Visual Interpretation of the Meaning of kcat/KM in Enzyme Kinetics

kcat/KM is a fundamental kinetic parameter as important as kcat in enzyme kinetics. Although the parameter provides critical information on how the enzyme lowers the energy barriers along the reaction pathway for catalysis, kcat/KM is frequently considered as a composite or derived parameter to assess both kcat and KM together rather than as a fundamental kinetic parameter. The main challenge of teaching the meaning and the value of kcat/KM to students is the difficulty in explaining how the reaction energetics of enzyme catalysis determine kcat/KM. This report demonstrates a step-by-step process to visualize the meaning of kcat/KM on the reaction free energy diagram. The reciprocal form of the expression of kcat/KM shows that kcat/KM is a harmonic sum of kinetic terms that correspond to the heights of the transition states relative to the free enzyme. Then, it is demonstrated that the height of the highest transition state has the dominant influence on kcat/KM. The visualization of the meaning of kcat/KM on the reaction free energy diagram offers an intuitive way to understand all the known properties of kcat/KM, including the Haldane relationship, which will greatly enhance the students' learning of the fundamental concepts in enzyme kinetics.