The enzyme involved in successive dehydration and hydration reactions of Krebs cycle is

During dehydration two molecules of citric acid  by losing two molecules of water, is changed into two molecules of another 6-carbon compound cis-aconitic acid, it is catalyzed by aconitase. And also during hydration two molecules of cis-aconitic acid is converted into two molecules of iso-citric acid by utilizing water, it is catalyzed by aconitase.