1. Write the properties of enzymes.
Answer: Enzymes are biological catalysts that accelerate chemical reactions in living organisms. They possess several unique properties that distinguish them from inorganic catalysts.
1. Protein Nature: Almost all enzymes are globular proteins. Their function is directly dependent on their three-dimensional structure. Denaturation of the protein leads to loss of catalytic activity.
2. Catalytic Efficiency: Enzymes are highly efficient catalysts. They can increase the rate of a reaction by a factor of 10⁶ to 10¹² compared to the uncatalyzed reaction. They are required in very small amounts.
3. Specificity: Enzymes are highly specific to their substrates. This specificity can be absolute (the enzyme acts on only one substrate) or relative (the enzyme acts on a group of similar substrates). The specificity is due to the precise shape and chemical properties of the active site.
4. Reversibility: Enzymes catalyze reversible reactions and do not alter the equilibrium position of a reaction. They simply speed up the rate at which equilibrium is reached.
5. pH Sensitivity: Each enzyme has an optimal pH at which its activity is maximum. Deviation from this pH can alter the enzyme's charge and shape, reducing its activity. Extreme pH values can cause denaturation.
6. Temperature Sensitivity: Enzymes have an optimal temperature. The rate of reaction increases with temperature up to this optimum. Above the optimum, the enzyme denatures, and the rate decreases sharply.
7. Cofactor Requirement: Many enzymes require non-protein components called cofactors (metal ions) or coenzymes (organic molecules) for their activity.
8. Regulation: Enzyme activity can be regulated by various mechanisms, including feedback inhibition, allosteric regulation, and
covalent modification (e.g., phosphorylation).
9. Turnover Number: The turnover number is the number of substrate molecules converted to product per enzyme molecule per unit time. It is a measure of catalytic efficiency.
10. Location: Enzymes are often compartmentalized within cells (e.g., in organelles) to separate metabolic pathways.
In summary, enzymes are highly specific, efficient, and regulated biological catalysts that are essential for all metabolic processes in living organisms.
2. Explain the role and component parts of the active site of an enzyme.
Answer: The active site is a specific region on the surface of an enzyme where substrate molecules bind and undergo a chemical reaction. It is a small pocket or groove, typically composed of only a few amino acid residues, but its unique three-dimensional shape and chemical properties are crucial for enzyme function.
Role of the Active Site:
Substrate Binding: The active site binds the substrate(s) through non-covalent interactions such as hydrogen bonds, ionic bonds, and hydrophobic interactions. This binding is highly specific, ensuring that the enzyme only acts on its intended substrate. Catalysis: The active site provides a microenvironment that is optimal for the chemical reaction. It brings the substrate molecules into close proximity and correct orientation, which lowers the activation energy. It may also involve amino acid residues that act as acids, bases, or nucleophiles to facilitate the reaction. Stabilization of Transition State: The active site stabilizes the transition state of the reaction, further lowering the activation energy.
Component Parts of the Active Site:
Binding Site: This is the region that recognizes and binds the substrate. It is formed by specific amino acid side chains that are complementary in shape and charge to the substrate. Catalytic Site: This is the region that actually carries out the chemical reaction. It contains amino acid residues that participate directly in the catalytic mechanism (e.g., by donating or accepting protons, or by forming temporary covalent bonds with the substrate). Cofactor Binding Site: If the enzyme requires a cofactor or coenzyme, the active site also includes a site for binding this non-protein component.
Factors Determining Active Site Structure:
Primary Structure: The sequence of amino acids determines how the protein folds. Secondary and Tertiary Structure: The folding of the polypeptide chain brings the relevant amino acid residues together to form the active site. Conformational Changes: In the induced fit model, the active site changes shape upon substrate binding to better fit the substrate.
In summary, the active site is a highly specialized region of an enzyme that is responsible for substrate binding and catalysis. Its structure is precisely tuned to its function, and any alteration can lead to loss of activity.
3. What are cofactors? Describe the two types of cofactors by giving examples.
Answer: Cofactors are non-protein chemical compounds or metallic ions that are required for an enzyme's biological activity. They assist in the catalytic process, often by helping to stabilize the enzyme's structure or by participating directly in the reaction. Cofactors can be classified into two main types: inorganic cofactors (metal ions) and organic cofactors (coenzymes and prosthetic groups).
