Every second of every day, the cells in our bodies perform billions of chemical reactions. Amino acids form peptide bonds, glycogen molecules break into simple sugars, and simple sugars are used to give cells energy. In the cell, these processes happen at lightning speed. However, if you were to try to run the same reactions in test tubes, they would occur much more slowly or not even start at all. The secret ingredient that cells use to perform these reactions are special molecules called enzymes. Enzymes increase the rate of chemical reactions in our cells. This topic will focus on what enzymes are and how they speed up chemical reactions.
Catalysts
A chemical reaction is a process in which one or more compounds, called the reactants, transform into one or more different compounds, the products. Every reaction proceeds with a characteristic rate that depends on given conditions (temperature, pH, chemical concentrations).
Catalysts are substances that speed up chemical reactions. They are not reactants themselves, but they help provide the conditions needed for reactions to go faster.
Consider an example. Some campers want to make a campfire, and they have all the needed reactants for combustion: wood and atmospheric oxygen. Smoke and ash will be the products of this reaction. They can make their fire burn faster if they add a splash of gasoline; however, the gasoline will burn out too. That is how you know that gasoline is not acting as a catalyst.
Activation energy
Even with gasoline, it is impossible to start a fire without an initial spark. A burning match or a lighter help the wood and oxygen overcome the activation energy of the reaction. Activation energy is the energy needed for a reaction to occur. Imagine lying under a cozy blanket on a Sunday morning. You're comfortable, so you need a little effort, an energy input, to get up and start your day. A dog jumping on the bed, makes it easier to throw off the blanket. It lowers your activation energy.
Why do chemical reactions need this initial energy input? Let's take a closer look at what happens during a chemical reaction. When the reaction occurs, chemical bonds in the reactant molecules break, and new bonds in the product molecules form. To break chemical bonds, a molecule must rearrange into an unstable or transient state, and it requires energy to do so. The energy needed to reach this transient state is the activation energy. Because this state is unstable, the molecule will not linger in that form for long, and it will proceed through the rest of the reaction to form the product.
If the activation energy of a reaction is high, its rate will be low, because it is less likely the molecules will have enough energy to overcome the energy barrier and enter into a reaction. The activation energy can be so high that some reactions will not proceed at all without external energy input. The common source of activation energy is thermal energy (heat). In the case of the campfire, a burning match provides thermal energy to start combusting the molecules in the wood. Heat makes molecules move faster, and their bonds start vibrating faster, and as the reactant molecules vibrate and collide, the bonds will begin to break.
However, living organisms can't use heat or other such sources to get activation energy because it is too dangerous. Applying heat can damage cell structures and denature their proteins. Thus, cells use biological catalysts to run their reactions. We'll examine them in detail in the next section.
Enzymes
The most common biological catalysts are protein enzymes, though some RNAs can also act as catalysts. Every enzyme binds to specific molecules called their substrates. The reactant molecules in each enzyme-catalyzed reaction in a cell will have their own specific enzymes. By binding the substrate, an enzyme lowers the activation energy of the reaction. But what exactly does the enzyme do?
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Enzymes destabilize the native state of substrates by distorting and bending them to make the reactant enter the transition state. They can also bring reactants close together, which in the crowded environment of a cell, is important for reactions to proceed.
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Enzymes stabilize the transition state and help it to be more energetically stable. This increases the chance that a molecule will reach its transition state and remain there long enough to react.
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Some enzymes form temporary chemical bonds with their substrate molecules. This creates alternative reaction pathways with lower activation energy.
After performing these actions, enzymes will always return to their native states, ready for the next round of catalysis.
The active site of an enzyme
Substrates do not interact with the entire enzyme, as these proteins can be very large and even contain several subunits. The part of the enzyme where the substrate binds is called an active site. The chemical properties, size, and shape of an active site depend on its amino acid sequence as well as its higher level structures. These distinct features make active sites specific to certain substrates. A common way to describe enzyme-substrate pairings is as a lock and key. However, the active site is not a rigid structure like a lock. When an enzyme binds its substrate, the geometry of its active site can change slightly to make the substrate fit more tightly. This adjustment is called induced fit.
Enzymes differ in specificity: some enzymes act on one particular substrate, and others accept any substrate that has a certain chemical group or type of bond that matches its active site. It does not matter how the head of a key looks if the blade fits! For example, DNA polymerase, the enzyme that catalyzes the synthesis of DNA molecules, works with four types of nucleoside triphosphates, one each for A, T, C, and G bases.
After a substrate enters the active site of the enzyme, the enzyme-substrate complex forms, catalysis occurs, the substrate converts into the reaction products, and the products leave the active site and diffuse away. This leaves the active site free to bind a new substrate and start catalysis again.
Conclusion
Enzymes are proteins that act as catalysts: they speed up chemical reactions in living organisms. By binding the substrate molecule, an enzyme lowers the activation energy and facilitates a reaction. Enzymes themselves are not consumed or altered permanently by the reaction. When one round of catalysis is over, the enzyme is ready to speed up the next reaction.
Active sites of enzymes interact with particular substrates, making enzymes specific to certain reactions.