Selenocysteine

Selenocysteine is a naturally occurring amino acid that is considered the 21st proteinogenic amino acid. It is a homologue of the more common amino acid cysteine, differing only by the substitution of a selenium atom for cysteine's usual sulfur atom.

Structure and Properties

Selenocysteine has the same structure as cysteine, but with a selenium atom replacing the sulfur atom. This gives it unique properties, such as a lower pKa and a higher reactivity, which allows it to participate in several unique biological processes.

Biological Role

Selenocysteine is found in several enzymes, including glutathione peroxidases, thioredoxin reductases, and iodothyronine deiodinases. These enzymes play crucial roles in antioxidant defense, redox homeostasis, and thyroid hormone metabolism, respectively.

Unlike other amino acids, selenocysteine is not directly coded for in the genetic code. Instead, it is encoded by a UGA codon, which is normally a stop codon. A specific mRNA structure known as a selenocysteine insertion sequence (SECIS) is required for the UGA codon to be read as selenocysteine.

Dietary Sources and Deficiency

Selenium, and by extension selenocysteine, is found in a variety of dietary sources, including Brazil nuts, seafood, and organ meats. Selenium deficiency can lead to diseases such as Keshan disease and Kashin-Beck disease, which are characterized by heart problems and osteoarthritis, respectively.

See Also

• Selenoprotein
• Selenomethionine
• Selenouracil