11th Class Biology Unit 5 Enzymes Long Questions and Answers

1.Describe the structure of enzymes, explaining the role and component parts of the active site.

Definition Enzymes may be defined as specific globular proteins that speed up specific chemical reactions by lowering the required activation energy, but remain unaltered in the process. Enzymes are also known as biocatalysts. Rates of enzyme-catalysed reactions may be 10³ to 10⁶ times greater than the rates of corresponding uncatalyzed reactions.

Structure Enzymes are three-dimensional globular proteins. They are made of polypeptide chains that are coiled upon themselves.

Active site There is a small cleft or depression on the surface of globular enzyme molecule. It consists of only a few amino acids. This site is known as active site. It is the location at which catalysis occurs.

Shape The shape of active site of each enzyme is very specific. So, only a certain substrate molecule can fit into it. It is three-dimensional and bears a specific charge.

Regions Active site has two distinct regions i.e., binding site and catalytic site.
(i) Binding site: Substrate molecule fits into binding site by weak chemical forces, such as hydrogen bonds.
(ii) Catalytic site: It catalyses the reaction and substrate is transformed into products

2.Differentiate among the three types of co-factors by giving examples.

Cofactors Many enzymes use additional chemical components to aid in catalysis. These additional non-protein components are called cofactors.

Types There are three kinds of cofactors: metal ions, prosthetic groups and coenzymes.

(i) Metal ions: Many enzymes use metal ions, such as Ca⁺², Mg⁺², Mn⁺², Cu⁺², and Zn⁺² as their cofactors. These metal ions change the non-functional active sites of enzymes into functional sites. The attachment of a cofactor also changes the shape of enzyme and allows it to combine with substrate.

Many trace elements such as molybdenum and manganese, which are necessary for our health, are used by enzymes as cofactors.

(ii) Prosthetic Groups: Some cofactors form covalent bonds with enzyme and are known as prosthetic groups. Prosthetic group may be an organic compound e.g., hematin. Hematin is also known as heme, is a part of hemoglobin and myoglobin, it acts as prosthetic group in enzymes like catalyses and peroxidases.

(iii)Coenzyme: When the cofactor is a non-protein organic molecule and is loosely attached with enzyme, it is called a coenzyme. Coenzymes participate in enzyme-catalysed reactions, often by transporting electrons (hydrogen atoms). from one enzyme to another.

• Many vitamins (e.g., niacin (vitamin B₃) and riboflavin (vitamin B₂)) function as coenzymes. Some are part of coenzymes. The most important coenzyme in cell is the hydrogen acceptor Nicotinamide Adenine Dinucleotide (NAD⁺).

Role of NAD⁺ in a cell

When NAD⁺ acquires a hydrogen atom from an enzyme, it reduces to NADH. The electron of hydrogen atom contains energy that NADH molecule carries. For example, when food is oxidized in cell, enzymes draw electrons from food molecules and transfer them to NAD⁺, which reduces to NADH.

3.Define activation energy and Explain through graph, how an enzyme speeds up a reaction by lowering activation energy?

The speed of a chemical reaction depends on the amount of activation energy required to initiate it. Activation energy is the energy which works to destabilize existing chemical bonds.

Effect of enzymes on activation energy: Enzymes bring reactants together in correct orientation or stress particular chemical bonds of reactants. Thus, they lower the activation energy required for new bonds to form and speed up the rate of reactions. Reactions proceed much faster than their normal speed.

Effect on End Product The presence of enzymes does not affect the nature or properties of end products. For example, sucrose (substrate) will always be hydrolysed into glucose and fructose (products) whether Sucrose enzyme is present or not.

Enzymes Substrate Complex Due to its specificity, an enzyme recognizes a specific substrate. The substrate binds with the active site of enzyme. In this way, an enzyme-substrate complex (ES complex) is formed and catalytic site is activated. The atoms of catalytic site stress and destabilize particular bonds of substrate. So, activation energy is lowered. This action initiates the reaction and substrate is transformed into products. After it, enzyme detaches itself from the products, in an unaltered state.

4.Explain the mechanism of enzyme action through induced fit model. Also compare it with lock and key model of enzyme action.

