Models
There are several models of how enzymes work.
Lock and Key Theory:
The specific action of an enzyme with a single substrate can be explained using a Lock and Key analogy. In this analogy, the lock is the enzyme and the key is the substrate. Only the correctly sized key (substrate) fits into the key hole (active site) of the lock (enzyme).
Smaller keys, larger keys, or incorrectly positioned teeth on keys (incorrectly shaped or sized substrate molecules) do not fit into the lock (enzyme). Only the correctly shaped key opens a particular lock.
Induced Fit Theory
The induced-fit theory assumes that the substrate plays a role in determining the final shape of the enzyme and that the enzyme is partially flexible. This explains why certain compounds can bind to the enzyme but do not react because the enzyme has been changed too much. Other molecules may be too small to induce the proper alignment and therefore cannot react. Only the proper substrate is capable of inducing the proper alignment of the active site.
In the graphic on the left, the substrate is represented by the magenta molecule, the enzyme protein is represented by the green and cyan colours. The cyan colours protein is used to more sharply define the active site. The protein chains are flexible and fit around the substrate.
The Factors that Effect Enzyme Activity
1. Temperature
Enzymes have an optimum temperature at which they work fastest.
Up to the optimum temperature the rate increases geometrically with temperature The rate increases because the enzyme and substrate molecules both have more kinetic energy so collide more often, and also because more molecules have sufficient energy to overcome the activation energy. The increase in rate with temperature can be quantified as a Q10, which is the relative increase for a 10°C rise in temperature. Q10 is usually 2-3 for enzyme-catalysed reactions and usually less than 2 for non-enzyme reactions.
Above the optimum temperature the rate decreases as more and more of the enzyme molecules denature. The thermal energy breaks the hydrogen bonds holding the secondary and tertiary structure of the enzyme together, so the enzyme loses its shape.
The substrate can no longer bind, and the reaction is no longer catalysed. At very high temperatures this is irreversible. Only the weak hydrogen bonds are broken at these mild temperatures; to break strong covalent bonds you need to boil in concentrated acid for many hours.
2. pH
Enzymes have an optimum pH at which they work fastest. The pH affects the charge of the amino acids at the active site, so the properties of the active site change and the substrate can no longer bind.
3. Enzyme concentration
As the enzyme concentration increases the rate of the reaction increases linearly, because there are more enzyme molecules available to catalyse the reaction. At very high enzyme concentration the substrate concentration may become rate-limiting, so the rate stops increasing. Normally enzymes are present in cells in rather low concentrations.
4. Substrate concentration
The rate of an enzyme-catalysed reaction shows a curved dependence on substrate concentration. As the substrate concentration increases, the rate increases because more substrate molecules can collide with enzyme molecules, so more reactions will take place. At higher concentrations the enzyme molecules become saturated with substrate, so there are few free enzyme molecules, so adding more substrate doesn't make much difference.
5. Inhibitors
Inhibitors inhibit the activity of enzymes, reducing the rate of their reactions. There are two kinds of inhibitors.
- A competitive inhibitor molecule has a similar structure to the normal substrate molecule, and it can fit into the active site of the enzyme. It therefore competes with the substrate for the active site, so the reaction is slower. Competitive inhibitors increase KM for the enzyme, but have no effect on vmax, so the rate can approach a normal rate if the substrate concentration is increased high enough.
(b) A non-competitive inhibitor molecule is quite different in structure from the substrate molecule and does not fit into the active site. It binds to another part of the enzyme molecule, changing the shape of the whole enzyme, including the active site, so that it can no longer bind substrate molecules. Non-competitive inhibitors therefore simply reduce the amount of active enzyme, so they decrease vmax, but have no effect on KM.
So this means that when I change my substrate concentration in my experiment, the rate of enzyme activity should also change. A high concentration of substrate should increase the chances of a successful collision, and therefore have a faster rate of reaction. There would be more substrate molecules to bind with the enzyme in the active site.
Details about the Main Experiment
The factor I am going to change is the concentration. This is simple because is it easy to control this variable. Unlike the temperature, it would change during the experiment which would affect my results. The concentration will never change during the experiment, so this is why I am going to change the concentration. I also did not use different temperatures as my variable because the water baths are usually not that accurate when trying to get an exact temperature.
So my concentration can change however the rest of the other factors must stay the same in order for the experiment to be a fair test.
I am going to use eight different Hydrogen Peroxide concentrations of equal volume (10ml).
The eight different Hydrogen Peroxide concentrations I am going to use are
0.4 Mol/dm3,
0.8 Mol/dm3,
1.2 Mol/dm3,
1.6 Mol/dm3,
2.0 Mol/dm3,
2.4 Mol/dm3,
2.8 Mol/dm3,
3.2 Mol/dm3.
For each acid concentration, I will repeat it twice to make the experiment more accurate, and so I can find an average.
My Prediction of the Main Experiment
My prediction is that the higher the concentration of the substrate, then the higher the rate enzyme activity will be. This means that the higher the concentration of Hydrogen Peroxide, the faster and more bubbles are produced. I say this because of my theory. As the substrate concentration increases, the rate increases because more substrate molecules can collide with enzyme molecules, so more reactions will take place. However I predict that the enzyme will be a limiting factor after a certain concentration, and therefore the fate of reaction will not go any faster. This is because there are no more available active sites to increase the rate of reaction. . At higher concentrations the enzyme molecules become saturated with substrate, so there are few free enzyme molecules, so adding more substrate doesn't make much difference
I believe my graph will look like this because of my prediction.
Equipment
8 Test Tubes with Bungs with holes in them
20 mls of 0.4, 0.8, 1.2, 1.6, 2.0, 2.4, 2.8, 3.2 Mol/dm3
Test Tube Rack
Distilled Water
100ml Gas Syringe
Stop Clock
100 mls of Catalyse
Pipette
Method
1. Using the Pipette, put 5mls of Catalyse into each of the 8 test tubes.
2. Then put 5mls of Hydrogen Peroxide at 0.4 Mol/dm3 into the first test tube.
3. Then quickly attach the 100ml Gas Syringe to the test tube.
4. Collect gas produced for 5 minutes. Use the stop clock to time your experiment.
5. Record your results, and then repeat the procedure.
6. Then repeat the whole procedure, this time for a different concentration of Hydrogen Peroxide.
