In this article we will discuss about the Michaelis-Menten Constant and Significance of Michaelis-Menten Constant.
In an enzyme catalysed reaction when there is large excess of substrate and the enzyme concentration is held constant, if substrate concentration (S) is plotted against velocity (V) or reaction rate, a hyperbolic curve is obtained (fig. 10.13). This type of plot is also known as saturation plot.
In the beginning, there is approximately direct proportionality between substrate concentration and reaction rate until the enzyme concentration becomes limiting and a steady state is obtained. In this situation addition of more substrate will not increase reaction rate because all the active sites of the enzyme are saturated with substrate molecules and the rate of reaction will increase only by addition of more enzyme.
The concentration of substrate required to half saturate the enzyme or in other words to cause half the maximal reaction rate (1/2 Vmax) called as Michaelis Constant or Michaelis-Menten Constant and is denoted by Km. Michaelis constant is a reflection of the affinity of enzyme for its substrate and is characteristic of a particular enzyme-substrate system.
The smaller the value of Km, the more strongly the enzyme binds the substrate. An enzyme that catalyses a reaction between two or more different substrates has different Km value for each of the substrate.
Kinetic values of enzyme catalysed reactions are usually measured under steady state conditions and described by a simple expression called Henri-Michaelis-Menten, equation.
V = Vmax[S]/Km+ [S]
where, V = velocity or reaction rate (in units such as moles l-1s-1)
Vmax = maximum velocity or maximal reaction rate (at oc substrate conc.)
S = Substrate concentration
Km = Michaelis constant.
Although Km values are more or less constants for particular enzyme-substrate systems, but these may vary slightly with pH, temperature, ionic strength and also with types and amount of coenzymes when required for the reaction.
The values of Km are measured in terms of molarity. Typically, the values of Km for most enzymes studied so far range between 10-3 to 10-6 molar (1mM – 1 µM). Km values for some enzyme-substrate pairs are given in Table 10.2.
Significance of Michaelis-Menten Constant:
There are many advantages of knowing the Km values of enzyme-substrate systems:
(i) By knowing the Km value of a particular enzyme-substrate system, one can predict whether the cell needs more enzymes or more substrate to speed up the enzymatic reaction.
(ii) If an enzyme can catalyse a reaction with two similar substrates (e.g., glucose and fructose) in the cell, it will prefer that substrate for which the enzyme has lower Km value.
(iii) Km value gives an approximate measure of the concentration of substrate of the enzyme in that part of the cell where reaction is occurring. For instance, those enzymes which catalyse reactions with relatively more concentrated substrates (such as sucrose), usually have relatively high Km value. On the other hand, the enzymes that react with substrates which are present in very low concentrations (such as hormones) have comparatively lower Km values for the substrates.