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D-glucose most common in nature and L-glucose is synthesized in the lab.

I know that humans can't use L-glucose in their aerobic pathways because it doesn't match the active site of the enzyme ...but I'd like to confirm and understand:

Why can't our cells use L-glucose?

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    $\begingroup$ You can't put a left shoe on your right foot. Same thing here. Enzymes are chiral too. $\endgroup$ – Ivan Neretin Oct 3 '20 at 18:53
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    $\begingroup$ Alice speaking to Kitty. ‘How would you like to live in Looking-glass House, Kitty? I wonder if they’d give you milk in there? Perhaps Looking-glass milk isn’t good to drink—' I have always felt that Lewis Carroll was aware of the mirror image world. Is milk (lactose) on the other side comprised of L-glucose and L-galactose? Is that why Alice questions whether it is good to drink? @Ivan Neretin $\endgroup$ – user55119 Oct 4 '20 at 3:18
  • $\begingroup$ Wikipedia has a pretty good answer; en.wikipedia.org/wiki/L-Glucose. It includes one species able to digest L-Glucose, and the information that the taste is the same (which makes you wonder about those chiral taste receptors). $\endgroup$ – Karsten Theis Mar 2 at 22:33
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It is like the problem of the key and the lock. If you put the good key in a given lock, it will go all right.It will open the lock. But if you make up a new key which is the image of the original key, it will not open the lock.

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  • $\begingroup$ Therefore, not so neither an enzyme cannot catalyze both L and D stereochemical structures. $\endgroup$ – onat Oct 3 '20 at 18:50
  • $\begingroup$ Right. An enzyme can react with only one variety, either L-glucose, or D-glucose, but not both. $\endgroup$ – Maurice Oct 3 '20 at 19:15
  • $\begingroup$ I don't know that this is 100% true. Sure, all known natural enzymes are enantiomeric, but that doesn't mean that if they react w/ D-glucose they won't react with L-glucose at all. They very well may react to some extent. $\endgroup$ – Curt F. Oct 4 '20 at 3:28
  • $\begingroup$ Hmm, the difference between L and D glucose is significantly bigger than between D-glucose and other natural simple sacharides, with which they do not react. Enzymes are very selective. $\endgroup$ – Poutnik Oct 4 '20 at 7:33
  • $\begingroup$ @Maurice. I agree with Curt F. Racemases and epimerases, by their very nature, are able to act on either stereoisomer. $\endgroup$ – Karsten Theis Mar 2 at 22:36

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