Study 22nd amino acid - pyrrolysine

Study of 22nd amino acid - pyrrolysine

10:56 PM, 20th November 2011
Study of 22nd amino acid - pyrrolysine
PylB with methylornithin (3MO) and co-factor S-adenosylmethionine (SAM).

 

GARCHING, GERMANY: Proteins are key players in many vital processes in living organisms. They transport substances, catalyze chemical reactions, pump ions or recognize signaling molecules. The complexity and variety of proteins is tremendous. In the human body alone there are more than 100,000 different proteins at work. But almost all of them are made up of just twenty different amino acids.

Only a few highly specialized proteins additionally contain selenocysteine, the rare 21st amino acid which was discovered in 1986 and ten years ago a 22nd, the pyrrolysine was discovered. Researchers at Technische Universitaet Muenchen have elucidated the structure of an important enzyme in the production of pyrrolysine. A big surprise was the discovery of a 22nd amino acid in methane-producing archaea of the family Methanosarcinaceae in 2002: pyrrolysine. It is genetically encoded in a similar manner as that of selenocysteine and other twenty amino acids. The archaea use unusual amino acid in proteins that they need for energy conversion.

Pyrrolysine is located in the catalytic centre of proteins and is essential for their function. In March 2011, scientists at Ohio State University succeeded in deciphering parts of the manufacturing process of pyrrolysine. They proposed a reaction mechanism suggesting that the enzyme PylB catalyzes the first step of pyrrolysine biosynthesis by converting the amino acid lysine to the intermediate product metyhlornithine.

They found the enzyme active, ie at the time of crystallization the reaction product, methylornithine, had not left the enzyme. It adhered to a confined space, a kind of “reaction vessel,” still in connection with centres of the enzyme. “That the product was still present in the enzyme, was something special and a great stroke of luck, We were not only able to directly detect the methylornithine, but also retroactively reconstruct how it is created from the source amino acid lysine,” said Felix Quitterer, Member of Scientific Staff, Department of Biochemistry and Lead Author of the publication.

This reaction was not only hitherto unknown, it is also very difficult to catalyze. “This is a really unusual enzymatic reaction. Up to now no chemist in the laboratory is able to synthesize methylornithine in a one-step reaction starting from lysine,” said Michael Groll, Professor of Biochemistry, TUM-Department of Chemistry.

A more fundamental reason for high interest in synthesis of 22nd amino acid is that scientists are hoping to find new clues to the evolutionary development of the amino acid-canon. Why does the vast complexity of proteins in living organisms descend from only a few natural amino acids, even though the genetic code would be able to encode many more? An answer on this fundamental question has thus far eluded scientists. Selenocysteine and pyrrolysine are exotic exceptions, but knowledge about their development from the standard amino acids helps to come little closer to the answer.

The original publication, “Crystal structure of methylornithine synthase (PylB): Insights into pyrrolysine biosynthesis is written by Felix Quitterer, Anja List, Wolfgang Eisenreich, Adelbert Bacher and Michael Groll, in the journal Angewandte Chemie.

© Technische Universitaet Muenchen News

 

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