Well, I am not specified in this area, but I can tell you that everything has a specific shape, respectively. So, knowing that, if a protein was not in the "Isomer L" position, and was changed to an "Isomer D" position, then it would now longer be a protein, but another molecule. I was discussing this with a teacher, but it seems that even if you switch the position of an isomer, although it has the same "shape" as the original isomer, it becomes and acts in a totally different way.
Hope this helps,
The Cancer Curer