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Chapter 6 Enzymes
- Short Questions
Q.1) Differentiate between the lock and key model and the induced-fit model.
Answer:
| Lock and key model | Induced fit model |
| This model was proposed by a German chemist, Emil Fischer in 1894. | This model was given by an American biologist, Daniel Koshland in 1958. |
| According to this model, the active site of the enzyme and the substrate have specific geometric shapes. | According to this model, when a substrate molecule fits in the enzyme’s active site, the enzyme molecule alters the shape of the active site slightly so that it fits more tightly around the substrate molecule. |
| This model suggests that the active site is a rigid and non-flexible structure. | This model suggests that the active site is a flexible structure. |
Q.2) In what way does an enzyme affect the chemical reaction it catalyzes?
Answer:
  Whenever a chemical reaction occurs, some amount of energy is spent to break the chemical bonds in reactants (substrates). This energy is called activation energy. Enzymes lower off the amount of activation energy.  For example, they change the shape of substrate molecules or bring them in the correct orientation. They may also disrupt the charge distribution on substrate molecules.
Q.3) What will be the effect on digestion if we take some digestive enzymes from outside?
Answer:
If we take some digestive enzymes from outside, the rate of digestion will increase. If the optimum pH of intake enzyme does not match with the pH of the stomach, the enzyme gets denature and rate of enzyme reaction remain unchanged.
Q.4) What is meant by denaturation of enzyme?
Answer:
When the temperature or pH rises above the optimum temperature, the rate of reaction decreases rapidly. This causes denaturation of enzyme which alters the structure of the enzyme. This denatured enzyme is unable to bind the substrate and product will not form.
Q.5) How are enzymes specific for the substrate?
Answer:
Specificity of enzymes:
The enzymes are specific for their substrate and the type of reaction. For example, the enzyme protease speed up the digestion of protein only. The specificity of different enzymes is due to the shape of their active sites. The active sites of each enzyme possess a specific shape that fits with a specific substrate.
Q.6) What are the terms used to describe the temperature and the pH at which an enzyme can work most effectively in a reaction?
Answer:
 The term used to describe the temperature at which an enzyme can work most effectively in a reaction is known as “optimum temperature” and for pH, the term “optimum pH” is used.
- Long Questions
Q.1) Describe the factors which affect the enzymes activities.
Answer:
Factors affecting enzyme activity:
 There are different factors which affect the enzyme activity. Some of them are given below:
1-Effect of Temperature on Enzyme Activity:
  Increase in temperature speeds up the rate of enzyme-catalyzed reactions but only to a point. Each enzyme works at the maximum rate at a specific temperature, which is called its optimum temperature. The optimum temperature of enzymes varies in different organisms. For most enzymes, the optimum temperature is about 40-45°C. Certain enzymes in plants have a high optimum temperature. A decrease in temperature lowers the rate of reaction.
  When the temperature rises above the optimum temperature, the rate of reaction decreases rapidly. This is due to the denaturation (alteration in the structure) of the protein of an enzyme. The average temperature of the human body is 37°C. It is the optimum temperature for human enzymes. Human enzymes start to denature quickly at temperatures above 40°C.
2- Effect of pH on Enzyme Activity:
  Enzymes are affected by the acidity or alkalinity of the solutions in which they act. Each enzyme works best at a specific pH, which is called its optimum pH. Some work best in slightly acidic pH while others require slightly alkaline pH. Most enzymes function between a pH of 6-8, however, pepsin in the stomach works best at a pH of 2 and trypsin at a pH of 8.
  Any change in pH can lower the enzymatic activity. Extreme changes in the pH of solutions denature the enzymes.
Read more: 9th Class Biology Notes Chapter 1 to 5 For Mardan Board
3- Effect of Concentration of Substrate on Enzyme Activity:
As the concentration of substrate increases, the rate of reaction increases. This is due to successful collisions with enzyme molecules. However, if the substrate concentration is increased beyond a saturation point, the active sites of the enzyme are not available and the rate of reaction does not increase more.
Q.2) Explain the mode of action of an enzyme in a reaction.
