Non-protein amino acids (also sometimes spelled without a space as “nonprotein”) are found both in living things and outside–in the realm of nature outside of biology. There are over 140 of the non-protein amino acids that have been catalogued thus far. One famous example is that watermelon develops naturally, as part of its life cycle, a non-protein amino acid called “citrulline”!
At the Wikipedia article at https://en.wikipedia.org/wiki/Non-proteinogenic_amino_acids — (accessed 1/14/2021), this helpful overview is provided: In biochemistry, non-coded or non proteinogenic amino acids are those not naturally encoded or found in the genetic code of any organism. Despite the use of … amino acids … by the translational machinery to assemble proteins (the proteinogenic amino acids), over 140 amino acids are known to occur naturally in proteins and thousands more may occur in nature or be synthesized in the laboratory. Many non- proteinogenic amino acids are noteworthy because they are…
● intermediates in biosynthesis,
● post-translationally formed in proteins,
● possess a physiological role (e.g. components of bacterial cell walls, neurotransmitters and toxins),
● natural or man-made pharmacological compounds,
● present in meteorites and in prebiotic experiments (e.g. Miller–Urey experiment).
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