Question

Why is kcat not necessarily the best way to evaluate or compare the activities of two enzymes? What “adjustment” can be made to make the term more useful and why? A) Define the rate constants k1, k2 and k-1. Given k1 of 2x10^-8/M s, k2 of 5x10^3/s, and k-1 of 1x10^3/s, what is KD? what is KM? B) Which substrate is preferred: R, for which enzyme X has a KM of 2mM? OR Q, for which X has a KM is 7mM? Explain. If k2 for Q is 2x10^4/s and for R is 4x10^5/s, which substrate yields the best catalytic efficiency? Explain.

Answer 1

Kcat means the turn over frequency or turn over number these only deals with the rate at which the enzyme-substrate complex dissociates to the product but this does not deals with the equilibrium between the enzyme and substrate with the enzyme-substrate complex. You can observe from the image below .so a new turm catalytic efficiency is introduced which is given by Kcat/ Km. Where km is Michaelis Menton constan ( see th formula in the image) . This is more effective because it deals with both the step of the reaction i.e the dissociation of ES to E and S and also the decomposition of Es to product.

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