Topic 2: Molecular Biology (Student)
2.5
Essential Idea: Living organisms control their composition by a complex web of chemical reactions.
 2.5 Enzymes
i.
Enzymes have an active site to which specific substrates bind.
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Enzymes are globular proteins that work as catalyst (speed up chemical reactions by lowering
activation energy of reaction)
They do not change themselves and are not used up.
Active site: the place on the enzyme where the substrate binds.
Each enzyme catalyzes a specific reaction for a specific substrate, Like a lock and key.
The binding of the substrate to the enzyme causes the chemical bonds of the substrate to
weaken, causes the reaction to take place and form products.
After the products are released, the enzyme can bind to another substrate, because
enzymes are not used up.
Draw Enzyme with it’s specific substrate. Label the Enzyme, active site, and substrate.
ii.
Enzyme catalysis involves molecular motion and the collision of substrates with the active site.
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Three stages:
1. Substrate binds to active site of the enzyme. Sometimes two substrates will bind to
different parts of the active site.
2. While bonded, the substrate will change into different chemical substances which are
products of the reaction.
3. The products separate form active site, leaving it vacant for another substrate to bind
again.
The coming together of substrate and active site is known as collision.
The substrate is usually dissolved in water around the enzyme, the substrate and enzymes are
in continuous motion.
The direction and movement s constantly changing and is random, collisions occur at random
between substrate and enzyme.
Successful reactions only occur if the substrate and the active site of the enzyme are correctly
aligned when they collide with sufficient kinetic energy.
Label the following empty boxes:
iii.
Temperature, pH and substrate concentration affect the rate of activity of enzymes.
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Temperature:
- In liquids the particles are in continuous motion
- When heated the particles gain more kinetic energy and move faster along with the
enzyme and substrate molecules.
- The chance of a substrate colliding with the active site is increased so the enzyme activity
increases.
- BUT: as temp increases and becomes to high the bonds of the enzyme vibrate and will
break, then shape of enzyme will change (including active site) , then can no longer
catalyze reactions. This is called denatured.
- When enzymes become denatured the reaction rate decreases dramatically. This
damage is permanent.
- Enzymes have optimum temperature in which the work the best.
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pH:
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ph scale measures acidity or alkalinity (base)
Low pH (0-6.9) is acidic, has higher concentration of hydrogen ions. The closer you get
to one the stronger the acid.
High pH (7.1-14) is basic, has more OH- ions
pH of 7 is neutral
Most enzymes have an optimum pH at which there activity is highest. If the pH is
changed the activity decreases and eventually stops because the structure of the
enzyme is altered (denaturation)
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 (breakdown of proteins in the
stomach).
Most enzymes have an optimum pH close to neutral (7) pH.
Substrate Concentration
- If concentration of substrate increases the substrate-active site collisions will happen
more and the rate at which the enzyme catalyzes its reaction increases.
- BUT, when active site is occupied it is unavailable until products are formed and
release. So, the rate of the reaction will increase with concentration to a point but then
it will reach a plateau and remain constant. Adding more substrate to the reaction will
not increase the rate. You can only increase more enzymes at this point to increase
rate.
 Guidance: Students should be able to sketch graphs to show the expected effects of
temperature, pH and substrate concentration on the activity of enzymes. They should be able to
explain the patterns or trends apparent in these graphs.
iv.
Enzymes can be denatured.
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v.
Immobilized enzymes are widely used in industry.
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Denaturation is a structural change in a protein (usually enzymes) that results in the loss of
its biological properties or functions.
This loss is permanent.
When denatures the active site shape is altered and the substrate can no longer bind and
reaction does not occur.
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)
Advantages to immobilization of enzymes:
i. Enzyme can be separated easily from products so reaction can be stopped at the
ideal time, preventing contamination of the products
ii. Enzyme can be recycled, giving cost savings.
iii. Immobilization increases the stability of enzymes to changes in temperature and pH,
reducing the rate at which they are degraded and have to be replaced.
iv. Substrates can be exposed to higher enzyme concentrations than with dissolved
enzymes, speeding up reaction rates.
Applications and skills:
Application: Methods of production of lactose-free milk and its advantages.
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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.