Question

Which of the following amino acids is not one of the 20 common amino acids found in cells?

1) leucine
2) isoleucine
3) glutamine
4) proline
5) pyrrolysine

Answer 1

Pyrrolysine is not one of the 20 common amino acids found in cells; it is a special amino acid used by certain archaea and bacteria, and was discovered in 2002.

Step-by-step explanation:

The amino acid that is not one of the 20 common amino acids found in cells is pyrrolysine. While the proteins in all living species, including humans, are made up of a core set of 20 amino acids, there are two additional amino acids that have been discovered in limited quantities in proteins: selenocysteine and pyrrolysine. Pyrrolysine was discovered in 2002 and is used by some methanogenic archaea and bacteria in enzymes that are part of the methanogenesis pathway. It is not considered a common amino acid in human cells.