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?