1. Inorganic Cofactors (Metal Ions):
Description: These are metal ions that bind to the enzyme and assist in catalysis. They can help to orient the substrate, stabilize charged intermediates, or participate in redox reactions. They are often tightly bound to the enzyme. Examples:
Magnesium (Mg²⁺): Required by kinases (e.g., hexokinase) to neutralize the negative charge of ATP and facilitate its transfer. Zinc (Zn²⁺): Found in carbonic anhydrase, where it helps to polarize a water molecule for attack on CO₂. Iron (Fe²⁺/Fe³⁺): Found in cytochromes and catalase, where it participates in electron transfer and redox reactions. Calcium (Ca²⁺): Acts as an activator for many enzymes, such as blood clotting factors.
2. Organic Cofactors:
Description: These are organic molecules that assist in catalysis. They can be further divided into coenzymes and prosthetic groups based on how tightly they are bound to the enzyme. Coenzymes: Loosely bound organic molecules that can dissociate from the enzyme. They often act as carriers of electrons, atoms, or functional groups.
Examples: NAD⁺ (carries electrons in redox reactions), FAD (carries electrons), Coenzyme A (carries acyl groups), and ATP (carries phosphate groups). Many coenzymes are derived from vitamins (e.g., niacin [source symbol unavailable] NAD⁺, riboflavin [source symbol unavailable] FAD).
Prosthetic Groups: Tightly and permanently bound organic molecules.
Examples: Heme (in hemoglobin and cytochromes), biotin (in carboxylases), and flavin mononucleotide (FMN).
In summary, cofactors are essential non-protein components that enable many enzymes to function. Inorganic cofactors are metal ions, while organic cofactors include loosely bound coenzymes and tightly bound prosthetic groups.
4. Explain the mechanism of enzyme action through induced fit model.
Answer: The induced fit model, proposed by Daniel Koshland in 1958, is a more accurate description of enzyme-substrate interaction than the earlier lock-and-key model. It proposes that the active site of an enzyme is not a rigid, pre-formed structure, but rather a flexible one that changes shape upon binding to the substrate.
Steps of the Induced Fit Model:
1. Initial Binding: The substrate approaches the enzyme's active site. The active site is not perfectly complementary to the substrate in its free form.
2. Induced Conformational Change: As the substrate binds, it induces a conformational change in the enzyme's active site. The active site molds itself around the substrate to become complementary in shape.
3. Transition State Stabilization: This conformational change brings specific amino acid residues into the correct positions to catalyze the reaction. It also strains chemical bonds in the substrate, making them easier to break. The active site stabilizes the transition state of the reaction.
4. Catalysis: The reaction occurs, converting the substrate into the product.
5. Product Release: The product(s) are released from the active site. The enzyme returns to its original conformation, ready to bind another substrate.
Significance:
Explains Enzyme Flexibility: It explains how enzymes can catalyze reactions for substrates that are not perfectly complementary in their ground state. Explains Broad Specificity: Some enzymes can act on a group of similar substrates. The induced fit allows the active site to adjust to slightly different substrates. Explains Allosteric Regulation: It provides a basis for understanding how allosteric regulators can bind to a site other than the active site and induce a conformational change that affects the active site. Explains Denaturation: It explains why enzymes are so sensitive to changes in pH and temperature: these factors can disrupt the conformational changes necessary for induced fit.
In summary, the induced fit model describes a dynamic interaction between enzyme and substrate, where the enzyme's active site
is flexible and adjusts its shape to better fit the substrate, thereby facilitating catalysis.
5. Explain the mechanism of enzyme action through lock and key model.
Answer: The lock and key model, proposed by Emil Fischer in 1894, was the first model to explain enzyme specificity. It proposes that the enzyme's active site is a rigid structure that is perfectly complementary in shape to its specific substrate, much like a lock fits a specific key.
Steps of the Lock and Key Model:
1. Substrate Recognition: The substrate molecule (the "key") has a specific shape that is complementary to the active site of the enzyme (the "lock").
2. Binding: The substrate fits precisely into the active site, forming an enzyme-substrate complex. This binding is stabilized by non-covalent interactions such as hydrogen bonds, ionic bonds, and hydrophobic interactions.
