Key Difference – Kd vs Km
Kd and Km are equilibrium constants. The key difference between Kd and Km is that Kd is a thermodynamic constant whereas Km is not a thermodynamic constant.
Kd refers to dissociation constant while Km is the Michaelis constant. Both these constants are very important in the quantitative analysis of enzymatic reactions.
What is Kd?
Kd is dissociation constant. It is also known as equilibrium dissociation constant due to its use in equilibrium systems. The dissociation constant is the equilibrium constant of reactions where a large compound is converted into small components reversibly. The process of this conversion is also known as dissociation. An ionic molecule always dissociates into its ions. Then the dissociation constant or Kd is a quantity expressing the extent to which a particular substance in solution dissociates into ions. Thus, this is equal to the product of the concentrations of the respective ions divided by the concentration of the un-dissociated molecule.
AB ↔ A + B
In the above general reaction, the dissociation constant, Kd can be given as below.
Kd = [A][B] / [AB].
Moreover, if there is a stoichiometric relationship, one should include the stoichiometric coefficients in the equation.
xAB ↔ aA + bB
The equation of dissociation constant, Kd for the above reaction is as follows:
Kd = [A]a[B]b / [AB]x
Specifically, in biochemical applications, Kd helps to determine the amount of products given by a chemical reaction in the presence of an enzyme. The Kd of an enzymatic reaction expresses the ligand-receptor affinity. In other words, it states the capability of a substrate to leave the receptor of an enzyme. On the other hand, it describes how strongly a substrate binds to the enzyme.
What is Km?
Km is the Michaelis constant. Unlike Kd, Km is a kinetic constant. Its main application is in enzyme kinetics, that is, to determine the affinity of a substrate to bind with an enzyme. The constant is expressed by relating the substrate concentration to the reaction rate in the presence of an enzyme. Accordingly, the Michaelis constant or Km is the concentration of the substrate when the speed of the reaction reaches the half of its maximum speed.
During a reaction between enzyme (E) and substrate (S), the formation of products (P) is as follows:
E + S ↔ E-S complex ↔ E + P
If the equilibrium constants of above reaction are as follows, you can derive Km from these constants.
Km = K-1 + K+2 / K+1
Determination of Km According to Michael’s Concept
Michaelis developed a relationship using the concentration of substrate, [S] and the maximum reaction velocity, Vmax. The relationship between substrate concentration and Km of an enzymatic reaction is as follows:
v = Vmax[S] / Km + [S]
v is velocity at any time, while [S] is the substrate concentration at a particular time, and Vmax is the maximum velocity of the reaction. Km is the Michaelis constant for the enzyme in the reaction. The value of the Michaelis constant depends on the enzyme. Consequently, a small value of Km indicates that the enzyme becomes saturated with a small amount of substrate. Then the Vmax is obtained at a low substrate concentration. In contrast, a high Km value indicates that the enzyme requires a high amount of substrate to become saturated.
What is the Difference Between Kd and Km?
Kd vs Km
|Kd is the dissociation constant.||Km is the Michaelis constant.|
|Kd is a thermodynamic constant.||Km is a kinetic constant.|
|Kd represents the affinity of a substrate towards an enzyme.||Km represents the relationship between substrate concentration and reaction speed.|
Summary – Kd vs Km
Kd and Km are equilibrium constants that describe properties of enzymatic reactions. The key difference between Kd and Km is that Kd is a thermodynamic constant whereas Km is not a thermodynamic constant.
1. “Michaelis–Menten Kinetics.” Wikipedia, Wikimedia Foundation, 10 Apr. 2018, Available here.
2. “Introduction to Enzymes.” Substrate Concentration (Introduction to Enzymes), Available here.
3. “Dissociation Constant.” Wikipedia, Wikimedia Foundation, 10 Apr. 2018, Available here.