A typical enzyme kinetics curve for a non-allosteric enzyme is shown in the graph: An explanation for the shape of the enzyme kinetics curve At low substrate concentration the reaction rate increases sharply with increasing substrate concentration because there abundant free enzyme available E to bind added substrate. At high substrate concentration, the reaction rate reaches a plateau as the enzyme active sites become saturated with substrate ES complex , and no free enzyme to bind the added substrate.
In the graph reaction rate vs substrate concentration, the reason that the curve reaches a plateau, and does not increase any further at high substrate concentration is that:. Cronk Syllabus Topics. Catalysis and enzymes. Reaction coordinate diagrams, mechanistic models, rate constants and kinetic parameters. The study of biological catalysis and enzymes is something near and dear to many biochemists. We look at how the principles of chemical kinetics apply to catalysts, including enzymes.
We'll find we can construct a simple mechanism for an enzyme-catalyzed reaction. Catalysts are notable for their ability to greatly speed up a reaction despite being in many cases present in substoichiometric amounts. Some of the important features of catalysts are summarized below:. Catalysts can be participants in a reaction mechanism, combine with reactants to form intermediates, but free catalyst is regenerated, which can then undergo another round of catalysis.
This relates primarily - again - to the participation of the catalyst. A catalyst can speed a bimolecular reaction by bringing the reacting molecules together - providing adjacent, properly-oriented binding sites for the reactants, for example - but it may also provide a completely different mechanistic pathway by reacting to form intermediates that are not accessible in an uncatalyzed mechanism.
This is a thermodynamic principle. Catalysts do not affect the thermodynamics of a reaction, which determine the equilibrium constant "Big K " for the reaction. Catalysts affect the reaction kinetics by increasing the rates little k 's for both the forward and reverse reactions. Enzymes are amazing from the standpoint of how greatly they accelerate reaction rates in biological systems, their exquisite specificity no unintended side products, as in synthetic organic chemistry!
Reaction coordinate diagrams for catalyzed reactions. An explanation for the ability of a catalyst to speed up a reaction is that it can lower the activation energy of the reaction. This can be nicely illustrated using a reaction coordinate diagram. Let us consider the diagram at left to represent an elementary reaction that can take place with or without catalysis.
The red curve shows the energy profile for the uncatalyzed reaction. The activation energy for uncatalyzed conversion to products is much greater than that for the catalyzed reaction indigo curve. This means that the rate constant for the catalyzed reaction, k cat , will be much greater than k uncat , the rate constant for the uncatalyzed reaction. An enzyme exhibits maximum activity over the narrow pH range in which a molecule exists in its properly charged form.
With the notable exception of gastric juice the fluids secreted in the stomach , most body fluids have pH values between 6 and 8. Not surprisingly, most enzymes exhibit optimal activity in this pH range. However, a few enzymes have optimum pH values outside this range.
For example, the optimum pH for pepsin, an enzyme that is active in the stomach, is 2. Initially, an increase in substrate concentration leads to an increase in the rate of an enzyme-catalyzed reaction.
As the enzyme molecules become saturated with substrate, this increase in reaction rate levels off. The rate of an enzyme-catalyzed reaction increases with an increase in the concentration of an enzyme. At low temperatures, an increase in temperature increases the rate of an enzyme-catalyzed reaction.
At higher temperatures, the protein is denatured, and the rate of the reaction dramatically decreases. An enzyme has an optimum pH range in which it exhibits maximum activity. In non-enzyme-catalyzed reactions, the reaction rate increases as the concentration of reactant is increased.
In an enzyme-catalyzed reaction, the reaction rate initially increases as the substrate concentration is increased but then begins to level off, so that the increase in reaction rate becomes less and less as the substrate concentration increases. Explain this difference. An enzyme has an optimum pH of 7. What is most likely to happen to the activity of the enzyme if the pH drops to 6.
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