And so, because we can't modify these amino acids in this way, the structure of collagen basically is destabilized. And what that means is it decreases the strength of our tissue. Now let's move on to carboxyl groups. So once again, many different types of proteins inside the body can be modified by the addition of carboxyl groups. And one particular example is a protein and enzyme in the blood clotting cascade we call prothrombin. So prothrombin is needed to basically stop bleeding.