3. Catalysis: Once bound, the substrate undergoes a chemical reaction within the active site. The enzyme provides a favorable microenvironment for the reaction, lowering the activation energy.
4. Product Formation: The substrate is converted into product(s).
5. Product Release: The product(s) have a different shape and no longer fit the active site, so they are released. The enzyme (the "lock") remains unchanged and can bind another substrate molecule.
Significance:
Explains Specificity: It explains why enzymes are highly specific. Only a substrate with the correct shape can fit into the active site, just as only the correct key can open a lock. Simple to Understand: It provides a simple, intuitive model for enzyme action.
Limitations:
Rigid Active Site: It implies that the active site is rigid and does not change shape. This is not entirely accurate, as many enzymes undergo conformational changes upon substrate binding. Does Not Explain Allosteric Regulation: It does not explain how molecules can bind to sites other than the active site and affect enzyme activity. Does Not Explain Transition State Stabilization: It does not fully explain how enzymes stabilize the transition state and lower activation energy.
Despite these limitations, the lock and key model was a significant step in understanding enzyme action and remains a useful introductory concept. The more refined induced fit model is now widely accepted.
6. Explain how an enzyme catalyzes specific reactions.
Answer: Enzymes catalyze specific reactions through several mechanisms that lower the activation energy of the reaction. These mechanisms are facilitated by the unique structure of the enzyme's active site.
1. Proximity and Orientation: The active site brings the substrate molecules into close proximity and correct orientation. This increases the likelihood of effective collisions, which is particularly important for reactions involving two or more substrates. By orienting the substrates correctly, the enzyme reduces the entropy of the system and makes the reaction more favorable.
2. Strain and Distortion: The binding of the substrate to the active site can induce strain or distortion in the substrate's chemical bonds. This weakens the bonds and makes them easier to break, reducing the energy required for the reaction.
3. Catalytic Groups: The active site contains specific amino acid residues that act as catalytic groups. These residues can act as acids, bases, or nucleophiles. For example, an acidic residue can donate a proton to the substrate, while a basic residue can accept a proton. A nucleophilic residue can form a temporary covalent bond with the substrate, creating a reactive intermediate.
4. Microenvironment: The active site provides a specialized microenvironment that is different from the surrounding solution. This
microenvironment can have a specific pH, polarity, or charge that is optimal for the reaction. For example, the active site may be hydrophobic, which can enhance the reactivity of certain substrates.
5. Transition State Stabilization: The active site is complementary in shape and charge to the transition state of the reaction, not the substrate. By binding and stabilizing the transition state, the enzyme lowers the activation energy required to reach it.
6. Cofactors and Coenzymes: Many enzymes require cofactors or coenzymes to catalyze reactions. These non-protein components can participate directly in the reaction by carrying electrons, atoms, or functional groups.
In summary, enzymes catalyze specific reactions by bringing substrates together, straining bonds, providing catalytic groups, creating a favorable microenvironment, and stabilizing the transition state. These mechanisms work together to dramatically lower the activation energy and increase the rate of the reaction.
7. Explain through graph how an enzyme speeds up reaction by lowering the energy of activation.
Answer: (Graph Description: A reaction coordinate diagram showing energy on the y-axis and reaction progress on the x-axis. Two curves are shown: one for the uncatalyzed reaction and one for the catalyzed reaction.)
The graph illustrates how an enzyme speeds up a reaction by lowering the activation energy (Ea).
Reactants and Products: The starting energy level (reactants) and the final energy level (products) are the same for both the catalyzed and uncatalyzed reactions. The overall free energy change (ΔG) of the reaction is unaffected by the enzyme. Uncatalyzed Reaction: The curve for the uncatalyzed reaction has a high peak. This peak represents the transition state. The energy difference between the reactants and the transition state is the activation energy (Ea) for the uncatalyzed reaction. This is a large energy barrier, so only a small fraction of molecules have enough energy to react at a given time, resulting in a slow reaction rate. Catalyzed Reaction: The curve for the enzyme-catalyzed reaction has a much lower peak. The enzyme provides an
alternative reaction pathway with a lower activation energy (Ea'). The enzyme stabilizes the transition state, reducing the energy required to reach it. Because the energy barrier is lower, a much larger fraction of molecules have sufficient energy to react at the same temperature. This leads to a dramatic increase in the reaction rate. Rate Enhancement: The ratio of the rates of the catalyzed and uncatalyzed reactions is related to the difference in activation energies. A small decrease in Ea can cause a very large increase in the reaction rate.
