functional group highlighted” By Clavecin – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Selenocysteine w functional group highlighted” By Clavecin – Own work (CC BY-SA 4.0) via Commons Wikimedia