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From: Athel Cornish-Bowden <me@yahoo.com>
Newsgroups: talk.origins
Subject: Re: origin of biological chirality?
Date: Mon, 19 Aug 2024 11:07:26 +0200
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On 2024-08-18 18:37:19 +0000, RonO said:

> 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.

Me neither. It had to be one or the other, and with appropriate enzymes 
D aminoacids would have been just as good, but having made the choice 
life had to stick with it.

An interesting case is that of lactate. Both D-lactate and L-lactate 
are important metabolites, and the lactate dehydrogenases that act on 
them are quite different from one another.

>   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


-- 
Athel -- French and British, living in Marseilles for 37 years; mainly 
in England until 1987.