https://www.science.org/content/article/why-are-all-proteins-left-handed-new-theory-could-solve-origin-of-life-mystery

I've never thought that the question of why left handed amino acids are
used in proteins to be much of any type of mystery. The use of left or
right would have been codified when the genetic code evolved and
proteins were made by ribosomes. Before that, proteins that were made
could have been left or right handed amino acids, but they would have
likely been one or the other if they were part of the first self
replicators. The thing about self replicators is that they make
themselves, and so rely on enzymatic catalysis of reactions to make more
of themselves. If part of a self replicator had peptide bonds, they
would have been able to create those peptide bonds in order to replicate themselves, and left and right handed mirror images do not fit into the
same catalytic site. This means that as soon as the self replicators no
longer relied on mineral surfaces or random peptide bonds forming the
necessary peptides, either left or right handed amino acids would have
been used to make the necessary peptides, and as I noted before once the genetic code started to evolve and peptide bonds were catalyzed by
ribosomal RNA left handed amino acids were the ones that fit into the
catalytic site of the ribosome, and the catalytic RNAs that were
charging the amino acids onto tRNAs, so all proteins created by the
initial renditions of our genetic code would have used left handed amino
acids. Such coded proteins would likely replace any other proteins that
were self replicating and had some other function needed by the evolving lifeform.

Ron Okimoto

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