Polarization of molecules in body

[sontag]

Why do all the protein molecules in our bodies
rotate polarized light in one direction only?
If I injected proteins of the opposite polarization
into my bloodstream would their be a strong immune reaction?
Also,what would happen if one of the organs in my
body - for example the liver - was made from proteins
with the"wrong" polarization?

 

[Monique]

There are different enantiomers because certain amino groups have chiral carbon atoms, the body only uses one of them because molecular interactions are very specific so the other shoe would not fit.

I am not sure how asymmetric the presence of enantiomers is in the cell, either the body only selectively produces one or actively degrades the other form (there are enzymes known that regulate this).

There would be no immune reaction, the immune system needs a 'danger signal' in order to mount an immune response. Although there will be a small number that might produce a response.

I am not an expert on enantiomers, but I don't think there always needs to be a problem. It depends on the kind of interaction between molecules. If it is only electrostatic, there might not be a problem?

There IS one very good example of how one molecule with two polarizations can have a very different function, that is the drug Thalidomide. One is active against morning sickness in pregnant women, the other leads to birth defects (Thalidomide children).

 

[Almeisan]

Because left hand and right hand molecules are different.

Generally, all amino acids(protein) are left handed. This is because we all evolved from a common ancestor. Or, we all evolved from that one left handed amino acid. If we would try to make a amino acids you have a 50/50 chance of it being lefthanded. Same goes for that first amino acid. It could just as well have been a right handed one


If I would eat an apple from a parallel universe that did have that right handed amino acid instead of the left handed one it would be impossible to digest the apple. My enzymes wouldn't fit on the apple protein, just as a left foot doesn't fit a right-handed shoe.

It is also impossible to replace a left handed amino acid with a right handed one and get the same protein with the same function. A protein made up of a thousand amino acids could fold very different or not at all if you replace only one left handed AA with a right handed one. So you can only make a protein with either only left or right handed AA. And it will only function in a left or right handed organism.

Now I am not a biochemist so I might miss some of the nuances and exceptions.

 

[DaveC426913]

Isn't that the difference between sugar and aspartame? Isn't aspartame the molecular mirror image of sugar? We can taste it but we can't metabolize it because our tastebuds are activated by one part of the molecule but our bodys need to interact with another part.

Wait, that seems implausible; sugar molecules are pretty simple.

 

[Monique]

There are a number of molecules that fall into the sugar category and aspartame is not a mirror image of any sugar. Aspartame DOES contain chiral components: phenylalanine and aspartic acid. When the aspartame molecules does not have the right shape it won't fit into the taste receptor and not taste sweet, that is why the molecule is purified to the L enantiomer. The artificial sweetner is 200 times sucrose, you need to use less so there are less calories. I think that it is metabolized just fine, it is hydrolyzed to aspartic acid, phenylalanine and methanol. People who have the genetic disorder phenylketonuria are advised not to use aspartame, since they can not metabolize phenylalanine.

 

[nipwoni]

It wasn't aspartame, but somebody a few years ago was trying to develop a left-handed sugar (living things use right-handed sugars) that would be sweet but would provide no calories. Unfortunately, I think the testing petered out because of problems getting FDA approval or something. I'd like to taste some!