Unit 5: Unit 5: Enzymes — Short Questions

Ans. Enzymes
Enzymes are three-dimensional globular proteins, that speedup specific chemical reactions by lowering the required activation energy.

Co-factor
The additional non-protein chemical components used by enzymes, to aid in catalysis are called co-factors. For example, metal ions, prosthetic groups and coenzymes.

Ans. Co-enzyme
• When the cofactor is a non-protein organic molecule and is loosely attached (hydrogen bond) with enzyme, it is called coenzyme.
• Coenzymes participate in enzyme-catalysed reactions, often by transporting electrons (hydrogen atoms), from one enzyme to another.
• Many vitamins (e.g., niacin and riboflavin) function as coenzymes.

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

Ans. Hydrolases
These enzymes catalyse hydrolysis reactions. They break their substrates into monomers by adding water

For example; lipase (hydrolyze lipids into fatty acids and glycerol), amylase (catalyse the hydrolysis of starch to maltose), peptidase, and other digestive enzymes catalyse the hydrolysis of food molecules.

Ans. 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. Enzymes bring reactants together in correct orientation or required for new bonds to form and speed up the rate of reactions.

Ans. Feedback inhibition is the phenomenon where the product of a process controls the process itself, often-times limiting the production of more products.

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.

Ans. Competitive inhibitors 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. Competitive inhibitors are used as antibiotics to kill bacteria.

Non-Competitive Inhibitors 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 non-competitive inhibitors and binds with enzyme. Thus enzyme is inhibited and no more succinyl-CoA is produced.

Ans. Every enzyme works 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).

Optimum pH of pepsin is 1.5 – 1.6, salivary amylase is 4.6 – 5.2 and of trypsin is 7.8 – 8.7

Ans. Enzymes are specific proteins that speed up specific chemical reactions by lowering the required activation energy. but are unaltered themselves in the process. Enzymes are also known as biocatalysts

Rates of enzyme-catalysed reactions may be 10⁶ to 10⁹ times greater than the rates of corresponding uncatalyzed reactions.

Ans. A reaction that is catalysed by an enzyme and is completed in 30 minutes, would take one year to get completed without being catalysed by enzyme. Thus, we can say that without enzymes there would have been no life at all.

Ans. All enzymes are synthesized inside cells by ribosomes. After their synthesis, either they stay and work inside cell or they are secreted out to work at other sites.

Ans. 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.

Regions Active site has two distinct regions i.e., binding site and catalytic site.

• Binding site: Substrate molecule fits into binding site by weak chemical forces, such as hydrogen bonds.
• Catalytic site catalyses the reaction and substrate is transformed into products.

Ans. The protein part of enzyme is called apoenzyme and complete enzyme including co-factor is called holoenzyme.

Ans. Metal ions
• Many enzymes use metal ions, such as Ca²+, Mg²+, Mn²+, Cu²+ and Zn²+ 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.

Ans. Some enzymes may prove harmful, if they become active at wrong place. For example; pepsin is a protein digesting enzyme. It can destroy protein-made structures present inside cells where it is synthesized. That is why it is produced in inactive form (pepsinogen) and is secreted out of cells. When it reaches its target site of action, it is activated (pepsin).

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

Ans. The most important coenzyme in cell is the hydrogen acceptor nicotinamide adenine dinucleotide (NAD+). 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.

Ans. In complex metabolic pathways e.g., respiration, photosynthesis, protein synthesis etc., many enzymes act in a sequence and regulate the steps of pathway. The successive enzymes controlling these steps are present together along with their cofactors. The products from one enzyme's catalysis serve as substrate for the enzyme of next step and are transformed into next products. The series goes on and finally end products are formed that inhibit (through feedback) the first enzyme.

Ans.

Lock and Key Model
• 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"
• 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.

Induced Fit Model
• 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 change in enzyme structure.
• 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.

Ans. Optimum Temperature Every enzyme works at its maximum rate at a specific temperature called its optimum temperature. 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.

Ans. When temperature is raised well above optimum temperature, the heat energy increases the vibrations of atoms of enzyme molecules. When vibrations become too violent, 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.

