How Do Enzymes Change the Speed of a Chemical Reaction

Enzymes speed up chemical reactions by lowering the activation energy, the energy barrier reactants must cross to become products. They bind reactants, called substrates, at an active site and help them reach the transition state, a short-lived arrangement that occurs during the reaction.

At a given temperature, lowering this barrier allows more substrate molecules to react in a given time. The enzyme does not supply energy or change the overall energy difference between reactants and products. It changes how quickly the reaction approaches equilibrium, not where equilibrium lies Chemistry LibreTexts.

How the active site speeds up a reaction

The active site is a region of the enzyme where substrates bind and the reaction takes place. Its shape and chemical properties help the reaction proceed. For example, the site can hold substrates in a useful orientation, provide chemical groups that assist the reaction, or create a local environment that favors the transition state.

Binding is not simply a matter of keeping molecules close together. The active site interacts in a way that makes it easier for substrates to reach the transition state. This lowers the activation energy and increases the reaction rate Khan Academy.

Once the reaction is complete, the products leave the active site. The enzyme is generally available to bind more substrate and catalyze another reaction. It is not used up as a reactant Enzyme - Wikipedia.

One example: an enzyme-catalyzed reaction

Imagine the same reaction taking place in two containers at the same temperature. In one, there is no enzyme. In the other, the enzyme binds the substrate at its active site.

Without the enzyme, relatively few substrate molecules may cross the activation-energy barrier in a given period. With the enzyme, the barrier is lower, so more molecules can react in that same period. The products and the overall energy change are the same; the reaction simply happens faster with the enzyme present.

What enzymes do not change

An enzyme changes the rate of a reaction, not its thermodynamic favorability. If a reaction is reversible, the enzyme speeds up both the forward and reverse reactions. The mixture reaches equilibrium sooner, but the enzyme does not shift the equilibrium position Chemistry LibreTexts.

This distinction matters: an enzyme cannot make an unfavorable reaction favorable on its own. Cells can make a larger process proceed by coupling reactions or removing products, but that involves the surrounding chemistry, not a change in the enzyme’s effect on equilibrium.

Factors that affect enzyme activity

An enzyme’s effect depends on its structure and the conditions around it. Temperature and pH can affect the shape of the enzyme or the chemical properties of its active site. If conditions interfere with substrate binding or with the active site’s role in the reaction, the rate may fall. Each enzyme has conditions under which it works well; higher temperature does not always mean a faster enzyme-catalyzed reaction.

Substrate concentration also affects the observed rate. With a fixed amount of enzyme, adding substrate can increase the rate while active sites are available. When most active sites are occupied, the rate approaches a maximum under those conditions. Adding more substrate then has less effect; increasing the amount of active enzyme can raise the rate.

Enzymes are usually selective. An active site is suited to particular substrates and reactions, so an enzyme generally accelerates specific chemistry rather than every reaction in its surroundings Khan Academy.

Key Takeaways

• Enzymes speed up reactions by lowering activation energy.
• The active site binds substrates and helps stabilize the transition state.
• Enzymes are not used up by the reactions they catalyze.
• Enzymes change reaction rates, not the overall energy difference or equilibrium position.
• Temperature, pH, and substrate concentration can affect the rate of an enzyme-catalyzed reaction.