Enzymes are large proteins that speed up chemical reaction. As globular protein, enzymes have a specific three-dimensional shape which is determined by their sequence of amino acids. Despite their large overall size, enzyme molecules only have a small region that is functional. This is known as enzyme’s active site. The substrate molecule is held within the active site by bonds that temporarily form between the R groups of the amino acids of the active site by bonds and all groups on the substrate molecules. This structure is known as enzyme-substrate complex.
Enzymes are classified into several categories, such as hydrolytic, oxidising, and educing. Depending on the types of reaction they control. In this case, the enzyme I will use in the investigation is catalase from celery extract, which is concluding as hydrolytic enzyme. This type of enzyme accelerates reactions in which a substrate is broken down into simpler compounds through reaction with adding up water molecules. Oxidising enzyme, known as oxidises, accelerate oxidation reaction; reducing enzyme speed up reducing reactions in which oxygen is removed.
A substrate is the molecule, which can bind into the active of enzyme. In this case, I will use hydrogen peroxide as the substrate.
Enzyme works in the same way as a key operates a lock. Enzymes’ active sites have a particular shape like a lock and only a particular key ( substrate) can fit into that lock. Enzymes are therefore specific in the reactions that they catalyse. This is known as the ‘lock and key theory’. In practice, unlike a rigid lock, the enzyme actually changes its form slightly to fit the shape of the substrate. In other works, it is flexible and moulds itself around the substrate. As it alters its shape, the enzyme puts a strain on the substrate molecule and thereby lowers its activation energy.
In this case, enzyme catalse has a specific active site, which just for the substrate hydrogen peroxide to fit in, then the reaction takes place. The reaction involved is hydrolysis.
The equation of reaction is:
2H2O2 2H2O + O2
Factor that would affect the reaction:
At low temperature, the reaction takes place very slowly, this because molecules are moving relatively slowly as have low kinetic energy. Substrate molecules will not often collide with the active site, and so binding between substrate and enzyme is a rare event. Therefore, reaction is slow. A rise in temperature increases the kinetic energy of molecules which therefore move around more rapidly and collide with one another more often. This means at a higher temperature, the reaction will take place faster than a lower temperature and an increasing or a decreasing of temperature will affect my reaction and results.
For this reason, I will control this effect by using a water bath to maintain the temperature the same during the whole reaction. As enzyme work best at a certain temperature, this is known as optimum temperature. If the temperature is too low, enzyme cannot work properly; if the temperature is too high, this may denature the enzyme active site and enzyme will lose its function. So to avoid this problem, I will keep the water bath has the temperature of 25?C, which is the room temperature. Therefore enzyme catalase will work properly and temperature will not be a factor to affect this investigation.
Most enzymes also have an optimum pH at which they function best. In human body, most enzyme work fastest at an optimum pH of about 7. For example, the digestive system Pepsin found in the stomach to digest proteins. A change I pH means a change in the concentration of hydrogen ions in the surrounding of the enzyme. This affects the ionisation of R group in the amino acid residues of the protein molecule and the shape of the active site to bind with the substrate. The lower the pH, the bigger the hydrogen ions’ concentration it is . Hydrogen ions can interact with the R group of amino acids, affecting the way in which they bond with each other and therefore affect their 3D shape. So the lower the pH, the higher the hydrogen ions’ concentration it is, and therefore the slower the reaction. Oppositely, the higher the pH, the faster the reaction it will take place. To control this factor, I will keep the pH all the way through the same by using a pH paper to check the pH number during the experiment.
The rate of reaction increases as the concentration of enzyme increase. Because higher concentration of enzyme means higher number of enzyme molecules, so that more enzyme molecule will collide with substrate molecule, therefore the reaction will take place faster. When there is plenty of substrate, the rate of reaction is not limited by the concentration of enzyme. Therefore if the concentration of enzyme is increased, the number of collision with the substrate molecule and hence the rate of reaction will increase. If the amount of substrate is limited, the rate of reaction decreases because if the amount of substrate is limited, the enzyme’s active site will not collide with substrate very often, therefore a few products will be made.
As the reaction progressing, the substance will be broken down slowly, therefore less substrate left. So enzyme molecules freer, therefore products will be produced more slowly. Hence the slower the reaction goes. In this experiment, the enzyme concentration is an independent variable which I will vary the concentration of celery extract each time to investigate how different concentration of celery extract( enzyme concentration) affect the rate of breaking down hydrogen peroxide.
Increasing in the substrate concentration will increase in the rate of reaction. Because increasing the substrate concentration, we increased the number of substrate molecules; therefore more collision will take place between substrate and enzyme’s active site. Hence mere products will be produced. Therefore the faster the rate of reaction takes place. However, if we continue increase the substrate keeping the enzyme concentration and volume the same. The rate of reaction will not continue to increase. This is because a higher substrate concentration active site of all the enzyme molecule is busy with substrates. The number of enzyme molecules because a limiting factor. The rate of reaction at higher substrate becomes constant. In this case, I will keep the volume of hydrogen peroxide the same for using each time.
Enzyme inhibitors are substances that directly or indirectly interfere with the functioning of the active of an enzyme and so reduce its activity.
There are two types of inhibitors:
Competitive inhibitors are the molecules which have a shape similar to the substrate, so they fit into the active site of the enzyme. Therefore the substrate cannot enter into the active site and so the enzyme cannot catalyse the reaction. Hence, the reaction is slow down. However, the inhibitors are not permanently bound to the active site and so, when it leaves another molecule can take its place. Sooner or later, all the substrate molecules will find an active site, but the greater the concentration of inhibitors, the longer the reaction will be.
Non-competitive inhibitors will bind to another part of the enzyme rather than the true active site. This changes the shape of the enzyme’s active site so that the substrate no longer fits. Therefore the reaction is slow down. As the substrate and the inhibitor are not competry for the same site, an increase in substrate concentration does not decrease the effect of inhibitors.
To control the factor of inhibitors. I will not add any substance except celery solution and hydrogen peroxide during the experiment.
I predict that the higher the concentration of celery extract I use, the fast the rate of breaking down hydrogen peroxide I will get.
As in higher concentration of celery extract, the more number of catalase molecules in the solution. So there are more collisions between substrate ( hydrogen peroxide) and enzyme( catalase) active site, and more products will be formed( water and oxygen). The rate of reaction can be measured by measuring the volume of oxygen produced in a period of time. The larger the volume of oxygen produced in the same period of time, the fast the reaction is taken place.
In order to get finest and reliable results, I did a preliminary work. This enable me not just find out a suitable range of celery extract of catalase concentration I will use in my real experiment, but also help me descried the time interval I will measure the oxygen get and justify the equipment and method. I think these all lead me to get reliable and accurate results in the end.
In the preliminary work, I choose to do three different concentration of celery extract. To get different concentrations of celery extract, I used distilled water to dilute the celery extract.
I tested the range of celery extract between 100% –20%, the highest concentration is 100%, the lowest concentration is 20% and the median is 60%. The reason I chose these three concentrations is because between each concentration, there is 40% difference and this enable me to test that in which level of concentration, reaction goes best and to find out which concentration of celery extract I will use in my real experiment.