And so when one of those active sites Is filled with the substrate molecules, such as the aspartate, the other active sites become much more likely to actually bind to that substrate molecule. That's why we have this sigmoil curve. So, this cooperative behavior implies that atcase must consist of multiple subunits and hence multiple active sites. It has multiple active sites. As a substrate binds onto one of those active sites, it changes the affinity of the other active sites for that substrate molecule. And this is due to the interaction between the different subunits.