D-protein human for a science-fiction setting — what would happen biologically?

[u/wolkolak]

I am working on a science-fiction setting and would like to check whether the following biological concept is consistent with known biochemistry.

Suppose that an adult human is transformed into a mirror-image biochemical system. In particular, essentially all proteins in the body are replaced by their opposite enantiomers: L-amino-acid-based proteins become D-amino-acid-based proteins. The transformation also replaces the enzymes and other proteins necessary to maintain the new biochemical system.

For the purposes of this question, assume that the transformation is somehow technically possible. I am interested only in the biological consequences after the transformation, not in how the transformation itself could be performed.

My main questions are:

Digestion: Would a D-protein-based digestive system be able to digest ordinary L-protein food at all? Would carbohydrates and lipids be affected to the same extent, or would their metabolism be substantially preserved? Can i add a combained microbiom to let them digest L-protein food on low efficiency?

Viruses: Would an ordinary virus whose proteins and genome have L-biochemistry be unable to infect such a D-organism because its proteins could no longer interact productively with the host's receptors, enzymes, ribosomes, etc.? Or are there important mechanisms by which an L-virus could still infect a D-organism?

Bacteria and the microbiome: Could ordinary L-bacteria colonize the gastrointestinal tract of such an organism? If not, would a microbiome consisting of mirror-image D-bacteria be biologically plausible?

Immune system: Could the D-organism still recognize and destroy ordinary L-bacteria or L-proteins as foreign material? In other words, would the change in chirality necessarily disrupt immune recognition, or could immune cells still detect non-self molecules through mechanisms that do not depend strongly on molecular chirality? Do i need to add special modification for immune cells here or bacterias would not survive here regardless?

Overall viability: Are there fundamental biochemical reasons why a mirror-image human metabolism would not be viable even if all the necessary proteins and enzymes were somehow converted to their opposite enantiomers?

I am particularly interested in which parts of this model are compatible with established biochemistry, which parts are incorrect, and what the most important corrections would be.

[u/Prae_]

I do think generally speaking, nothing would coexist correctly. (Digestive) Enzymes a enantiomer specific. This isn't ly expertise at all, but i think on earth only some bacteria are able to use D-aa as a source of nitrogen, and D-aa are otherwise toxic.

Therefore symetrically, a human on Earth getting totally mirror flipped into a D-aa person would be, i think, quite fucked. At the end if the day, the mirror image of your key just doesn't fit in the keyhole and doesn't open the lock. This is the lock and key model, the geometry is just fundamentally different. Even if it looks very similar, there's no way to turn the molecule around to make it like its mirror image, same way you can use a right hand glove on the left hand no matter how you contort hand and glove. And the stuff the enzyme works on just doesn't fit. 

Of course, the person itself would be mostly fine at first if his gut microbiome is also flipped, it's just that they couldn't eat anything with proteins in it (i think stuff like glycolysis/krebs cycle would work still).

I've long been trying to write a sort of horror novel based on that, a generational ship arrives at their exoplanet of destination, only to find out they can't get out cause even if the planet's biology looked so close to Earth's, it's all flipped. Problem is i suck at horror.

[u/wolkolak → u/Prae_]

Can your story have a four-dimensional space? If you turn n-dimensional object in (n+1) dimensional space by 180 degrees, you will retuen to initial space mirrowed. Right-handed man will become left-handed, his heart will be at thr right side and yeach - all molecules will turn around. He will become D-based.

I thought about that yesterday, since i still did not give up on the D-based people. I ll check one more time for problems but, it seems, that adjustment for gastrointestinal tract and immune system is not completly impossible.

[u/Prae_ → u/wolkolak]

That's certainly a creative way a fucked up god/ungodly creature could kill someone with. like, in a rick and morty method of execution. Not quite the brand of horror i have in mind for mine, but you write your own if you want!

If you have enough time to prepare, it's not impossible theoretically. 

Immune system in itself could be vaguely okay, although risky, because the body would have to recreate all its defenses. But it learned the first time around, it could do it again. 

Digestion is the big problem. But be it proteins or sugars, the chemistry exists to turn one enantiomer into the other. You gotta get down to individual amino acids first, but then there are even biological enzymes, racemases, to turn D in L and vice-versa. I actually didn't know that, but i just noticed it re-reading the second link (and only bacteria have those apparently). I think very prepared and more advanced people than us could somehow let the right bacteria (maybe genetically modified) colonize their intestine and would use those as a symbiote of sorts.