Now what happens when the concentration of tryptophan rises? Now let's suppose there is a very high intracellular concentration inside that cell. So the tryptophan concentration inside is very high in this case because we do have a lot of the tryptophan molecules. We have an excess of these tryptophan molecules floating around in the cell. Some of these tryptophan molecules will act as a co repressor. They will bind onto the allosteric side of that inactive repressor protein and that will activate the protein by changing its three dimensional shape.