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CIE IGCSE Enzymes
Transcript of CIE IGCSE Enzymes
Structure and action of enzymes
Factors affecting enzyme action
Enzymes are proteins that act as a catalyst in biological reactions.
A catalyst is a substance that speeds up the rate of a reaction without being changed.
At any moment in your body millions of chemical reactions are taking place, building new molecules, breaking others down and changing others.
All of these reactions are catalysed by enzymes.
Most of these chemical reactions could happen without enzymes but would go too slowly for the organism to survive.
Enzymes are used to break down large molecules into smaller molecules. This is especially useful in digestion where large, insoluble molecules need to be broken up into smaller, soluble molecules. In our body this happens in our digestive system but bacteria and fungi secrete these enzymes on to their food and absorb the products.
Other enzymes catalyse the opposite type of reactions, making large molecules such as protein or starch from smaller molecules.
Other enzymes can turn one molecule into another (for example glucose into fructose)
All enzymes share 5 important properties which we will look at in detail. These are:
They are all proteins
Each enzyme catalyses only 1 reaction, they are very specific due to their shape
They can be used multiple times
They are influenced by temperature
They are influenced by pH
All enzymes are made from long protein strands. Proteins are made out of amino acids. As there are twenty different amino acids there a lot of possible protein strands. The sequence of amino acids results in the protein folding into a particular shape.
Each enzyme is folded into a shape that allows the substrate(s) to fit into an area of the enzyme called the active site, which is where the reaction occurs. Other substrates do not fit into this active site. This is why each enzyme can only catalyse a specific reaction.
A substrate is a molecule that enzymes work on, they are the molecule(s) before the arrow in a word equation.
Substrates also have a specific shape.
We call this the "lock-and-key" model of enzyme action. The substrate (key) fits into the active site of this specific enzyme (lock) but not into any others. This forms the enzyme-substrate complex (ESC). Once it fits into the active site the reaction is catalysed.
The products are a different shape than the substrate and therefore no longer fit into the active site and are released. The enzyme is unchanged and can fit another substrate molecule.
A small quantity of enzyme can catalyse a very large amount of substrate.
We can measure the efficiency of an enzyme-catalysed reaction by looking at the rate of the reaction.
The rate of a reaction is the speed by which the substrate(s) (or reactants) are converted into the product(s).
You can work this out by looking at either the amount of substrate that disappears in a certain time frame or the amount of product that appears.
One of the key factors that influences enzyme activity is temperature.
This graph shows the rate of reaction against the temperature.
To explain this we need to think about the effect of temperature on particles.
If you increase the temperature particles get more energy. In a solid this means that the particles vibrate more and take up more space, in a fluid (liquid or gas) this means that the particles have more kinetic energy and move more quickly.
Enzymes work in cells whose contents are largely liquid. At low temperatures the particles have less kinetic energy. This means they move around less and enzymes and substrates are less likely to collide. This means that there is less chance of the substrate going into an active site and the reaction occurring. The rate of reaction is low.
As the temperature goes up the particles get more kinetic energy and the chance of a collision between the substrate and the enzyme occurring increases.
Enzymes have an optimum temperature at which they work best. In humans this temperature is approx 37C.
When the temperature gets above the optimum temperature the bonds that hold the enzyme together start to break because the particles are vibrating too much. This changes the shape of the enzyme and the enzyme no longer has an active site into which the substrate can fit. We say that the enzyme is denatured (DO NOT say that the enzyme has been killed, it is not alive)
Another factor that influences enzyme activity is pH.
This graph show enzyme activity against pH
Enzymes have an optimum pH at which they work best. An enzyme that is produced in the stomach (e.g. pepsin) will work best at pH 1.5. An enzyme that is produced in your saliva (e.g. amylase) will work best at pH7.
The pH level of the solution will also affect the bonds in the enzyme. If the pH changes too much the active site will be lost and the enzyme will be denatured.
When the amylase from your saliva enters your stomach it will denature and stop working because the pH is very different.
The same happens to the pepsin when it enters the small intestine and bile is added.
How do you think substrate and enzyme concentration might affect the rate of enzyme activity?
Effect of temperature on enzyme action practical:
Set up water baths at 4 different temperatures (20, 30, 40, 50 C)
Another person can set up 4 test tubes with 2cm of 1% starch solution
The third person can set up 4 tubes with 2cm of 1% diastase solution (an enzyme that digests starch)
Put the tubes into the water baths and leave to equilibrate for 5 minutes. In the mean time set up two dropper plates by adding a drop of iodine to each well.
Mix the contents of the tubes together and immediately take a sample using the long pipettes and add it to your dropper plate. Note down your observation.
Repeat this process every 2 minutes for 30 minutes (or until no more starch is present).