Substrate (chemistry) - Wikipedia
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Enzymes are usually proteins, though some ribonucleic acid RNA molecules act as enzymes too. Enzymes perform the critical task of lowering a reaction's activation energy —that is, the amount of energy that must be put in for the reaction to begin.
Enzymes work by binding to reactant molecules and holding them in such a way that the chemical bond-breaking and bond-forming processes take place more readily. Reaction coordinate diagram showing the course of a reaction with and without a catalyst. With the catalyst, the activation energy is lower than without.
Instead, enzymes lower the energy of the transition state, an unstable state that products must pass through in order to become reactants.
The transition state is at the top of the energy "hill" in the diagram above. Active sites and substrate specificity To catalyze a reaction, an enzyme will grab on bind to one or more reactant molecules. These molecules are the enzyme's substrates. In some reactions, one substrate is broken down into multiple products. In others, two substrates come together to create one larger molecule or to swap pieces. In fact, whatever type of biological reaction you can think of, there is probably an enzyme to speed it up!
A substrate enters the active site of the enzyme. This forms the enzyme-substrate complex.
The reaction then occurs, converting the substrate into products and forming an enzyme products complex. The products then leave the active site of the enzyme.
Image modified from " Enzymes: The catalytic site of the enzyme is empty, waiting for substrate to bind, for much of the time, and the rate at which product can be formed is limited by the concentration of substrate which is available.
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.
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.
Enzymes: principles and biotechnological applications
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. If two enzymes, in different pathways, compete for the same substrate, then knowing the values of Km and Vmax for both enzymes permits prediction of the metabolic fate of the substrate and the relative amount that will flow through each pathway under various conditions.
In order to determine the amount of an enzyme present in a sample of tissue, it is obviously essential to ensure that the limiting factor is the activity of the enzyme itself, and not the amount of substrate available.
This means that the concentration of substrate must be high enough to ensure that the enzyme is acting at Vmax.