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I remember reading that scientist were making cells (I assume bacteria), that used differently oriented proteins to create a whole new class of life. Because apparently right and left aligned proteins don't interact in same way, cells made in such manner would behave similarly but couldn't be interacted with differently aligned bacteria/virus.

However I can't find any proof this story was true? I could be wrong about details. Is there a similar mechanism that would allow a whole case of twin (contain DNA, and most molecules that regular cells posses) cells that couldn't interact with existing bacteria?

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    $\begingroup$ That seems extremely far fetched to do with current technology... in any case, natural aminoacids are called L-aminoacids and their "mirror counterparts" are the D-aminoacids. Maybe that can help with your search? $\endgroup$
    – nico
    Mar 29, 2014 at 14:42
  • $\begingroup$ I can see an "easy" experiment involving generation of auxotrophic d-amino acids for any given gene. You can now make certain bacterial stop codons code for non-canonical amino acids. Of course for an entire genome...now that would be something. $\endgroup$
    – jwillis0720
    Apr 1, 2014 at 10:05

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The problem with this is something called the structure-function relationship. The function of a protein or enzyme is completely dependent on its structure. For example, take a look at this representation of the active site of chymotrypsin:

The side chains of D102, H57, and S195 all need to be in a perfect conformation in order for the enzyme to function properly, with the hydrogen bonds being essential. If any one of those amino acids was in the D-form, the enzyme would lose its activity completely. Having every single amino acid in the D-form wouldn't help either, as the entire protein's structure (and hence its function) would be altered.

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  • $\begingroup$ This assumes lots of things. There may be instances in which the active site is a bit "loose" to allow for multiple substrates and in which a D-aminoacid would not harm too much. Plus if everything, including the substrate, was D-form I bet there would be a high chance of it working. There are examples of antibiotics becoming more specific or selective when one aminoacid is changed from L to D (e.g. this and this) $\endgroup$
    – nico
    Mar 31, 2014 at 5:52
  • $\begingroup$ @nico - a few D-amino acids here and there may not affect structure/function too much, I agree. But I really can't imagine that a fully D organism could exist. I didn't get into this in my answer, but I doubt that all tRNAs could bind the D isoforms of their respective payloads. Secondary and tertiary structures often depend on H-bonding, and a change in chirality could destroy those bonds, leading to incorrect folding. $\endgroup$
    – MattDMo
    Mar 31, 2014 at 15:38
  • $\begingroup$ I don't see why. If everything in nature was specular, well, it could in theory work very well! I think we simply do not have any proof in favour or against to say more... $\endgroup$
    – nico
    Mar 31, 2014 at 16:15

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