This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Structure and function of protein, Subject name: Biochemistry
Enzyme catalyzing reversible step in glycolysis is/are 
Select and explain. 
A. Phosphofructokinase, B. Enolase, C. Pyruvate kinase, D. Phospho - glyceromutase
B: Enolase
Answer: B: Enolase 
Explanation: In step 8 of glycolysis i.e 2- phosphoglycerate is conveed to phosphoenol pyruvate by the enzyme enolase.One water moleculeis removed. A high energy phosphate bond is produced.The reaction is reversible. Enolase requires Mg ++ , and by removing magnesium ions, fluoride will irreversibly inhibit this enzyme.Thus, fluoride will stop tge whole glycolysis.So when collecting blood for sugar estimation, fluoride is added to blood.If not , glucose is metabolised by the blood cells, so that lower blood sugar values are obtained. REFERENCE: DM.VASUDEVAN.TEXTBOOK SEVENTH EDITION ; page no: 111