Natural occuring D-isomer & L-isomer?

Question

Why do natural occuring carbohydrates are always in D-isomeric form whereas natural occuring amino acids are usually occuring as L-isomers?

Urgent for answers!

Answer 1

There really isn't much of a reason.  Enzymes and other biological molecules are chiral - they work on one optical isomer and not the other.  So, all molecules of a type that end up being in the biological system will have the same chirality.  But D and L refer to which way the molecule (the parent of the series) rotates light.  This is fairly arbitrary, when you change types of molecules it can change even if some other measure of one or the other isomer is staying the same.   Amino acids are very different structures from carbohydrates.  Also carbohydrates have more optical centers in them.  So, the fact that all amino acids and all carbohydrates are similar to each other has a reason, but there is no reason to expect the way the two different types of molecules rotate light to be the same or to expect it to be different.  There's not a relation.