Answer:
Mechanism of Enzyme Action:
 There is a small portion in the enzyme molecule that is actually involved in catalysis. It is in the form of depression or “pocket” on the surface of the enzyme molecule. This catalytic region is called an active site. When an enzyme attaches to the substrate, a temporary enzyme-substrate (ES) complex is formed. The active site catalyzes the reaction and the substrate is converted to product. The ES complex breaks and the enzyme and the product are released.
            Enzyme + Substrate→Enzyme-Substrate→ Enzyme + ProductsÂ
 Two models have been proposed to explain the mechanism of enzyme action. These are:
 1. Lock and key model
2. Induced fit model
1. Lock and key model:
  In order to explain the mechanism of enzyme action a German chemist Emil Fischer, in 1894, proposed the lock and key model. According to this model, the active site of the enzyme and the substrate have specific geometric shapes. The substrate molecule exactly fits in the active site of the enzyme just like a lock and a key (enzyme is the lock and the substrate is the key). This model explains enzyme specificity and suggests that the active site is a rigid and non-flexible structure.
2. Induced fit model:
  In 1958 an American biologist Daniel Koshland suggested a modification to the lock and key model and proposed the induced-fit model. according to this model, when the substrate fits into the active site of the enzyme, the enzyme molecule alters the shape of their active site slightly, so that it fits more tightly around the substrate molecule.
Q.3) Write the various properties of enzymes.
Answer:
Characteristics of Enzymes:
- Enzymes are proteins in nature and are secreted by cells.
- Enzymes function as catalysts and increase the rate of chemical reactions. They do so by lowering the activation energy.
- Enzymes are specific in action. They are specific not only for substrate but also for the kind of reaction.
- A small amount of an enzyme is needed to catalyze relatively large quantities of the substrate. They are not used during chemical reactions.
- All enzymes are synthesized is inside the cells. Some enzymes work inside the cells and are called ‘intracellular enzymes’ e.g. mitochondrial enzymes. Some enzymes are released from the cell and they work outside. Such enzymes are called extracellular enzymes e.g. pepsin enzyme working in the stomach cavity.
- In complex metabolic reactions, many enzymes work together in a sequence. One enzyme takes the product of another enzyme as a substrate. After speeding up the reaction, the new product is passed on to another enzyme.
- Some enzymes require non-protein molecules called cofactors for their action. The main types of cofactors are prosthetic groups, coenzymes, and activators.
a) Prosthetic groups are the organic molecules that make a permanent attachment with the enzyme.
b) Coenzymes are organic molecules, which are not permanently attached to the enzyme. Most coenzymes are derived from vitamins.
c) Activators:Â It is another type of cofactors. These are metal ions (e.g. zinc, copper, or iron) that form a temporary attachment with the enzyme.
Q.4) The diagram below shows the relationship an enzyme, a substrate and the product of an enzyme-catalyzed reaction.
Q.4 a) What is represented by the parts labeled A, B, and C in the diagram?
Answer:
Part A shows “substrate”, part B shows “Enzyme” and part C shows “products”.
Q.4 b) Name two properties of enzymes that are represented in this diagram. What will happen to the chemical reactions if the enzyme is removed?
Answer:
- Enzymes are specific in action. They are specific not only for substrate but also for the kind of reaction.
- Enzyme remains unchanged after the reaction is completed.
The chemical reaction will stop, if enzyme is removed.
Q.4 c) What will happen to the rate of reaction if the reaction temperature is raised steadily
i) From 250C to 350C
ii) From 400C to 600C?
Answer:
- i) If the reaction temperature is changed from 250C to 350C, the rate of reaction will increase.
- ii) If the temperature increases from 400C to 600C, the enzyme starts to denature and reaction will stop.
Q.4 d) What term is used to describe the condition of the enzyme when it is heated to a temperature of 600C and above?
Answer:
 When enzymes are heated at 600C and above, the enzymes stop working and the term used is enzyme denaturation.
Q.6 a) According to graph A, at which temperature the enzyme activity is highest. When temperature increases above this point, what will happen to the enzyme?
Answer:
 According to graph A, the enzyme activity is highest at 400C. When temperature increases above 400C, the enzyme starts denaturation and the rate of reaction decreases.
Related: Biology Notes FA FSc Chapter No 3 Enzymes