In summary, the graph clearly shows that enzymes do not change the energy of reactants or products, nor the equilibrium constant. They simply provide a lower energy pathway, thereby lowering the activation energy and increasing the rate at which equilibrium is reached.
8. Describe the effect of temperature on the rate of enzyme action.
Answer: Temperature has a profound effect on the rate of enzyme-catalyzed reactions. The relationship between temperature and enzyme activity is typically represented by a bell-shaped curve.
1. Increase in Rate with Temperature (Up to Optimum): As temperature increases, the kinetic energy of both the enzyme and substrate molecules increases. This leads to more frequent and more energetic collisions between enzyme and substrate, resulting in the formation of more enzyme-substrate complexes. Consequently, the rate of the reaction increases. For most human enzymes, this increase is observed up to an optimum temperature of around 37°C.
2. Optimal Temperature: The optimal temperature is the temperature at which the enzyme's activity is highest. At this temperature, the rate of collision is optimal, and the enzyme's structure is stable. The optimum temperature varies for different enzymes depending on the organism's environment. For example, thermophilic bacteria have enzymes with optima around 70-80°C.
3. Denaturation at High Temperatures:
Above the optimal temperature, the rate of reaction decreases sharply. This is due to denaturation of the enzyme. High temperatures provide enough energy to break the hydrogen bonds, ionic bonds, and hydrophobic interactions that maintain the enzyme's three-dimensional structure. The enzyme unfolds, and the active site is distorted, losing its complementary shape to the substrate. Denaturation is usually irreversible.
4. Effect of Low Temperatures: At low temperatures, the rate of enzyme-catalyzed reactions is slow. The kinetic energy of the molecules is low, so collisions are less frequent and less energetic. However, low temperatures do not denature the enzyme; they merely inactivate it. If the temperature is raised again, the enzyme can regain its activity.
In summary, the rate of enzyme action increases with temperature up to an optimum, then decreases sharply due to denaturation. This is a critical factor for the survival of organisms, which must maintain their internal temperature within a narrow range.
9. Describe how the concentration of enzyme affects the rate of enzyme action.
Answer: The concentration of enzyme is a key factor that affects the rate of an enzyme-catalyzed reaction. The relationship depends on the availability of substrate.
1. When Substrate is in Excess (Not Limiting): If the substrate concentration is very high (i.e., all enzyme active sites can be occupied), the rate of reaction is directly proportional to the enzyme concentration. This is because the enzyme is the limiting factor. As you increase the number of enzyme molecules, you increase the number of active sites available to bind substrate. This leads to more enzyme-substrate complexes being formed per unit time, and thus a higher rate of product formation. The graph of rate vs. enzyme concentration is a straight line passing through the origin.
2. When Substrate is Limiting:
If the substrate concentration is low, adding more enzyme may not increase the rate of reaction significantly. This is because the substrate molecules are the limiting factor. All available substrate molecules are already bound to enzyme active sites. Adding more enzyme will not help because there is no substrate for them to bind. The rate will reach a maximum and remain constant regardless of further increases in enzyme concentration.
3. Saturation: At very high enzyme concentrations, if substrate is not limiting, the rate will continue to increase linearly. However, if substrate is limiting, the rate will plateau.
Practical Implications:
In clinical diagnostics, enzyme concentrations in blood are measured to detect tissue damage (e.g., elevated liver enzymes indicate liver damage). In industrial processes, the amount of enzyme used is optimized for cost-effectiveness. In cells, enzyme concentrations are tightly regulated to control metabolic flux.
In summary, the effect of enzyme concentration on reaction rate depends on whether the substrate is present in excess. If substrate is abundant, the rate is directly proportional to enzyme concentration. If substrate is limiting, the rate is independent of enzyme concentration.
10. Explain the effect of substrate concentration on the rate of enzyme action.
Answer: The effect of substrate concentration on the rate of an enzyme-catalyzed reaction is described by a saturation curve, which is often hyperbolic in shape.
