Biology Notes FA FSc Chapter No 3 Enzymes

The unique professor notes Biology FA FSc Chapter No 3 Enzymes pdf & images format download. MCQs, solved exercise, short questions, and long question.

Biology Notes FA FSc Chapter No 3 Enzymes pdf kpk

What is an Enzyme? Describe its different characteristics.

Chapter No 3 Enzymes

Definition: –Enzymes are complex organic compounds that speed up a biological reaction without any change in itself.” The term enzyme was used by F.W. Kuhne in 1876.

Characteristics of Enzymes:
Enzymes are the biochemical catalysts and possess the following important Characteristic of all is important for MCQs.

Catalyst:
They act as catalysts and speed up a biochemical reaction.

Protein nature:
Enzymes are partly or completely made up of proteins.

What is a cofactor?

Cofactors – are atoms, groups of atoms and molecules that join with enzymes altering their shape and making them functional. One can think of these cofactors as an” on-off switch for activating an enzyme. If the cofactor is a non-protein like a metallic ion (i.e. zinc, copper, or iron) it is directed to as a prosthetic body. Some cofactors are small organic molecules called coenzymes. Like enzymes, they are not permanently altered in the reactions. Many of these coenzymes are derived from vitamins and minerals that are essential for life. The absence of these cofactors can lead to vitamin and mineral deficiency diseases e.g lack of Vitamin B produces beriberi. Examples of coenzymes are NAD+, FAD+, NADP.

Q.2 ii) What are metal activators? Give three examples.

Answer:
Co-factors (metal activators):
  “A cofactor is a non-protein chemical compound or metallic ion that is required for an enzyme’s activity.”
    Cofactors are atoms, groups of atoms and molecules that join with enzymes altering their shape and making them functional.
Metal activators:
   Metal Activators are cations that are positively charged metal ions.


 Example: 
         K+, Fe++, Fe+++etc.
Prosthetic group:
 If the cofactor is a non-protein like a metallic ion (i.e. zinc, copper, or iron) it is referred to as a prosthetic group.
Co-enzymes:
   Some cofactors are small organic molecules called coenzymes. Like enzymes, they are not permanently altered in the reactions. Many of these coenzymes are derived from vitamins and minerals that are essential for life. The absence of these cofactors can lead to vitamin and mineral deficiency diseases e.g lack of Vitamin B produces beriberi. 

Examples of coenzymes:
       NAD+, FAD+, NADP.

Q.2 iii) Differentiate the key difference between the Lock and Key Model and Induced Fit Hypothesis?

Answer: 

Induced- fit theoryLock-and-key theory
An induced-fit theory is a modified version of the lock and key theoryLock-and-key theory is the initial theory of enzyme action.
An induced-fit theory does not depend on the precise contact being made between the active site and substrate. It depends on the precise contact being made between the active site and substrate. 
The enzyme shape is affected by the substrate.Substrate shape is affected by the enzyme. 
The actives sites have a precise shape.The active site initially do not have a precise shape, but later the site shape is formed according to the substrate, which is going

Q.2 iv) How pH of a cell affects the enzyme activity?

Answer:
Effect of pH on enzyme activity:
  Every enzyme operates most effectively over a limited range of pH understood as the optimum pH. A small change in pH can revise the ionization of the amino acids at the active site. Moreover, it may influence the ionization of the substrates. Beneath these changed needs enzyme activity is either retarded or stopped completely. Extreme differences in pH cause the bonds in the enzyme to break, resulting in the enzyme denaturation.


Example:
   For most enzymes, optimum pH is about 7-8 (physiological pH of most cells) but a few enzymes can work at an extreme pH such as protease enzymes in the animal stomach which have an optimum pH of 1. The pH affects the charge of the amino acids at an active site more specifically, changes in pH ionizes amino acids forming an enzyme so the properties of the active zone change and the substrate can no extended bind.


Long Questions notes for biology class 11

Q.3 i) Describes the characteristics of enzymes.

Answer:
Characteristics of enzymes:
   Enzymes, the biochemical catalysts possess the following important characteristics.

  • All enzymes are globular proteins.
  • Enzymes are required in a very small quantity for the reaction.
  • Enzymes only speed up a reaction and do not affect the equilibrium of the reaction.
  • They are sensitive to even a minor change in pH, temperature and substrate concentration.
  • Their presence does not affect the nature or properties of end products.
  • Small amounts of an enzyme can accelerate a chemical reaction.
  • They are very specific in their action; a single enzyme catalyzes only a single chemical reaction or a group of related reactions.
  • Some enzymes require a co-factor for their proper functioning.
  • They lower the activation energy of the reactions.

Q.3 ii) What do you know about inhibitors of enzymes?

Answer:
Inhibitors:
“An enzyme inhibitor is a molecule that binds to an enzyme and decreases its activity.”
  Inhibitors inhibit the action of enzymes, reducing the rate of their responses. They are found inherently but are also used artificially as drugs, pesticides and analysis tools.


Types of inhibitors:
  There are two kinds of inhibitors.

