Because if the water molecule attacked this carbon, we would have simply reformed the citrate molecule. But if the water attacks this carbon, which is basically less hindered because it contains a smaller group on this side compared to this large group here, the water molecule is able to actually attack from this side because of less hindrance. And so once it attacks that side, we form the isocitrate molecule. So the entire point of this step is to basically prepare the citrate molecule for oxidative decorboxylation that takes place in step three as well as step four. Now, this double bonded intermediate molecule is known as cisaconitate. And because of this cysticonitate, the enzyme that catalyzes step two is known as aconitase.