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IB Biology - Curriculum Notes 

2.5 Enzymes  

∑ - Understandings:

∑ - Enzymes have an active site to which specific substrates bind.



Active site: The area or the pocket on the enzyme where the substrate binds.
Enzyme: Proteins that catalyze chemical reactions (increase the rate by lowering the activation energy)
Each enzyme catalyzes a specific reaction for a specific substrate
Enzymes are not used up during the chemical reactions
Enzymes are very specific because both the enzyme and the substrate possess specific complementary shapes that fit into one another.
The binding of the substrate to the enzyme causes the chemical bonds of the substrate to weaken.
This eventually causes the reactions that take place that form the products.
After the products are released, the enzyme can bind to another substrate, because enzymes are not used up in these chemical reactions.



 Good Video on Enzymes: https://www.youtube.com/watch?v=qgVFkRn8f10​

∑ - Enzyme catalysis involves molecular motion and the collision of substrates with the active site.



When a substrate comes close to the active site of the enzyme, it can collide and bind to the active site of the enzyme
Since the substrate is dissolved in water around the enzyme, the substrates and enzymes are in continuous motion
The direction and movement is constantly changing and is random
Collisions occur at random between the substrate and enzyme
Successful reactions only occur if the substrate and the active site of the enzyme are correctly aligned and the collide with sufficient KE



 

∑ - Temperature, pH and substrate concentration affect the rate of activity of enzymes.

Temperature



When heat is added to a liquid the particles speed up, thus giving them more kinetic energy.
In a liquid that contains substrates and enzymes, the increase in kinetic energy will cause more collisions between substrates and enzymes thereby increasing enzyme activity and reaction rates.
However, as temperature increases and becomes too high, the bonds of the enzyme begin to vibrate and eventually break.
This causes the enzyme to lose its 3D shape, including the shape of the active site.
When the enzyme loses its shape and can no longer catalyze reactions, the enzyme is said to be denatured.
When the enzymes in solution become denatured, the reaction rate decreases dramatically.
Enzyme denaturation is usually permanent.
































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The optimum rate of reaction is when the graph reaches the top of the curve which is around 40ºC for most enzymes.

pH:



pH is dependent on the number of H+ ions compared to the number of OH- ions.
When a solution has a high number of H+ ions the solution it is said to have a low pH (acid). If a solution has a high number of OH- ions the solution is said to have a high pH (base).
Enzymes have an optimum pH at which they work the best.
When deviations occur away from this optimum pH, the enzyme’s activity or reaction rate decreases.
When the pH moves too far away from the enzyme’s optimum pH, the enzyme will lose its shape and denature, drastically decreasing enzyme activity.
For example, the optimum pH for the enzyme pepsin is around 2-3. If the pH increases to 5 or 6, the enzyme loses its ability to catalyze reactions (the breakdown of proteins in the stomach).
Most enzymes have an optimum pH close to neutral (7) pH.
















































Substrate Concentration



With a fixed amount of enzymes, as substrate concentration increases, the rate of reaction will increase, because more collisions between enzymes and substrates will occur.
However, as substrate-level increases, more and more enzyme active sites are being filled.
At a certain substrate concentration, all active sites on the enzymes are being used or are filled.
At this point, the reaction rate levels off and remains constant.
Adding more substrate to the reaction will not increase the reaction rate. The reaction rate can only be increased with the addition of more enzymes.



 




































∑ - Enzymes can be denatured.



Denaturation is a structural change in a protein (usually enzymes) that results in the loss (usually permanent) of its biological properties. When an enzyme denatures, the bonds that hold together its three-dimensional shape begin to vibrate and eventually break. This causes the enzyme to unfold and lose its shape, thereby eliminating the enzyme’s ability to catalyze reactions.


 

∑ - Immobilized enzymes are widely used in industry.



More than 500 enzymes are now used for commercial purposes
The majority of these enzymes used in industry are immobilized; meaning they are attached to another material (such as glass) or grouped together (in a calcium alginate gel)
The benefits of using immobilized enzymes are as follows



 



Convenience – only small amounts of proteins dissolve in the reactions leaving only solvent and the products. This means the enzymes and products can be easily separated
Economics – The immobilized enzymes can be easily removed and recycled from the solution, saving money. Eg. Particular useful in the removal of lactase in the production of Lactose-Free Milk.
Stability – Immobilized enzymes generally have a greater thermal and chemical stability than the soluble form of the enzyme
The reaction rate is faster because substrates can be exposed to a higher concentration of enzymes



 
∑ - Applications and skills:

∑ - Application: Methods of production of lactose-free milk and its advantages.



Lactose is a disaccharide sugar present in milk composed of monosaccharides glucose and galactose.
Lactase is the enzyme that breaks down lactose into its two monosaccharides.
Humans are born with the ability to digest milk (lactase produced) but as we grow older, most humans lose the ability to produce lactase in significant amounts.
If the lactose is broken down in milk before it is consumed, people that are lactose intolerant can drink the milk.
Some types of yeasts produce lactase.
Biotechnology companies can culture these yeasts and remove the lactase.
Milk is treated with lactase before distribution, allowing lactose-intolerant people to consume milk and milk products.






 



B Skill: Design of experiments to test the effect of temperature, pH and substrate concentration on the activity of enzymes.

 

B  Skill: Experimental investigation of a factor affecting enzyme activity.


(Practical 3)



LAB: Students can be given a choice to investigate a factor (pH, temperature, substrate or enzyme concentrations) that affects enzyme activity. Students should come up with their testing and recording methods. The use of data loggers such as pressure sensors, pH sensors or colorimeters can be used to satisfy ICT requirements.