Example:
    The sulphonamide to antibacterial drugs which act as competitive inhibitors.
2- Non-Competitive inhibitors:
      A non-competitive inhibitor molecule is quite different in structure from the substrate molecule and does not fit into the active site. It binds to another part of the enzyme molecule changing the shape of the whole enzyme, including the active site, so that it can no longer bind substrate molecules.
a. Reversible inhibitors:
    Inhibitors that bind weakly and can be removed out easily are sometimes called reversible inhibitors.
b. Irreversible inhibitors:
      The inhibitors that bind tightly and cannot be removed out are called irreversible inhibitors.
Example:
  Poisons like cyanide, heavy metal ions and some insecticides are all non-competitive inhibitors.
    The activity of some enzymes is regulated by certain molecules critical to a specific regulatory (or allosteric) place on the enzyme, different from the busy site. Different molecules can interfere or activate the enzyme, allowing sophisticated control of the rate. Only a few enzymes can do this, and they are often at the start of a long biochemical pathway. They are generally activated by the substrate of the pathway and inhibited by the product of the pathway, thus only turning the pathway on when it is needed.


Feedback Inhibition:
   Another kind of inhibition is called feedback inhibition. In feedback inhibition, there is a second binding site on the enzyme where the inhibitor binds so that the inhibitor is not necessarily similar in structure to the substrate. The absence or presence of the inhibitor at this second binding site activates or deactivates the enzyme, by changing the conformation of the enzyme so that the active site is made available or unavailable to the substrate. The inhibitor is usually the product of a reaction formed during the metabolic pathway.

Feedback Inhibition

Q.3 iii) Write briefly the mode of action of an enzyme.

Answer:
Enzymes:
  “Enzymes are proteins that act as catalysts within a living cell to bring about a specific biochemical reaction.”
Structure of enzymes:
   Enzymes are proteins, and their function is determined by their complex structure. The reaction takes place in a small part of the enzyme called the active site, while the rest of the protein acts as a framework. The amino acids around the active site attach to the substrate molecule and hold it in position while the reaction takes place. This makes the enzyme specific for one reaction only, as other molecules would not fit into the active site.


Hypothesis about enzyme action:
   Following two hypotheses explains the mode of enzyme action.
1- Lock and Key Hypothesis
2- Induced fit hypothesis


1- Lock and Key Hypothesis:
       Fischer in 1890 suggested that each enzyme had a particular shape into which the substrate fits exactly. This was known as the lock and key hypothesis. According to this hypothesis, the substrate is imagined like a lock while the enzyme is imagined like a key. As one specific key can open only a specific lock, similarly a specific enzyme can break up only one specific substrate. The active site is regarded as a rigid structure that does not modify or change during the reaction process. However, later studies did not support this hypothesis in all type of reactions and therefore the hypothesis was modified into Induced fit hypothesis.

Read more: Cell structure and function Biology Class 11 Notes Chapter 1

Lock and Key Hypothesis
Lock and Key Hypothesis

2- Induced-Fit Hypothesis:
        The attraction of the substrate and enzyme form an enzyme-substrate complex. It was originally referred to as the Lock and Key Enzyme Theory. The current theory suggests that the enzyme molecules are in an inactive form. To become active they must undergo a slight change in structure to form.
Process of enzyme catalysis:
  There are three ways of thinking about enzyme catalysis which are:
i- Reaction mechanism
ii- Molecular geometry
iii- Energy changes
i- Reaction mechanism:
       In any chemical reaction, a substrate (S) is converted into a product (P):
                                            S→P
   There may be more than one substrate and more than one product, but that doesn’t matter. In an enzyme-catalyzed reaction, the substrate first binds to the active site of the enzyme to form an enzyme-substrate (ES) complex, then the substrate is converted into product while attached to the enzyme, and finally the product is released.
ii. Molecule Geometry:
        The substrate molecule fits into the active site of the enzyme molecule like a key fitting into a lock. Once there, the enzyme changes shape slightly, distorting the molecule in the active site, and making it more likely to change into the product.
Example:
   If a bond in the substrate is to be broken, that bond might be stretched by the enzyme, making it more likely to break. Alternatively, the enzyme can make the local conditions inside the active site quite different from that outside (such as pH, water concentration, charge), so that the reaction is more likely to happen.
iii- Energy changes:
         The way enzymes work can also be shown by considering the energy changes that take place during a chemical reaction. Consider a reaction where the product has lower energy than the substrate, so the substrate naturally turns into a product (in other words the equilibrium lies in the direction of the product).
Activation energy:
   Before it can change into a product, the substrate must overcome an “energy barrier” called the activation energy (EA). The larger the activation energy, the slower the reaction will be because only a few substrate molecules will by chance have sufficient energy to overcome the activation energy barrier.
   Enzymes dramatically reduce the activation energy of a reaction, so that most molecules can easily get over the activation energy barrier and quickly turn into a product.
Example:
  For the reaction (2H2O2 → 2H2O + O) the activation energy is 86 kJ mol, with no catalyst and just 1 kJ mol, in the presence of the enzyme catalase.
   The activation energy is actually the energy required to form the transition state, so enzymes lower the activation energy by stabilizing the transition state, and they do this by changing the conditions within the active site of the enzyme.

Q.3 iv) How do the enzyme and substrate concentrations affect the rate of enzyme action.

Answer:
1. Enzyme concentration:
As the enzyme concentration increases the rate of the reaction increases linearly because there are more enzyme molecules available to catalyze the reaction. At very high enzyme concentration the substrate concentration may become rate-limiting, so the rate stops increasing. Normally enzymes are present in cells in rather low concentrations.
2. Substrate concentration:

The rate of an enzyme-catalyzed reaction shows a curved dependence on substrate concentration. As the substrate concentration increases, the rate increases because more substrate molecules can collide with enzyme molecules, so more reactions will take place. At higher concentrations, the enzyme molecules become saturated with the substrate so there are few free enzyme molecules so adding more substrate doesn’t make much difference.

Substrate concentration

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