1. Low Substrate Concentration: At low substrate concentrations, the rate of reaction is directly proportional to the substrate concentration. This is because the
enzyme active sites are largely empty, and the rate is limited by the frequency of collisions between enzyme and substrate. The more substrate you add, the more frequently collisions occur, and the faster the reaction.
2. Increasing Substrate Concentration: As substrate concentration increases, the rate continues to increase, but the increase becomes progressively smaller. This is because more and more enzyme active sites become occupied. The probability of an enzyme molecule encountering a substrate molecule increases, but the enzyme molecules are becoming saturated.
3. Saturation (Vmax): At a sufficiently high substrate concentration, all the enzyme active sites are occupied by substrate at any given moment. The enzyme is said to be saturated. At this point, the rate of reaction reaches its maximum value (Vmax) and does not increase further with additional substrate. The reaction rate is now limited by the rate at which the enzyme can process the substrate (the turnover number), not by the substrate concentration.
4. Michaelis-Menten Kinetics: The relationship between substrate concentration and reaction rate is described by the Michaelis-Menten equation. The Km (Michaelis constant) is the substrate concentration at which the reaction rate is half of Vmax. It is an inverse measure of the enzyme's affinity for the substrate; a low Km indicates high affinity.
5. Practical Implications:
In cells, enzyme activity is often regulated by substrate availability. In clinical assays, Vmax and Km are used to characterize enzyme activity and to diagnose diseases.
In summary, the rate of an enzyme-catalyzed reaction increases with substrate concentration until the enzyme becomes saturated, at which point the rate reaches Vmax. This saturation kinetics is a hallmark of enzyme-catalyzed reactions.
11. Describe enzymatic inhibition, its types and its significance.
Answer: Enzymatic inhibition is the process by which a molecule (an inhibitor) binds to an enzyme and decreases its catalytic activity. It is a fundamental mechanism for regulating enzyme activity in cells and is also the basis for the action of many drugs and toxins.
Types of Inhibition:
1. Reversible Inhibition: The inhibitor binds non-covalently and can be removed.
Competitive Inhibition: The inhibitor binds reversibly to the active site, competing with the substrate. It has a similar shape to the substrate. It increases the apparent Km (lowers affinity) but does not change Vmax. Example: Malonate inhibits succinate dehydrogenase. Non-competitive Inhibition: The inhibitor binds to a site other than the active site (allosteric site). This causes a conformational change in the enzyme that reduces its activity. It does not affect substrate binding (Km unchanged) but lowers Vmax. Example: Cyanide inhibits cytochrome oxidase. Uncompetitive Inhibition: The inhibitor binds only to the enzyme-substrate complex. It lowers both Km and Vmax.
2. Irreversible Inhibition: The inhibitor binds covalently and permanently inactivates the enzyme.
Example: Penicillin inhibits transpeptidase, an enzyme needed for bacterial cell wall synthesis.
Significance of Enzymatic Inhibition:
Metabolic Regulation: Feedback inhibition is a key mechanism for regulating metabolic pathways, preventing the overproduction of end products. Drug Action: Many drugs are enzyme inhibitors. For example, statins inhibit HMG-CoA reductase to lower cholesterol. Aspirin inhibits cyclooxygenase to reduce pain and inflammation. Toxicology: Many poisons are enzyme inhibitors. For example, cyanide inhibits cytochrome oxidase, and heavy metals inhibit various enzymes. Research Tools: Inhibitors are used to study enzyme mechanisms and to identify metabolic pathways. Antibiotics: Some antibiotics act by inhibiting bacterial enzymes that are not present in humans, providing selective
toxicity.
In summary, enzymatic inhibition is a vital process for cellular regulation, and its principles are widely applied in medicine and toxicology.
12. Explain feedback mechanism with reference to enzymes.
Answer: Feedback inhibition (also called end-product inhibition) is a regulatory mechanism in which the final product of a metabolic pathway inhibits an enzyme that catalyzes an earlier step in the pathway. This is a common form of negative feedback that helps to maintain homeostasis and prevent the wasteful overproduction of metabolites.
Mechanism:
A metabolic pathway consists of a series of enzymatic reactions, where the product of one reaction becomes the substrate for the next. The final product of the pathway acts as an allosteric inhibitor. It binds to a specific regulatory site (allosteric site) on the first enzyme of the pathway (or a key regulatory enzyme). This binding induces a conformational change in the enzyme, reducing its affinity for its substrate and thereby slowing down the entire pathway. When the concentration of the final product decreases, it dissociates from the enzyme, and the enzyme regains its activity, allowing the pathway to resume.
