Why? Well, because only the isocitrate can actually undergo the decarboxylation step that takes place in step three. So, in step two, we have an isomerization reaction that is catalyzed by a connotase. And what a connotase does is it basically transfers this hydroxyl group, shown in orange, from this carbon onto this carbon here. So that's the only difference between this citrate molecule and the isomer isocytrate. But now this molecule is actually activated, and it's ready to undergo the decryptoxylation step that takes place in step three.