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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 19:31:29 +0200
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On 2024-08-19 14:01:55 +0000, RonO said:

> On 8/19/2024 4:07 AM, Athel Cornish-Bowden wrote:
>> 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.
> 
> As I indicated these carbon molecules transition between forms in 
> solution.  For some important amino acids cells have enzymes that 
> convert D amino acids to the L form.  For lactate it probably became 
> important to deal with the terminal product of glycolysis, so instead 
> of evolving a system to keep changing one to the other, enzymes evolved 
> to deal with both D and L forms so that the amount of lactate could be 
> regulated more efficiently.
> 
Yes, but there aren't many examples of reactions that convert achiral 
substrates into chiral products (or vice versa). Come to that, apart 
from pyruvate there aren't many important achiral metabolites.

>>   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.