Examples:
Isoleucine Synthesis in Bacteria: The pathway for isoleucine synthesis begins with threonine. The enzyme threonine deaminase is inhibited by isoleucine, the final product. ATP Synthesis: ATP inhibits key enzymes in glycolysis (e.g., phosphofructokinase) when energy levels are high. Cholesterol Synthesis: Cholesterol inhibits HMG-CoA reductase, the rate-limiting enzyme in its own synthesis.
Significance:
Economy: It prevents the waste of energy and raw materials. Homeostasis: It maintains the appropriate concentration of metabolites in the cell. Regulation: It provides a rapid, reversible mechanism for adjusting metabolic flux in response to cellular needs.
In summary, feedback inhibition is a crucial regulatory mechanism that ensures metabolic pathways operate efficiently and respond to the cell's changing requirements.
13. Classify enzymes on the basis of reactions catalyzed.
Answer: Enzymes are classified by the International Union of Biochemistry and Molecular Biology (IUBMB) into six main classes based on the type of chemical reaction they catalyze.
1. Oxidoreductases: These enzymes catalyze oxidation-reduction reactions. They transfer electrons or hydrogen atoms from one molecule (the reductant) to another (the oxidant).
Examples: Dehydrogenases, oxidases, peroxidases, catalase.
2. Transferases: These enzymes catalyze the transfer of a functional group (e.g., methyl, amino, phosphate, acyl) from one molecule (the donor) to another (the acceptor).
Examples: Kinases (transfer phosphate groups), transaminases (transfer amino groups), methyltransferases.
3. Hydrolases: These enzymes catalyze the hydrolysis of various bonds (e.g., ester, ether, peptide, glycosidic) by adding water.
Examples: Proteases, lipases, amylases, nucleases, phosphatases.
4. Lyases: These enzymes catalyze the removal of a group from a substrate (non-hydrolytically), often creating a double bond, or the
addition of a group to a double bond.
Examples: Decarboxylases (remove CO₂), aldolases (cleave aldol), dehydratases (remove water).
5. Isomerases: These enzymes catalyze structural rearrangements within a molecule, converting one isomer to another.
Examples: Isomerases (e.g., glucose isomerase), epimerases, mutases.
6. Ligases: These enzymes catalyze the joining of two molecules to form a new bond, coupled with the hydrolysis of a high-energy bond (e.g., ATP).
Examples: DNA ligase, RNA ligase, synthetases.
This classification system provides a systematic way to name and categorize enzymes based on their function. Each class is further divided into subclasses and sub-subclasses, and each enzyme is assigned a unique Enzyme Commission (EC) number.
14. Classify enzymes on the basis of the substrate they use.
Answer: Enzymes can be classified based on the type of substrate they act upon. This classification is often used in clinical and industrial contexts.
1. Carbohydrases: These enzymes act on carbohydrates (sugars and polysaccharides).
Examples: Amylase (breaks down starch), maltase (breaks down maltose), lactase (breaks down lactose), sucrase (breaks down sucrose).
2. Proteases (Proteolytic Enzymes): These enzymes act on proteins, breaking them down into smaller peptides or amino acids.
Examples: Pepsin, trypsin, chymotrypsin, erypsin, papain.
3. Lipases: These enzymes act on lipids (fats and oils), breaking them down into fatty acids and glycerol.
Examples: Pancreatic lipase, lipoprotein lipase.
4. Nucleases: These enzymes act on nucleic acids (DNA and RNA), breaking them down into nucleotides or smaller fragments.
Examples: DNase (acts on DNA), RNase (acts on RNA).
5. Nucleotidases and Nucleosidases: These enzymes act on nucleotides and nucleosides, respectively, breaking them down into their components.
6. Phosphatases: These enzymes remove phosphate groups from substrates (e.g., proteins, lipids, nucleotides).
7. Esterases: These enzymes hydrolyze ester bonds (e.g., in lipids, acetylcholine).
This classification is useful because it indicates the general type of reaction the enzyme catalyzes and the class of biomolecule it targets. However, it is less precise than the IUBMB classification, which is based on the specific reaction mechanism.