Enzyme-substrate complex Due to its specificity, an enzyme recognizes a specific substrate. The substrate binds with the active site of enzyme. In this way, an enzyme-substrate complex (ES complex) is formed and catalytic site is activated. The atoms of catalytic site stress and destabilize particular bonds of substrate. So, activation energy is lowered. This action initiates the reaction and substrate is transformed into products. After it, enzyme detaches itself from the products, in an unaltered state. The mechanism of enzyme action can be summarized as follows.

E + S -----> ES -----> E + P
Enzyme Substrate Enzyme-Substrate Enzyme Product
complex

Models for Mechanism of Action of Enzymes

1. Lock and key Model

History In 1894 a German chemist Emil Fischer proposed lock-and-key model. According to this model, "as a specific key can open only a specific lock, in the same manner a specific enzyme can transform only one specific substrate into products".

Postulates This model postulates that active site is a rigid structure and there is no modification or flexibility in it before, during or after the enzyme action

2. Induced Fit Model (Modified form of Lock and Key Model)

History Later studies did not support lock-and-key model in all reactions. On the basis of new evidence, an American biochemist Daniel Koshland in 1958 presented induced fit model. According to this model, "when a substrate combines with the binding site of an enzyme, it induces changes in enzyme structure. These changes enable the enzyme to perform its catalytic activity more effectively."

Postulates This model postulates that active site is not a rigid structure and is capable of going under modification and flexibility, before the enzyme action (catalysis) starts

Comparison of induced fit model with lock and key model

Active site According to lock and key model active site is a rigid structure but according to induced fit model it is flexible.

Enzyme regulation Lock and key model is followed by non-allosteric enzymes, while induced fit model is followed by allosteric enzymes.

Reactions Lock and key model is usually for simple reactions (e.g., one step reactions) while induced fit model is usually for complex reactions allowing feedback regulation.

5.Describe the effect of temperature on the rate of enzyme action. / Compare the effect of temperature on enzymes of human and thermophilic bacteria.

Effect of Temperature on Enzyme Activity in Humans

The shape of a protein is determined by the hydrogen bonds and hydrophobic interactions that hold its polypeptide chains in particular position. Both the hydrogen bonds and hydrophobic interactions are easily disrupted by slight changes in temperature.

Optimum Temperature Every enzyme works at its maximum rate at a specific temperature called its optimum temperature.
e.g., The optimum temperature for human enzymes is 37 °C.

At low Temperature When temperature fall below optimum temperature, the bonds that determine enzymes shape become less flexible. They do not permit the induced change in active sites that is necessary for enzyme action and so reaction rate is slow.

At High Temperature When temperature is raised up to a certain limit, the heat adds in substrate and enzyme and so reactions are accelerated. Heat also provides kinetic energy to enzyme molecules. It causes them to move rapidly. Thus, they collide more frequently and reaction rate is increased.

Denaturation When temperature is raised well above optimum temperature, the heat energy increases the vibrations of atoms of enzyme molecules. When vibrations become too violent, the bonds cannot hold polypeptide chains in the proper position and globular structure of enzyme is lost. This phenomenon is known as denaturation of enzyme. It results in a rapid decrease in the rate of enzyme action and it may be blocked completely.

★Effect of Temperature on Enzyme Activity in Thermophilic Bacteria

Thermophilic bacteria live in hot springs. They have proteins with stronger bonding between their polypeptide arms and can function at temperature of 70°C or higher. So, they are more thermostable than human enzymes

6.How pH affects the rate of enzyme action? Write the optimum pH of various human enzymes.

Effect of pH on Enzyme Activity

All enzymes work at their maximum rate at a narrow range of pH.

Optimum pH Every enzyme works at its best at a specific pH, called its optimum pH. For example, pepsin is active in acidic medium (low pH) while trypsin shows its optimum activity in alkaline medium (high pH). Some enzymes like papain from green papaya work both in acidic and alkaline media.

Effect of changed pH In the globular structure of an enzyme, polypeptide chains are held by bonds between oppositely charged amino acids, such as glutamic acid (-) and lysine (+). These bonds are sensitive to hydrogen ion concentration. Any change in pH can change the ionization of amino acids at active site. Moreover, it may affect the ionization of substrate.

