B As the concentration of substrate increases, the enzyme becomes saturated with substrate. As soon as the catalytic site is empty, more substrate is available to bind and undergo reaction.
The rate of formation of product now depends on the activity of the enzyme itself, and adding more substrate will not affect the rate of the reaction to any significant effect. The rate of reaction when the enzyme is saturated with substrate is the maximum rate of reaction, Vmax. The relationship between rate of reaction and concentration of substrate depends on the affinity of the enzyme for its substrate.
This is usually expressed as the Km Michaelis constant of the enzyme, an inverse measure of affinity. For practical purposes, Km is the concentration of substrate which permits the enzyme to achieve half Vmax. An enzyme with a high Km has a low affinity for its substrate, and requires a greater concentration of substrate to achieve Vmax. The Km of an enzyme, relative to the concentration of its substrate under normal conditions permits prediction of whether or not the rate of formation of product will be affected by the availability of substrate.
An enzyme with a low Km relative to the physiological concentration of substrate, as shown above, is normally saturated with substrate, and will act at a more or less constant rate, regardless of variations in the concentration of substrate within the physiological range. An enzyme with a high Km relative to the physiological concentration of substrate, as shown above, is not normally saturated with substrate, and its activity will vary as the concentration of substrate varies, so that the rate of formation of product will depend on the availability of substrate.
AChE is a serine hydrolase that reacts with acetylcholine at close to the diffusion-controlled rate. The Michaelis-Menten model is used in a variety of biochemical situations other than enzyme-substrate interaction, including antigen-antibody binding, DNA-DNA hybridization, and protein-protein interaction.
It can be used to characterize a generic biochemical reaction, in the same way that the Langmuir equation can be used to model generic adsorption of biomolecular species. When an empirical equation of this form is applied to microbial growth. The rate of product formation is dependent on both how well the enzyme binds substrate and how fast the enzyme converts substrate into product once substrate is bound.
For a kinetically perfect enzyme, every encounter between enzyme and substrate leads to product and hence the reaction velocity is only limited by the rate the enzyme encounters substrate in solution. These included:. It also gives a quick, visual impression of the different forms of enzyme inhibition.
The reaction between nicotineamide mononucleotide and ATP to form nicotineamide—adenine dinucleotide and pyrophosphate is catalyzed by the enzyme nicotinamide mononucleotide adenylyltransferase. The following table provides typical data obtained at a pH of 4. Figure Using the y -intercept, we calculate V max as.
The following data are for the oxidation of catechol the substrate to o -quinone by the enzyme o -diphenyl oxidase. A competitive inhibitor binds to the activation site of an enzyme blocking the substrate. In this way, the inhibitor competes with the substrate to bind to the enzyme site. Allowing high concentration of the competitive inhibitor ensures the binding to the site.
Hence, the competitive inhibitor changes the dynamics of the enzymatic rate. First, the inhibitor modifies the slope and the x-intercept Km creating a much steeper slope. However, the maximum rate, Vmax, stays the same. On the other hand, a noncompetitive inhibitor binds at a different site than the activation site of the enzyme and does not compete with the substrate. The inhibitor modifies the structural components of the activation site preventing the substrate or another molecule from binding to the site.
This change impacts the affinity of the substrate to the enzyme. Noncompetitive inhibitors change the slope and the y-intercept of the Lineweaver-Burk plot, decreasing the Vmax while increasing the y-intercept with a steeper slope. However, the x-intercept remains the same. While the Lineweaver-Burk plot is useful in many ways, the line plot has limitations. Unfortunately, the plot begins to distort rates at very high or low substrate concentrations, creating extrapolations on the plot.
0コメント