Re: origin of biological chirality?

On 8/18/2024 12:01 PM, Bob Casanova wrote:

On Sun, 18 Aug 2024 00:08:49 +0100, the following appeared
in talk.origins, posted by Ernest Major
<{$to$}@meden.demon.co.uk>:
 

A study has found that lipid membranes can be selectively permeable to
one or the other sugar or amino acid enantiomer. The study used membrane
models inspired by the membranes of modern organisms, so is not directly
relevant to abiogenesis. However it still raises the possibility that
membrane selectivity was the source of chirality in biological
molecules. One possible issue is does this effect require chiral
membrane lipids; if so it only move the question of the origin of
chirality from sugars and amino acids to lipids.
>

ISTM that this is similar to the "matter/antimatter"
imbalance; neither is inherently more "natural" than the
other, but one became more prevalent. And IIRC, the m/am
imbalance is now assumed to be a matter of chance in the
original ratio. I could; of course, be mistaken in that;
it's been years since I followed it even casually.

>
https://www.biorxiv.org/content/10.1101/2024.04.23.590732v2.full.pdf
>

For chirality there is an equilibrium ratio between the mirror images. D sugars have been known to exist in solution at higher concentrations that L forms.  My guess is that L forms of amino acids are likely to exist at higher concentrations in solution, but it doesn't matter.  The chirality of life was set by the first enzymatic reactions used by life to get started.  The use of L amino acids would have been set by the first functional proteases that could produce peptide bonds or for the RNA world scenario it would have been L amino acids that were probably used to make the first nucleotides.  The active sites of the first replicated enzymes would have set the chirality, and that chirality would have been maintained due to subsequent enzymes would have to be compatible for the ones that came before.  Only one form fits into the active site of an enzyme that uses that amino acid or carbohydrate. Enzymes have evolved to convert one form into the other because they spontaneously change from D to L and if left to themselves you would get a mix at a certain ratio in solution.  I really do not understand why anyone is worried about why life on earth uses D sugars and L amino acids.  It would have been set, probably, by the enzymes of the first self replicators, and would have likely been maintained by selection as everything would have worked better if new functions could use the same materials.
I found this paper that L amino acids would have been more efficiently incorporated into our current translation system (making proteins using ribosomes, mRNA and tRNAs) because both D and L amino acids transition between the 2 and 3 position of the ribose (at the end of the tRNA) several times a second, but L forms are found more often at the 3 position that is used in the translation system.  It is a reason to use L amino acids to make proteins using our current translation system, but L amino acids would have been selected long before by their use in making nucleotides and other essential biochemicals for the lifeform before the translation system existed.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC281674/
Ron Okimoto