• A slight change (increase or decrease) in this pH causes retardation in enzyme activity or blocks it completely.

• Extreme change in pH can break the bonds in enzymes, resulting in enzyme denaturation.

Table Optimum pH of important human enzymes

7.Describe how the concentration of enzyme affects the rate of enzyme action?

Enzymes are very efficient in low concentrations and a small number of enzyme molecules can catalyse reactions of large amount of substrate. The overall rate of enzyme-controlled reactions depends directly on the amount of enzyme present at a specific time (if substrate concentration is unlimited).

Effect of increase in Enzyme Concentration: When enzyme concentration increases, there are more enzyme molecules and more active sites. So, more substrate molecules bind with new active sites and are transformed into products. If enzyme concentration goes on increasing but substrate concentration remains the same, no more substrate molecules will be available to attach with enzymes. So, the rate of reaction stays constant and does not increase further.

8.Explain the effect of substrate concentration on the role of enzyme action?

If there are enzyme molecules with vacant active sites, an increase in substrate concentration will increase the rate of reaction.

At Constant Enzyme Concentration If enzyme concentration is kept constant and the amount of substrate is increased, a point is reached where any further increase in substrate does not increase the rate of reaction anymore.

Low substrate concentration

• When enzyme molecules are free (at low substrate concentration) new substrate molecules bind with the available active sites and so more products are formed in the given time i.e., rate of enzyme action is increased.

At High Substrate Concentration

• But when all active sites of enzymes are occupied (at high substrate concentration), any more substrate molecules do not find free active sites and so reaction rate does not increase and this point is called saturation.

9.Describe Enzymes inhibition. / Describe enzymatic inhibition, its types and its significance.

Inhibitor A chemical that interferes and blocks an enzyme's activity is called an inhibitor.

Inhibition In this process inhibitors attach with enzymes but are not transformed into products and thus block active sites temporarily or permanently. This phenomenon is known as enzyme inhibition. The final products of complex enzymatic reactions also act as the inhibitors of the enzyme of the first step (allosteric inhibition).

Types of Inhibitors Two general types of inhibitors are recognized competitive and non-competitive inhibitors.

1. Competitive: A competitive inhibitor resembles structurally with the enzyme's substrate. It competes with substrate for the same binding site on enzyme.

Example The enzyme succinic dehydrogenase catalyses the oxidation of succinic acid to fumaric acid. Malonic acid has structural similarity with substrate (succinic acid). So, both of them compete for active site of enzyme. Malonic acid is selected by active site and thus blocks it.

2. Non-competitive: A non-competitive inhibitor has no real structural similarity to substrate. So, it does not enter active site. Instead, it binds enzyme at other places. Its binding alters the shape of enzyme so that active site does not fit substrate and so enzyme is inhibited.

For example Two substrates i.e., succinic acid and CoA react to form succinyl- CoA. This reaction is catalysed by enzyme succinyl-CoA synthetase. After its formation, the product i.e., succinyl CoA acts as a non-competitive inhibitor and binds with enzyme. Thus, enzyme is inhibited and no more succinyl-CoA is produced.

3. Reversible and Irreversible inhibitors

The action of any inhibitor can be irreversible or reversible, depending upon the kind of bond formed between inhibitor and enzyme.

Irreversible inhibitors make covalent bonds with enzyme. Such inhibitors cannot be released by dilution or dialysis or by increasing the concentration of substrate.

• For example, penicillin permanently disables the enzyme responsible for building bacterial cell walls.

Reversible inhibitors make weak bonds. (e.g., hydrogen bonds) with enzyme. Such inhibitors can be released and the inhibition caused by them can be neutralized by increasing the concentration of substrate.

4. Significance of Enzyme inhibition

Enzyme inhibition is crucial in various biological processes.

(i) Enzyme inhibition plays a vital role in regulating metabolic pathways. By inhibiting specific enzymes, the rate of a metabolic reaction can be controlled.

(ii) Many drugs work as inhibitors. For example, antibiotics inhibit the enzymes of bacteria, while cancer drugs may inhibit enzymes involved in cell division.

