Cells are full of chemical reactions, but nature doesn't like waiting any more than we do. Enzymes help speed up these reactions, and over time, scientists have modified their theories on how this works. In this lesson, we'll explore the two main theories that explain this.
See this piece of wood? If I place it here on the ground and leave it alone, eventually it will completely disintegrate until nothing but the basic carbon molecules are left. But, this could take a very, very long time, and I am rather impatient. So instead of waiting, I'll just set it on fire and achieve the same result in a much shorter time period. Apart from satisfying my pyromania, do you see what I did there? I used fire as a catalyst, which is just a substance to increase the rate of a chemical reaction. The wood would have disintegrated in time, but the fire sped up that process.
Well, as it turns out, nature is just as impatient as I am. These sorts of catalysts are all around us, and even within us. Nearly all cells contain large molecules that catalyze chemical reactions within the cell, called enzymes. Enzymes are important. The cell could wait for nutrients to dissolve into energy, but frankly the cell could die waiting for this to happen. Enzymes speed up the process. Sometimes it pays to be a little impatient.
How Enzymes Work
We know that enzymes are catalysts, but how exactly do they work? Well, this here is glucosidase, an enzyme that processes specific kinds of sugars. It starts when the cell ingests a complex sugar molecule, like this one here (see video). The complex molecule in this process is called the substrate. Over time, this complex sugar molecule will decompose into smaller molecules called products. In this case, the complex sugar becomes individual glucose molecules, which is a much simpler compound that the cell can use for energy. But, the cell can't just wait for the substrate to naturally dissolve into the products, so that's where the enzyme comes in. Glucosidase turns the substrate, the complex sugar, into two smaller products, simple glucose molecules.
See how that works? Now, it's important to note that this specific reaction can only be produced by glucosidase. Another enzyme cannot break complex sugars into glucose, and glucosidase cannot break down other substrates. Each chemical reaction can only be catalyzed by a specific kind of enzyme. Considering that there are roughly 5,000 chemical reactions that enzymes are known to catalyze, we've got a lot of enzymes with very specific focuses. So how do they know which molecules they can catalyze and which ones they can't? After all, real enzymes don't actually have hands and eyes like our cute little cartoons here. This is a major scientific debate, and there are two leading theories to explain this complex, but very important, process.
The Lock and Key Hypothesis
Scientists have long wondered exactly how enzymes know which substrates to process and which to ignore. In 1894, German chemist Emil Fischer proposed the lock and key theory, which states that enzymes have a specific shape that directly correlates to the shape of the substrate. Basically, substrates fit into an enzyme the way a key fits into a lock. If the substrate is not the correct shape, it won't fit into the enzyme, and no chemical reaction can occur. Only those substrates that exactly fit into the enzyme can be catalyzed.
The Induced Fit Model
As scientific technology improved, researchers began to notice a small problem with the lock and key theory. Enzymes don't actually maintain a rigid shape; they change slightly to accommodate their substrate. This means that enzymes aren't precisely shaped to be exact fits for their substrates, and in 1958, another scientist named Daniel Koshland proposed a modification to the lock and key theory. This new theory, called the induced fit model, states that enzymes are partially flexible and that the active site of the enzyme will reshape to fit the substrate. So if the active site can change shape, how does the enzyme know which substrates it can process? Well, the enzyme itself is still only partially flexible. The active site can change a bit to accommodate the correct substrate, but if it changes too much, then the enzyme can't function. The basic theory behind the lock and key model, the idea that substrates have to fit the enzyme, is still the same, but in the induced fit model the active site is simply less rigid. You can reshape it enough to accommodate variations in the individual substrate but not enough to fit an entirely new substrate. Why does the enzyme need to be more flexible? Maybe it was just too impatient to try and create a perfect fit every time. After all, the enzyme's job is to get it done quickly. Turns out, not even our cells like to wait.
While cells could ingest molecules and wait for them to naturally dissolve, this process could take way too long, so instead cells use a catalyst, a substance to increase the rate of chemical reaction. This catalyst is called an enzyme, a molecule that catalyzes chemical reactions within a cell. Each enzyme can only catalyze specific complex molecules, called substrates, so scientists have often wondered how enzymes recognize substrates. In 1894, Emil Fischer proposed the lock and key theory, which states that enzymes have a specific shape that directly correlates to the shape of the substrate. This model was accepted for a long time, until scientists noticed that enzymes were not actually so rigid. Then, in 1958, Daniel Koshland modified the lock and key theory with the induced fit model, claiming that enzymes are partially flexible and that the active site of the enzyme will reshape to fit the substrate. However, this model still acknowledged that only specific substrates could fit into the active site, which would reject substrates of dramatically different shapes. This theory explains how our cells speed up the process of chemical reactions. Turns out, our cells are just as impatient as we are.