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It's fairly common to see discussion of different chirality for amino acids and sugars whenever people ask for advice on plausible alien designs or biochemistry (e.g.: metabolizing left-handed sugars and with proteins made of right-handed amino acids, compared to our R-sugars and L-amino acids). However, I've never seen anyone ask about a species using both chiralities in some way, so aliens who can digest both R-sugars and D-sugars, and/or who who have proteins built from a roughly even amount of R- and D-amino acids.

I've been considering trying to develop aliens who use roughly equal proportions of both chiralities of amino acids to build proteins. My questions then are A) is there any reason life couldn't incorporate both L- and D-amino acids into its biochemistry (this is of more interest to me), and B) could life evolve to digest both L- and D-sugars?

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    $\begingroup$ Reality is much more complicated than "Earth life forms absolutely cannot metabolize L-sugars". Some bacteria are known to be able to metabolize L-glucose, for example. Whole-body metabolism experiments have shown that rats can extract energy from L-fructose (and we haven't yet got any idea how they do it). (The link is to G. Livesey, J. C. Brown, "Whole body metabolism is not restricted to D-sugars because energy metabolism of L-sugars fits a computational model in rats", in J. Nutr., 1995 Dec, 125(12):3020-9, doi: 10.1093/jn/125.12.3020.) $\endgroup$
    – AlexP
    Commented Nov 25, 2021 at 13:22
  • $\begingroup$ Very interesting, thank you! $\endgroup$
    – passerby
    Commented Nov 25, 2021 at 16:09

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The reason why chirality is a thing in living organisms is because amino acids and sugars and so are made by enzymes and other such bits of biochemical machinery in cells, and those have chirality too. Thus they would assemble molecules preserving their chirality - which includes the next generation of cells. Since all living things have a common ancestor, you end up with every descendant cell keeping the same chirality. There isn't any evolutionary reason to create a whole new biologically incompatible set of systems.

You could potentially obtain a both-chirality alien if the planet the aliens live on have some sort of non-biological process that produces sugars or amino acids. Such processes would produce molecules of both chiralities, and perhaps organisms might evolve to take advantage of both being around. It's very unlikely that such an organism would use both chiralities the same way though. It would be more likely that you'd end up with an organism that initially evolved to use one chirality and then developed a secondary ability to use the second in some way so as not to be poisoned as its environment became rich in the other chiral type.

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  • $\begingroup$ Great, thank you! Would you happen to have any ideas of what abiotic processes could those be? Would the presence of life cause issues with those processes occurring? $\endgroup$
    – passerby
    Commented Nov 26, 2021 at 18:23
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    $\begingroup$ Scientists have found ribose and other sugars in meteorites so possibly you could have a planet that was seeded with a lot of the stuff through meteor impacts. Otherwise I dunno. $\endgroup$
    – Fhnuzoag
    Commented Nov 28, 2021 at 1:17
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We have both righthand and lefthand gloves to go on the respective hands.

If your creature uses both, because of the issue of fitting together like the glove, it would probably be an obligate user of both. It could not simply slot a right-handed molecule into a left-handed process.

This is imaginable but not more efficient.

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