Pyrrolysine

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Pyrrolysine (abbreviated as Pyl or O) is a naturally occurring, genetically coded amino acid used by some methanogenic archaea and one known bacterium in enzymes that are part of their methane-producing metabolism. It is similar to lysine, but with an added pyrroline ring linked to the end of the lysine side chain. Produced by a specific tRNA and aminoacyl tRNA synthetase, it forms part of an unusual genetic code in these organisms, and is considered the 22nd proteinogenic amino acid.

The joint nomenclature committee of the IUPAC/IUBMB has officially recommended the three-letter symbol Pyl and the one-letter symbol O for pyrrolysine.

Contents

Introduction and context

One key function of the genome is to direct production of proteins using genetic sequences that determine when or if each protein will be produced; what cells will produce it; and where it is located in the cell. Proteins form much of the physical structure of the body and catalyze a wide variety of chemical reactions, giving the genome the ability to control the body's biochemistry. Nearly all proteins are made using only 20 standard building blocks called amino acids, which are often assembled in very long sequences according to a standard genetic code. Specialized chemical reactions often require alterations of proteins after the fact by posttranslational modification, or protein binding to specific cofactors. Yet the genetic code itself is exactly the same among very many organisms, so that when researchers sequence DNA from new or unknown sources they can often immediately draw conclusions about the chemical activity it carries out based on the assumption that a standard genetic code applies. The discovery of unusual amino acids specified by an expansion of the genetic code can call this assumption into question, so it is important to understand any such aberrations. Additionally, these variations indicate that the process of evolution that led to the establishment of the genetic code did not end before the universal common ancestor perhaps some three to four billion years ago, but remains accessible to study even in the present day.

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