This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of carbohydrate, Subject name: Biochemistry
In TCA cycle, citrate is conveed into cis-aconitate by which mechanism? 
Select and explain. 
A. Loss of H+, B. Loss of phosphate, C. Loss of H2O molecule, D. Loss of carbon dioxide molecule
C: Loss of H2O molecule
Answer: C: Loss of H2O molecule 
Explanation: 2nd Step: Formation of Isocitrate, Citrate is isomerized to isocitrate by aconitase (Step 2, Fig.18.2). This reaction is a two-step process. At first, one water molecule is removed from citrate forming cis aconitate; a transient compound with a very sho half-life. Immediately, one water molecule is added to aconitate to form isocitrate. Thus the position of the hydroxyl group is shifted.Ref: DM Vasudevan, 7th edition