MET - Enzymes [LESSON]

Enzymes

In Module 1, we learned about the structure and function of enzymes. Please go back and review this lesson before proceeding.

Now, we will learn how enzymes affect reactions. Recall from the previous lesson that exergonic reactions are reactions that have a negative change in free energy (-∆G) from reactants to products.  Please also recall that it takes energy input to break bonds and achieve a movement to the transition state, which is why all reaction pathways have a hill in the middle. This is called the activation energy.

Activation Energy

Activation energy is the energy required for a reaction to occur and also determines its rate. Many chemical reactions, and almost all biochemical reactions, do not occur spontaneously and must have an initial input of energy (called the activation energy) to get started. Activation energy must be considered when analyzing both endergonic and exergonic reactions. Exergonic reactions have a net release of energy, but they still require a small amount of energy input before they can proceed with their energy-releasing steps. This small amount of energy input necessary for all chemical reactions to occur is called the activation energy (or free energy of activation) and is abbreviated EA.

The activation energy of a chemical reaction is closely related to its rate. Specifically, the higher the activation energy, the slower the chemical reaction will be. This is because molecules can only complete the reaction once they have reached the top of the activation energy barrier. The higher the barrier is, the fewer molecules that will have enough energy to make it over at any given moment. During chemical reactions, certain chemical bonds are broken and new ones are formed. For example, when a glucose molecule is broken down, bonds between the carbon atoms of the molecule are broken. Since these are energy-storing bonds, they release energy when broken. However, to get them into a state that allows the bonds to break, energy must be added to achieve the transition state. It is a high-energy, unstable state. For this reason, reactant molecules don't last long in their transition state, but very quickly proceed to the next steps of the chemical reaction.

Enzymes

When an exergonic (already spontaneous) chemical reaction occurs, it often occurs too slowly to provide products fast enough for the needs of the organism. Recall that enzymes are proteins that function as biological catalysts. They affect the activation energy of the reaction by lowering it. 

Image shows 2 reaction pathways

When an enzyme binds its substrate, it forms an enzyme-substrate complex. This complex lowers the activation energy of the reaction and promotes progression by providing certain ions or chemical groups that actually form covalent bonds with molecules as a necessary step of the reaction process. Enzymes also promote chemical reactions by bringing substrates together in an optimal orientation, lining up the atoms and bonds of one molecule with the atoms and bonds of the other molecule. This can contort the substrate molecules and facilitate bond-breaking. The active site of an enzyme also creates an ideal environment, such as a slightly acidic or non-polar environment, for the reaction to occur. All of these ensure that the transition state of the reaction is stabilized, reducing free energy (G) of the transition state.  As you can see in the image above, the “hill” or activation energy of the reaction is lowered. 

Watch the Enzyme Energetics video below to learn more about how enzymes affect exergonic reactions.

Feedback Inhibition

Feedback inhibition is when a reaction product is used to regulate its own further production. Cells have evolved to use feedback inhibition to regulate enzyme activity in metabolism, by using the products of the enzymatic reactions to inhibit further enzyme activity. Metabolic reactions, such as anabolic and catabolic processes, must proceed according to the demands of the cell. In order to maintain chemical equilibrium and meet the needs of the cell, some metabolic products inhibit the enzymes in the chemical pathway while some reactants activate them.

The image shows a series of three enzymes in succession.

 

Metabolic pathways are a series of reactions catalyzed by multiple enzymes. Feedback inhibition, where the end product of the pathway inhibits an earlier step, is an important regulatory mechanism in cells. We will look at examples of pathways that include multiple enzymes in sequence later in this module.

Try the Enzymes Practice below to see what you know.

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