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Enzyme kinetics studies enzyme-catalysed processes. Enzyme kinetics measures the reaction rate and the impact of varying reaction circumstances. Enzyme kinetics can disclose its catalytic mechanism, involvement in metabolism, how its activity is controlled, and how a drug or modification (inhibitor or activator) might influence the rate.
Enzyme kinetics
Enzyme kinetics is the study of how fast chemical reactions happen when an enzyme is present. In enzyme kinetics, the rate of a reaction is measured, and the effects of changing the reaction’s conditions are looked into. When you study the kinetics of an enzyme in this way, you can learn about its catalytic mechanism, its role in metabolism, how its activity is controlled, and how a drug or a modifier (an inhibitor or an activator) might change the rate.
An enzyme (E) is usually a protein molecule that helps another molecule, called its substrate, do what it needs to do (S). This binds to the active site of the enzyme to make an enzyme-substrate complex (ES), which changes into an enzyme-product complex (EP) and then to a product (P) via a transition state (ES*). The name for the set of steps is the mechanism:
E + S ⇄ ES ⇄ ES* ⇄ EP ⇄ E + P
This example is based on the simplest kind of reaction, where there is only one substrate and one product. There are enzymes that do this. For example, phosphoglucomutase transfers a phosphate group from one place to another. Isomerase is a more general term for an enzyme that speeds up any one-substrate, one-product reaction, like triosephosphate isomerase. But these enzymes don’t happen very often, and they are outnumbered by enzymes that speed up reactions with two substrates and two products. For example, NAD-dependent dehydrogenases like alcohol dehydrogenase speed up the oxidation of ethanol by NAD+. Less often, reactions have three or four substrates or products, but they do happen. There is no rule that says the number of products has to match the number of substrates. For example, glyceraldehyde 3-phosphate dehydrogenase has three substrates and two products.
When an enzyme like dihydrofolate reductase (shown on the right) binds to more than one substrate, enzyme kinetics can also show the order in which these substrates bind and the order in which products are made. Proteases, which cut one protein substrate into two polypeptide products, are an example of enzymes that bind to a single substrate and make more than one product. Others, like DNA polymerase, join two substrates together. For example, DNA polymerase links a nucleotide to DNA. Even though these mechanisms are usually made up of a long list of steps, there is usually one step that controls the rate of the whole thing. This step can be a chemical reaction or a change in the shape of the enzyme or substrates, such as those that cause the enzyme to release its product(s).
Kinetic data are easier to understand if you know how the enzyme is put together. For example, the structure can show how substrates and products bind during catalysis, what changes happen during the reaction, and even what role certain amino acid residues play in the mechanism. Some enzymes change shape a lot during the mechanism. In these cases, it is helpful to figure out the structure of the enzyme with and without bound substrate analogues that do not go through the mechanism.
Not all catalysts in living things are protein enzymes: Many cellular processes, like RNA splicing and translation, need RNA-based catalysts like ribozymes and ribosomes. The main difference between ribozymes and enzymes is that ribozymes are made of nucleotides and enzymes are made of amino acids. Ribozymes can only do a smaller number of reactions, but the same methods can be used to study and classify their reaction mechanisms and kinetics.
General Principles
As more substrate is added to a reaction, the binding sites on the enzymes are filled up to the limit of V max V max. Once this limit is reached, the enzyme is full of substrate, and the reaction rate stops going up.
The reaction that an enzyme speeds up uses the same reactants and makes the same products as the reaction that doesn’t use an enzyme. Like other catalysts, enzymes do not change where substrates and products are in equilibrium. But enzyme-catalysed chemical reactions are different from those that don’t have a catalyst because they show saturation kinetics. For a given concentration of enzymes and relatively low concentrations of substrates, the reaction rate goes up linearly with the concentration of substrates. This is because the enzyme molecules are mostly free to speed up the reaction, and as the concentration of substrates goes up, the rate at which enzyme and substrate molecules meet each other goes up as well. But when there are a lot of substrates, the reaction rate gets close to the theoretical maximum. This is because almost all of the active sites on the enzyme are filled with substrates, and the reaction rate is set by how fast the enzyme turns over on its own. KM stands for the concentration of substrate that is halfway between these two limits. So, KM is the concentration of the substrate at which the reaction speed is half as fast as it can be.
Michaelis-Menten Kinetics
Michaelis-Menten kinetics is a model of enzyme kinetics that shows how the rate of a reaction that an enzyme speeds up depends on how much of the enzyme and its substrate are present. Let’s look at a reaction in which a substrate (S) binds reversibly to an enzyme (E) to make an enzyme-substrate complex (ES), which then reacts irreversibly to make a product (P) and release the enzyme.
S + E ⇌ ES → P + E
In Michaelis-Menten kinetics, there are two important terms:
Vmax is the rate of the reaction at its fastest point, when all of the enzyme’s active sites are full of substrate.
Km, also called the Michaelis constant, is the concentration of the substrate at which the reaction rate is half of the maximum rate, Vmax. The affinity an enzyme has for its substrate is measured by Km. The lower the value of Km, the better the enzyme is at doing its job at a lower concentration of substrate.
For the reaction above, the Michaelis-Menten equation is:
This equation shows how the initial concentration of the substrate affects the initial rate of the reaction (V) ([S]). It assumes that the reaction is in its steady state, in which the amount of ES stays the same.
Conclusion
From the following article we can conclude that Enzyme kinetics is the study of the rates of chemical processes catalysed by enzymes. In enzyme kinetics, the reaction rate is evaluated and the consequences of changing the reaction conditions are studied. This method of studying the kinetics of an enzyme can show the enzyme’s catalytic mechanism, its involvement in metabolism, how its activity is regulated, and how a drug or modifier (inhibitor or activator) might influence the rate.
