Enzymes are proteins that act as catalysts

As we consider how biochemistry can explain how cells function and provide solutions to some of society’s major challenges, it is important to think about how reactions take place inside cells.

To do this, first of all we should think about what actually happens during a chemical reaction. In order for reactions to take place, molecules need to bump into one another. In fact, the molecules not only have to bump into one another, but they also have to be facing the right way and the bump must have more than a minimum energy. This is illustrated in the following image, which summarises how many enzymes interact with their substrates.

^{Representation of substrate binding to the active site of an enzyme molecule (Image credit: Essays in Biochemistry). Creative Commons Attribution Licence 3.0.}

When thinking about how energy changes during reactions, chemists (and biochemists) like to draw graphs where the horizontal axis plots the different steps in the reaction and the vertical axis describes the energy at each stage of the reaction. These types of graphs – like the image shown in the next step – are often referred to as “energy landscapes” because they look like the world that we may see around us.

In such energy landscape graphs, the reactions start on the left and proceed towards the right. The path from the molecules that are reacting (the reactants) to the products goes over a hill. On the other side of the hill, where products have been formed, the valley floor is at a lower altitude than where the reactants are. We can think of the altitude of the hill as the amount of energy that the molecules have. If they have enough energy to get over the top of the hill and down the other side, then the reaction happens. The energy needed to get over the hill is referred to as the activation energy.

This analogy provides a good overview of how energy influences whether a reaction will take place, but, of course, biochemical situations are more complicated than this. For example, the reactants might collide with enough energy to get over the hill, but they might not be facing the right way to go over the hill, in which case the reaction will not take place.

If the reaction takes place and the molecules reach the other side of the hill they will have formed products. If the products have a lower energy than the reactants (or lower altitude in our analogy), then the energy difference is given off to the surroundings. In chemical reactions we often experience this energy release as heat, from a burning candle, for example.

The link between energy and temperature highlights another way that we can influence whether a reaction will take place. Heating the reactants up provides them will more energy, meaning that at any time a higher proportion of them will have enough energy to make it over the hill and yield products. That is why reactions usually go faster when they are heated up.

Fortunately, heating molecules up is not the only way to increase the likelihood that a reaction will take place. There are many molecules and substances that are able to increase the chances that a chemical reaction will take place and these are not consumed during the overall reaction. Such substances are known as catalysts.

Many experiments performed by biochemists have shown that similar types of rules link energy and reactions in the biological situations that are found within cells. In fact, cells also contain their own versions of catalysts in the form of enzymes. These types of proteins are found in all cells and they usually act on one specific type of reaction, increasing the chances that the reaction will take place inside cells.