Abstract
The chalcogen elements oxygen, sulfur, and selenium are essential constituents of side chain functions of natural amino acids. Conversely, no structural and biological function has been discovered so far for the heavier and more metallic tellurium element. In the methionine series, only the sulfur-containing methionine is a proteinogenic amino acid, while selenomethionine and telluromethionine are natural amino acids that are incorporated into proteins most probably because of the tolerance of the methionyl-tRNA synthetase; so far, methoxinine the oxygen analogue has not been discovered in natural compounds. Similarly, the chalcogen analogues of tryptophan and phenylalanine in which the benzene ring has been replaced by the largely isosteric thiophene, selenophene, and more recently, even tellurophene are fully synthetic mimics that are incorporated with more or less efficiency into proteins via the related tryptophanyl- and phenylalanyl-tRNA synthetases, respectively. In the serine/cysteine series, also selenocysteine is a proteinogenic amino acid that is inserted into proteins by a special translation mechanism, while the tellurocysteine is again most probably incorporated into proteins by the tolerance of the cysteinyl-tRNA synthetase. For research purposes, all of these natural and synthetic chalcogen amino acids have been extensively applied in peptide and protein research to exploit their different physicochemical properties for modulating structural and functional properties in synthetic peptides and rDNA expressed proteins as discussed in the following review.
Highlights
In early days of X‐ray crystallography of proteins, one of the main difficulties was the trial and error soaking procedure for the production of heavy atom derivatives
It was noticed that the proteins have to be biosynthesized in the folded form since oxidation of the selenium and even more of the tellurium occurs in the unfolded states as exists in inclusion bodies.[3,13,14,15,16]
Thereby, the redox chemistry was found to represent the largest difference between the two chalcogens, a fact that has been exploited in nature to enhance and adjust redox properties of selenium‐containing enzymes and proteins, and in the laboratory to direct oxidative folding of cysteine‐rich peptides and enhance their thermodynamic stability in view of potential therapeutic applications
Summary
In early days of X‐ray crystallography of proteins, one of the main difficulties was the trial and error soaking procedure for the production of heavy atom derivatives. It looks as a promising probe in X‐ray crystallography as the electron density of tellurium is known to generate isomorphous and anomalous difference Patterson maps In this context, its properties should be superior to those of telluromethionine that suffers from a more facile oxidation when exposed on the protein surface (see Section 2.1) but with the drawback of a generally higher number of Phe residues in proteins compared with Met. In the serine/cysteine series, selenocysteine is a proteinogenic amino acid that is inserted into proteins by a special translation mechanism,[35,36] while the tellurocysteine is again only tolerated by the cysteinyl‐tRNA synthetase.[11].
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