3.7.1. Michaelis-Menten Kinetics

3.7.1.1. Significance of Various Parameters in Michaelis-Menten Equation
Significance of K_m- Measure of Substrate affinity:

• From Michaelis-Menten equation:   If vo is set equal to 1/2 V_max, then the relation   V_max /2 =   V_max[S]/K_m + [S] can be simplified to K_m + [S] = 2[S], or K_m = [S].  This means that at one half of the maximal velocity, the substrate concentration at this velocity will be equal to the Km.
• The significance of K_m change depends on the different rate constants and which step is the slowest rate-limiting step. In the simplest assumption, the rate of ES breakdown to product (k₂) is the rate-determining step of the reaction, so k_-1 >> k₂ and Km = k_-1/k₁.  This relation is also called dissociation constant for the ES complex and can be used as a relative measure of the affinity of a substrate for an enzyme (identical to K_d).  However if k₂ >> k_-1 or k₂ and k_-1 are similar, then K_m remains more complex and cannot be used as a measure of substrate affinity.
• K_m is a dissociation constant, so the smaller the K_m the stronger the interaction between E and S.
• Each enzyme has a characteristic K_m for a given substrate that show how tight the binding of the substrate is to the enzyme.
• Experimentally, K_m is a useful parameter for characterizing the number and/or types of substrates that a particular enzyme will utilize.
• It is also useful for comparing similar enzymes from different tissues or different organisms. Also, it is the K_m of the rate-limiting enzyme in many of the biochemical metabolic pathways that determines the amount of product and overall regulation of a given pathway.
• Clinically, K_m comparisons are useful for evaluating the effects mutations have on protein function for some inherited genetic diseases.