Ans. The overall rate of enzyme-controlled reactions depends directly on the amount of enzyme present at a specific time (if substrate concentration is unlimited). 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 remaining to attach with enzymes. So, the rate of reaction stays constant and does not increase further.

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

Ans.

Inhibitor
A chemical that interferes and blocks active site, hence blocking an enzyme's activity is called an inhibitor.

Inhibition
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.

Ans. The poison cyanide is an irreversible enzyme inhibitor that combines with copper and iron in the active site of enzyme cytochrome oxidase and blocks cellular respiration.

Ans.

Reversible
• Reversible inhibitors make weak bonds i.e. hydrogen bonds with enzyme.
• Such inhibitors can be released and the inhibition caused by them can be neutralized by increasing the concentration of substrate.
• For example, malonate is a reversible inhibitor. It temporarily slows down the reaction by blocking the enzyme succinate dehydrogenase, which is involved in cellular respiration. This inhibition can be reversed when malonate is removed.

Irreversible
• 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.

Ans. Significance of Enzyme inhibition

1. Many drugs work as inhibitors. For example, antibiotics inhibit the enzymes of bacteria, while cancer drugs may inhibit enzymes involved in cell division.
2. Enzyme inhibitors are used to manage various medical conditions. For example, some inhibitors of enzymes involved in blood clotting are used as anticoagulants.
3. Some toxins and poisons inhibit important enzymes in the body. Understanding how these inhibitors affect enzymes is critical in treating cases of poisoning.

Ans. The classes of enzymes on the basis of reactions:
1. Oxidoreductases
2. Transferases
3. Hydrolases
4. Lyases
5. Isomerases
6. Ligases

Ans.

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.

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.

Ans.

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.

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

Ans.

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

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 (DNA polymerase, RNA polymerase) join monomers by using ATP.

Ans. Enzyme Ligases

Subclasses
(i) C-C ligases (ii) C-O ligases
(iii) C-N ligases

Ans.

Lipases
• These enzymes act upon lipids and catalyse their breakdown.
• For example, pancreatic lipase hydrolyses lipids into fatty acids and glycerol.

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.

Ans. 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.

Ans. 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.

• Aminopeptidases further breakdown small polypeptides into dipeptides.
• Erypsim breaks dipeptides into amino acids.

Ans.

Enzymes Oxidoreductases
Sub-Classes:
• Dehydrogenases
• Oxidases
• Reductases

Transferases
Sub-Classes:
• Phospho-transferases
• Amino-transferases
• Acyl-transferases

Ans.

Enzymes Hydrolases
Sub-Classes:
• Peptidases
• Lipases
• Glycosidases

Lyases
Sub-Classes:
• Decarboxylases
• Aldolases
• Synthases

Ans.

Enzyme Isomerases

Reaction Type A ↔ A

Sub-classes • Epimerases
• Mutases
• Cis trans isomerases

Ans. See Q1 of theory.

Ans. See Q2. of theory.

Ans. See Q4. of theory.

Ans. See Q3. of theory.

Ans. See Q5. of theory.

Ans. See Q5. of theory

Ans. See Q7. of theory.

Ans. See Q8. of theory.

Ans. See Q9. of theory.

Ans. See Q9. of theory.

Ans. See Q10. of theory.

Ans. See Q11 of theory.

Ans. See Q12. of theory.

Ans. Yes, physical exercise involves both anabolic and catabolic processes.

Evidence for Catabolism
During exercise, energy is required for muscle contraction, our cells breakdown glucose aerobically to form ATP. Doing exercise is the evidence that we are continuously being provided with energy, which is brought by catabolism of glucose.

Evidence for anabolism
In doing physical exercise daily, we gain muscle mass, which is an anabolic process providing the evidence of muscle repair and growth.

Ans. A reaction that is catalysed by an enzyme and is completed in 30 minutes, would take one year to get completed without being catalysed by enzyme. Thus, we can say that without enzymes there would have been no life at all.

Ans.

Competitive Inhibitors
• Malonic Acid
• Sulfa drug
• Anti-biotics

Non-Competitive Inhibitors
• Aspirin
• Cyanide
• Succinyl CoA