Selenocysteine

 

Source: Wikipedia

Selenocysteine is a structure formed when the sulfur in cysteine is replaced by selenium. As a result of this substitution, selenocysteine has a lower reduction potential than cysteine and is more suitable as an antioxidant. Selenocysteine is a crucial amino acid in the composition of selenoproteins, which are found in both prokaryotes and eukaryotes. Consequently, selenocysteine is now considered one of the basic amino acids that make up proteins. In other words, proteins are no longer composed of just 20 amino acids, but 21!

A well-known selenoprotein is glutathione peroxidase, which is responsible for reducing hydrogen peroxide (H2O2) to water. Although selenocysteine is a basic amino acid in protein composition, unlike the other twenty amino acids, it doesn't have its own genetic codon. Instead, it uses UGA (a stop codon) along with a roughly sixty-base "selenocysteine insertion sequence" (SECIS element) as a signal for inserting selenocysteine. In prokaryotes, the SECIS element is located close to the UGA codon. In archaea and eukaryotes, the SECIS element is often found in the 3' untranslated region (3'-UTR) of the messenger RNA (mRNA).

Source: Wikipedia

When selenium is absent, the translation of selenoproteins stops at the UGA codon, resulting in incomplete, non-functional proteins. Only when selenium is present can complete proteins be produced. Selenocysteine was discovered in 1976 by Thressa Stadtman of the National Institutes of Health (NIH). She and her husband were the first married couple at NIH, each with their own laboratory, and the two labs also collaborated.

For the origin of cysteine, please refer to "Cysteine and Bladder." The "seleno-" in selenocysteine obviously comes from selenium, though "seleno-" is not a prefix. However, due to its association with selenoproteins, it seems to have been widely adopted.

References:

Wikipedia. Selenocysteine, Thressa Stadtman, SECIS element.

硒半胱胺酸（Selenocysteine）

硒半胱氨酸。圖片來源：wiki

硒半胱氨酸（Selenocysteine）是半胱氨酸的硫（sulfur）被硒（selenium）所取代，產生如上的結構。以硒取代硫的結果，硒半胱氨酸比半胱氨酸的還原電位更低，也更適合作為抗氧化劑。

硒半胱氨酸是組成含硒蛋白（selenoproteins）的重要氨基酸，從原核到真核生物都有含硒蛋白，所以硒半胱氨酸已經被認為是組成蛋白的基本氨基酸之一；也就是說，現在不是20個氨基酸組成蛋白質了，而是21個氨基酸組成蛋白質喔！有名的含硒蛋白如穀胱甘肽過氧化物酶（glutathione peroxidase），負責將過氧化氫（H₂O₂）還原為水。

雖然是組成蛋白的基本氨基酸，但是硒半胱氨酸不像其他二十個氨基酸有自己的遺傳密碼（codon），而是以UGA（終止密碼）加上一個約六十個鹼基的「硒半胱氨酸插入序列」（SECIS element）作為插入半胱氨酸的信號。在原核生物，硒半胱氨酸插入序列距離UGA很近；在古菌以及真核生物，硒半胱氨酸插入序列常常位在信息RNA（messenger RNA，mRNA）的3'非轉譯區（3'-untranslated region，3'-UTR）。

SECIS element的構造。圖片來源：Wiki

當沒有硒的時候，含硒蛋白的轉譯會停止在UGA，產生不完整沒有功能的蛋白質；只有在硒存在的時候，才會有完整的蛋白質產生。

硒半胱氨酸是在1976年由美國國家衛生署（NIH）的Thressa Stadtman發現的。她和她的丈夫是NIH的第一對夫妻檔，兩人各有自己的實驗室，兩個實驗室之間也互相合作。

半胱氨酸的由來請參考「半胱氨酸（cysteine）與膀胱（bladder）」，而硒半胱氨酸的seleno-很明顯是從硒（selenium）而來，不過seleno-不是字首，但由於含硒蛋白的關係，似乎已經被廣泛的使用了。

參考文獻：

Wikipedia. Selenocysteine, Thressa Stadtman, SECIS element.