Re: origin of biological chirality?

On 8/18/2024 5:11 PM, Bob Casanova wrote:

On Sun, 18 Aug 2024 13:37:19 -0500, the following appeared
in talk.origins, posted by RonO <rokimoto557@gmail.com>:
 

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/
>

OK; thanks.
 So if I understand what you wrote, while there are
similarities between chirality selection and m/am in the
sense that both involve equally-likely (chemically) forms,
the former involves, at least somewhat, preferential
selection for incorporation into living things while the
latter is (AFAIK) a matter of pure chance. Sound right?

The forms do not have to be equally likely in solution.  As I noted D glucose is found at over 60% at equalibrium with the L form.  If you have ever worked with the stick models the carbon backbone has to twist into the mirror image in order to change from one form to the other.  It makes them structurally different in 3 dimensions.  For enzymes that use these molecules usually only one mirror image fits into an active site. A molecule has to get into position so that one of it's bonds can be broken or two molecules have to be brought into close enough proximity for their reaction to be catalyzed by the enzyme.
Whether the D or L form was utilized would have been set by the first enzymes.  The first self replicators would have likely had some type of catalytic site or sites within their structures.  These catalytic sites would have preferred one form over the other.
Both D and L forms existed in solution, but only one would work in any enzyme that evolved to deal with those carbon molecules.
Ron Okimoto

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