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Our body only digest dextro(Right handed) glucose and levo(Left handed) proteins and can't digest Levo glucose and dextro proteins. Why?

Our body synthesis levo proteins. What is the reason?

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  • $\begingroup$ Because of different proteins taking care of each. $\endgroup$ – AliceD Jan 20 '17 at 11:04
  • $\begingroup$ Great question - AliceD is correct that our enzymes exhibit stereospecificity for each, but there is also another vital reason why humans utilize D-glucose and L-amino acids... will post as an answer $\endgroup$ – Vance L Albaugh Jan 20 '17 at 17:58
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    $\begingroup$ I am not clear what your question is. Are you asking why only one stereoisomer is used in each case, or are you asking why it is D-sugars and L-amino acids (rather than D-amino acids)? $\endgroup$ – David Jan 21 '17 at 13:04
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Our body digests only certain type of enantiomers like L amino acid and D sugars because enzymes are specific for their substrates stereochemistry meaning they will only recognize a particular enantiomer.So I hope this answers to your first question. Now, why we only see L amino acids and D sugars, there is some interesting hypothesis listed in this article.Here is the link http://guava.physics.uiuc.edu/~nigel/courses/569/Essays_Fall2006/files/Rajan.pdf

I am adding some answers I found from research gate here, which includes 1)I think the prevalence of L-amino acids in biological systems is a quirk of our evolutionary history; there's no reason D-amino acids wouldn't work chemically, but once early life forms started building proteins out of L-amino acids, L-amino acids became abundant an it became selectively advantageous to use L-amino acids because then you could use the amino acids you obtained by eating someone else. It's a matter of compatibility; if you use D-amino acids, you have to build them all from scratch, because you're not going to find them in your environment. As long as you use L-amino acids, you don't even need to maintain the de novo synthetic pathways; you can just eat something that has those pathways (hence the 'essential' dietary amino acids). 2)There are a couple of reasons for biological homochirality. One, our galaxy has a chiral spin and a magnetic orientation, which causes cosmic dust particles to polarize starlight as circularly polarized in one direction only. This circularly polarized light degrades D enantiomers of amino acids more than L enantiomers, and this effect is clear when analyzing the amino acids found on comets and meteors. This explains why, at least in the milky way, L enantiomers are preferred. Two, although gravity, electromagnetism, and the strong nuclear force are achiral, the weak nuclear force (radioactive decay) is chiral. During beta decay, the emitted electrons preferentially favor one kind of spin. That's right, the parity of the universe is not conserved in nuclear decay. These chiral electrons once again preferentially degrade D amino acids vs. L amino acids.

Thus due to the chirality of sunlight and the chirality of nuclear radiation, L amino acids are the more stable enantiomers and therefore are favored for abiogenesis. 3) Morozov, L. 1979. Mirror symmetry breaking in biochemical evolution. Origins Life Evol Biosphere 9: 187-217

But I think we have D amino acids also, as this paper says https://www.hindawi.com/journals/scientifica/2016/6494621/

Hope this helps!

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