And these are shown in red. And notice that each one of these enzymes contains this Crevice and that's the active site to which a substrate can actually bind. Now, if we take this enzyme mixture and we add a concentration of substrate that is equal to Km, the Macalus constant, exactly half of these active sites will be filled. So in this case, if we add two of these blue molecules, then that means two of these enzymes will form the enzyme substrate complex, as shown in this diagram. And so this means that exactly half of the active sites will be filled. And another way of saying this is in the following manner.