In
biochemistry,
non-coded,
non-proteinogenic, or "
unnatural"
amino acids are those not naturally encoded or found in the genetic code of any organisms. Despite the use of only 23 amino acids (21 in eukaryotes) by the translational machinery to assemble proteins (the
proteinogenic amino acids), over 140 natural amino acids are known and thousands of more combinations are possible. Several non-proteinogenic amino acids are noteworthy because they are: