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It is now known that life on earth uses D-form sugars and L-form amino acids. However, it is also known that life on other worlds might be the other way around; using L-form sugars or D-form amino acids. If this is the case, then such a biosphere would be incompatible with earthborn life.

I have an alien biosphere which uses D-form amino acids and stores genetic information in a compound called threonucleic acid, or TNA, instead of DNA or RNA. Other than that, there is no important biochemical difference between life on this world and life on earth.

Now, let’s say (for a laugh) I want to move a Terran species to this planet. I can use genetic engineering to adapt this species to the planet’s environment to some degree, but this animal needs to eat. And since the biochemistry of this world is incompatible with it, this animal cannot get nourishment from anything it eats. It could possibly digest the sugars, (which are also D-form, as on earth) but it would not be able to process the amino acids, meaning no proteins. And as any biologist worth his sodium chloride will tell you, proteins are kind of important.

So, (say the novices), since you’ve already got the gene-o-tron up and running, why not play god a little more and modify this animal so that it’s body can use and process D-form amino acids? Well, I’m not sure that this is possible. There are limits to genetic engineering after all, and I’m uncertain as to whether this is too fundamental a change to the animal’s biochemistry to be altered by simply tweaking its genetics, and whether or not modifying this animal’s amino acids is as impossible as genetically-modifying it to use liquid helium instead of water.

So, in essence, is it possible to genetically-engineer an animal which can use amino acids with different chirality?

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Do you want to alter the entire organism to switch it from being built out of D-form amino acids instead of L-form amino acids? Or do you just want to make the minimal changes to allow it to derive nutrition from D-form amino acids? Because those are very different things.

If you want to "translate" the entire organism, then the problem is much bigger than just genetic engineering. In fact, the genetic changes would be a relatively small component of the work you would have to do. So... no, you could not genetically modify a life form for that. You would have to build synthetic ribosomes that can assemble D-form proteins, and build a whole working D-form cell that you could insert the original organism's genome into, and if it's not a single-celled organism or something that develops from free-swimming gametes anyway, then you would need to build an artificial incubator 'cause it won't survive in a womb or an egg from an existing parent. Now, you could use other genetically-modified organisms to produce the components, and maybe even to assemble them, rather than doing the whole thing "in silico", but it's not a one-step process, and it's mostly not a problem of genetic engineering.

If you just want to make an organism that can exploit the native resources, though, that's much easier, and yes, you could achieve it through genetic modification. You would just need to provide the new organism with some extra enzymes that will break down D-form proteins, and then either further break down D-form amino acids for atoms and synthesize new L-form amino acids, or else simply catalyze a flip to convert them into their L-isomers.

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  • $\begingroup$ "So... no, you could not genetically modify a life form for that. " And then you proceeded to explain how you could do just that... It is not THAT hard. We could do it, if we spend a few decades working on such project. So there is no reason why a civilization that can travel the stars couldn't do it. $\endgroup$
    – Negdo
    Jun 19, 2023 at 9:52
  • $\begingroup$ @Negdo No, I go on to describe what would actually be necessary, which goes far beyond genetic engineering. Genetic modification can't get you there on its own, just like a software patch can't convert an Intel machine to an ARM machine--you need to build a chip fab and produce new hardware that can work with your modified code. $\endgroup$ Jun 19, 2023 at 15:52
  • $\begingroup$ Em, no? Playing with ribosomes and codone is still considered genetic engineering, and is something we have done already (changing one of the stop codones to encode something else). Inserting DNA into a different cell is quite easy. Building a D-form cell would be a challenge, but it should be solvable. Everything you listed is theoretically possible, and probably reachable for us in the next half a century at most. $\endgroup$
    – Negdo
    Jun 21, 2023 at 6:44
  • $\begingroup$ @Negdo I didn't say it was impossible, just difficult, and not limited to genetic modification. It's synthetic biology, but you could do the entire translation without changing a single gene, and just inserting a natural set of chromosomes into a fully synthetic D-form cell with appropriately designed transcription factors and ribosomes. It would probably be simpler to also alter genes, but that's a small part of it, and when you're not chanting genes, it seems rather silly to call it "genetic" engineering. $\endgroup$ Jun 21, 2023 at 15:13
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Luckily for you, biology always seems to be one step ahead:

Check out D-amino racemases.

These are enzymes found in a variety of bacterial species that convert D-amino acids into L-amino acids via a pathway that involves the destruction of the stereogenic center. Also, it's a misunderstanding that D-amino acids are not used by life on earth — while rare, they do occur in a variety of biological compounds, notably bacterial peptidoglycan, of which D-Ala and D-Glu serve important functions in cell wall integrity.

Your terran organism simply needs to have the genes encoding this protein spliced somewhere into its genome, to convert whatever amino acids sequestered from the environment into L-isomers which it can then use.

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