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| Source: Wikipedia |
The world is constantly changing, altering our understanding from past textbooks. The nine planets became eight, twelve pairs of cranial nerves are now thirteen, and the discovery of selenocysteine increased the standard protein amino acids from twenty to twenty-one. However, the discovery of pyrrolysine in archaea and some bacteria in 2002 once again challenged the composition of standard protein amino acids. Some scientists argue that pyrrolysine should be considered the twenty-second amino acid, while others disagree; after all, only methane-producing archaea and some bacteria (including E. coli) use pyrrolysine.
In 2002, scientists discovered pyrrolysine at the active site of methyltransferase in Methanosarcina barkeri. Later, pyrrolysine was found in many methyltransferases; its function in enzymes may be to help adjust the position of methyl groups for reaction with cofactors. Unlike hydroxyproline, pyrrolysine has a genetic code, UAG. Readers might exclaim: Wait! Isn't UAG one of the stop codons? Yes, it is. However, like selenocysteine using UGA, pyrrolysine's use of UAG requires special equipment.
Microorganisms capable of using pyrrolysine carry a special transfer RNA (tRNA) that bears pyrrolysine and has an anticodon to recognize UAG. Moreover, this unique tRNA has its own dedicated aminoacyl-tRNA synthetase (aaRS) responsible for attaching pyrrolysine. Pyrrolysine is synthesized from two lysines and, due to its pyrrolidine structure (see proline), it has the prefix "pyrro-" in its name.
The microbial world is fascinating, and the microorganisms we can cultivate in laboratories likely represent only a small fraction of all microbes. Perhaps in the future, with a better understanding of microorganisms and deep-sea life, we might discover more unique amino acids, and the list of standard protein amino acids may grow even longer!
References:
Srinivasan, G; James, C. M.; Krzycki, J. A. 2002. Pyrrolysine encoded by UAG in Archaea: charging of a UAG-decoding specialized tRNA. Science. 296 (5572): 1459–1462. doi:10.1126/science.1069588
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