(iii) Enzyme inhibitors are used to manage various medical conditions. For example, some inhibitors of enzymes involved in blood clotting are used as anticoagulants.

(iv) Some toxins and poisons inhibit important enzymes in the body. Understanding how these inhibitors affect enzymes can be critical in treating cases of poisoning.

(v) Enzyme inhibitors serve as valuable tools in pharmaceutical research. They are used to study the function of specific enzymes, and potential drugs. Enzyme inhibition is an important part of studying enzyme kinetics; it helps to understand the factors that influence enzyme activity.

10.Explain feedback inhibition of enzymes.

Definition Feedback inhibition is the phenomenon where the product of a process controls the process itself, oftentimes limiting the production of more products.

Feedback Inhibition of Enzymes In metabolic pathways, the product of one reaction becomes the substrate for the next reaction. At the end of pathway, a desired product is synthesized. In order to regulate the concentration of that product, pathway needs to be shut down. This is done through feedback inhibition. The final product of pathway acts as inhibitor. It reacts with some initial enzyme and changes its conformation.

That enzyme can no longer bind to its substrate. So, pathway closes and no more product is prepared.

For example When a cell has a greater number of ATP than its requirement. ATP itself acts as a non-competitive inhibitor and blocks the enzyme that catalyses ATP synthesis.

11.Classify enzymes on the basis of reactions catalyzed.

According to the general type of reaction, enzymes are classified into six classes.

1. Oxidoreductases: These enzymes catalyse the oxidation / reduction of their substrates. They add or remove H⁺ ions or electrons from substrates. For example, cytochrome oxidase catalyses the oxidation of cytochrome.

2. Transferases: The enzymes of this class catalyse the transfer of a specific functional group (e.g., methyl, acyl, amino or phosphate) from one substrate to another
For example, hexokinase, transfers phosphate group from ATP to glucose.

3. Hydrolases: These enzymes catalyse hydrolysis reactions. They break their substrates into monomers by adding water.
For example, lipase, amylase, peptidase and other digestive enzymes catalyse the hydrolysis of food molecules.

4. Lyases: These enzymes catalyse non-hydrolytic addition to removal of groups (e.g., CO₂, NH₂ etc.) from substrates.
For example, pyruvate decarboxylase removes CO₂ from pyruvic acid.

5. Isomerases: These enzymes catalyse the intra-molecular rearrangement i.e., one isomer is converted into another.
For example, hexose isomerase converts glucose to fructose.

6. Ligases: These enzymes catalyse the reactions in which two molecules join by forming new C-C, C-N, C-O, or C-S bonds, using energy from ATP
For example, Polymerase enzymes join monomers by using ATP, like DNA polymerase join nucleotides.

12.Classify enzymes on the basis of substrates. Give examples of enzymes naming according to substrates.

Enzymes are also classified into following groups on the basis of their substrates.

1. Proteases: This group included the enzymes which catalyse the breakdown of proteins.

• For example, pepsin and trypsin enzymes catalyse the breakdown of large polypeptides into smaller polypeptides. Similarly, aminopeptidases further breakdown small polypeptides into dipeptides and erypsin breaks dipeptides into amino acids.

2. Lipases: These enzymes act upon lipids and catalyse their breakdown.

• For example, pancreatic lipase hydrolyses lipids into fatty acids and glycerol.

3. Carbohydrases: These enzymes act upon bigger carbohydrates and break them into smaller units.

For example

• Amylase acts upon starch or glycogen and breaks them into maltose.

• Cellulase breaks cellulose into cellobiose (a disaccharide) or glucose.

• Maltase breaks down maltose into glucose.

• Sucrase breaks sucrose into glucose and fructose.

• Lactase breaks lactose into glucose and galactose.

4. Nucleases: These enzymes act upon nucleic acids and catalyse their breakdown. For example, RNAse, DNAse, ATPase are responsible for the breakdown of RNA, DNA and ATP respectively

13.Classify enzymes in form of table on the basis of reactions they catalyze. Also write their important subclasses.

Enzymes are classified into following groups on the basis of reaction they catalyze.

Unit 5: Enzymes — Long Questions