![]() First of all, Cys thiols are capable of unique reactivity in the protein world: covalent interactions with other thiols create intra- and intermolecular disulfide bonds. What are the physicochemical features of Cys that make this residue such a unique case? The main feature appears to be the high reactivity and chemical plasticity of its sulfur-based functional group. The unexpected aspect is that incorporation of Cys occurs specifically through the Sec insertion machinery: Cys is synthesized by Sec synthase directly on Sec-tRNA (from serine and thiophosphate) and inserted at UGA codons. It has been found that Cys can be inserted in proteins in place of Sec ( 12). A recent study made the relation between Cys and Sec even more intriguing. This functional interplay is unique in the protein world for example, the relationship between pyrrolysine (the 22nd amino acid) and lysine is not functional ( 11). Sec is the only natural amino acid thought to be located exclusively in catalytic sites (although a possible exception to this rule has been reported ( 10)), and its function can be partially preserved when Cys, but not any other amino acid, replaces Sec. Sec, known as the 21st amino acid in the genetic code, is a selenium-containing amino acid that differs from Cys by a single atom ( i.e. This feature has to be associated with several unique biological functions of Cys, as detected by genome-wide analyses of its tendency to form functional clusters, such as structural disulfides and metal-binding sites ( 8, 9).Īnother unique property of Cys is its ability to functionally interchange with Sec. Although the biological interpretation of this relationship is debated ( 5– 7), it highlights the fact that Cys residues in proteins appear to be under strict evolutionary control. With respect to aging, several studies showed that Cys content in mitochondrially encoded proteins inversely correlates with life span in animals ( 4). The functional importance of Cys in biology is also highlighted by the observation that, in humans, Cys mutations lead to genetic diseases more often than expected on the basis of its abundance ( 3). Although Cys is thought to be a later addition to the genetic code ( 1), it has also been shown to accumulate more than any other amino acid in present day organisms ( 2), implying that the usage of Cys may further expand in their descendents. It is one of the least abundant residues (often the least abundant) in organisms, yet it is frequently observed in functionally important sites of proteins, where it serves catalytic, regulatory, structure-stabilizing, cofactor-binding, and other functions. ![]() Among the 20 common amino acids in proteins, Cys is often an outlier credited with several unique features.
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