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
All the following enzymes catalyze physiologically irreversible reactions of glycolysis except 
Select and explain. 
A. Hexokinase, B. Phosphofructokinase, C. Pyruvate kinase, D. Enolase
D: Enolase
Answer: D: Enolase 
Explanation: GLYCOLYSIS IS REGULATED AT THREE STEPS INVOLVING NONEQUILIBRIUM REACTIONS Although most of the reactions of glycolysis are freely reversible, three are markedly exergonic and must, therefore, be considered to be physiologically irreversible. These reactions, catalyzed by hexokinase (and glucokinase), phosphofructokinase, and pyruvate kinase, are the major sites of regulation of glycolysis. Reference: Harper; 30th edition; Page no: 172