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3
CHAPTER
ENZYMES Major Concepts:
Number of allotted teaching periods: 9
3.1
Structure of Enzymes (2 Periods)
3.2
Mechanism of Enzyme Action (1 Period)
3.3
Factors Affecting the Rate of Enzymatic Action (3 Periods)
3.4
Enzyme Inhibition (1 Period)
3.5
Classification of Enzymes (2 Periods)
During the early nineteenth century, two French chemists, Payen and Persoz grounded up barley seeds in water to make a crude mixture that would digest starch. They gave the name diastase whatever it was that digested the starch.
Fig. 3.1 Enzymatic Action
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The sum of all the chemical reactions going on in cell is known as metabolism. It is divided into two types: anabolism and catabolism. There are chains of reactions going on in the cells called metabolic pathways. Many such pathways are going on at the same time in the cell. It may take months or years to complete the reactions but the reactions take place very quickly in the cells. How? It is due to enzymes. What is an enzyme? A catalyst is a substance, which speeds up a chemical reaction, but remains unchanged itself at the end. Enzymes are biological catalysts because they are proteins made by living cells. The chemical on which an enzyme works is called its substrate. An enzyme combines with its substrate to form a short lived enzyme- substrate complex. Once a reaction has occurred, the complex breaks up into products and enzyme. The enzyme remains unchanged at the end of the reaction and is free to interact again with other molecules of the substrate.
How can you define enzyme? Enzymes are thermolabile catalysts, protein in nature, which can work in living tissues and also outside the tissues. Q. What happens to a enzyme when it takes part in chemical reaction?
3.1 STRUCTURE OF ENZYMES All the enzymes are proteins, so each enzyme has its own specific conformation, which is very essential for its catalytic activity. The structure of enzymes can be studied under the following headings. Physical Nature of Enzymes High Molecular Weight: The enzymes have a relatively high molecular weight e.g. peroxidase-40,000 and catalase 250,000 approx. Colloidal Nature: Enzymes form colloidal suspension in the cytosol. Denaturation: Enzyme molecules can be denatured by high temperature and lose their enzymatic activity.
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Catalytic Nature: Enzymes being organic catalysts have many of the properties of inorganic catalyst, such as: (a) Enzymes are required in very small quantity for the reaction e.g. an enzyme molecule may breakdown other compounds of more than 100,000 molecules per second. (b) Activities of enzymes are affected by pH, temperature etc. (c) Enzymes are highly specific, that is an enzyme will generally catalyze only a single reaction e.g. amylase only breaks the starch amylose. (d) Enzymes can be studied in vivo (living cells) as well as in vitro (glasswares). (e) There is no change in enzyme before and after the reaction. Enzymes only speed up a reaction and do not affect the equilibrium of the reaction.
Science Titbits
Chemical Nature of Enzymes All the enzymes are proteins, so each enzyme has its own tertiary structure and specific conformation which is very essential for its catalytic activity. The functional unit of the enzyme is known as holoenzyme which is often made up of the protein part called apoenzymes and the non-protein part known as cofactor.
The ordered components of the cell provide the informational system that directs the kinds of energy transformations to oc足 cur. For example the ordered structure of the active site of an enzyme controls a way in which the catalyzed reaction takes place.
Dimensional Structure of Enzymes The simplest protein consists of only one long polypeptide chain. The chain is usually coiled and twisted to form globular molecules. For example 124 amino acid residues (when an amino acid is in chain it is called residue) are present in only one chain of enzyme ribonuclease a globular protein. The kinds of amino acid and sequence in which they are arranged determine the three dimensional structure of an enzyme. Active Site The site where the substrate binds in the enzyme is known as the active site. Enzymes are large protein molecules whereas the substrates are often of low molecular weight. Only a small part of the protein of the enzyme is in the catalytic process. Active site of an enzyme is a complex threedimensional cleft or cavity. An enzyme may have one to several active sites. The substrate molecule fits into the cleft by excluding water. Active site consists of few amino acids and their side groups, which are brought together in a particular fashion due to secondary and tertiary folding of the protein
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Fig: 3.2 Active Site: (a) Which substrate fits the active site? (b) Grouping of amino acids of a polypeptide during the formation of tertiary structure to produce an active site.
molecule, e.g. the active site for aldolase is glycine-histidine-alanine. Thus active site of an enzyme is also the binding and catalysing site i.e. it is the area of an enzyme which is capable of attracting and holding (binding site) the reactant to go chemical change (catalytic site). Enzyme Co- Factors Some enzymes consists only of protein e.g. pepsin, amylase, urease etc. Other enzymes have two components: a protein referred to as apoenzyme and an additional chemical component called a cofacter. Neither the apoenzyme nor the co-factor alone has catalytic activity; only when the two are combined does the enzyme function. A cofactor may be inorganic or it may be an organic molecule.There are three types of co-factors: (a) inorganic ions (b) prosthetic group (c) co-enzymes. Inorganic ions These are thought to mould either the enzyme or the substrate into a shape that allows enzyme-substrate complex to be formed. The detachable co-factor is known as activator if it is an inorganic ion. Many enzymes associated with the glycolysis require metallic cofactors. The cofactors of enzyme systems are copper, iron, manganese, zinc, calcium, potassium and cobalt.
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Prosthetic Group The non-protein groups that combine covalently with apoenzymes and share their active sites are called prosthetic groups. It is tightly bound to the enzyme on permanent basis. Heme is an iron containing prosthetic group which acts as an electron carrier in cytochrome and oxygen carrier in haemoglobin and myoglobin. Hematin is found in catalases and peroxidases which catalyse the decomposition of hydrogen peroxide into water and oxygen. Co-enzyme When the cofactor is a non-protein organic molecule it is called coenzyme. Coenzyme never becomes part of its enzyme. It works just in close association with its enzymes. Most coenzymes can be categorized as transfer agents that transfer some component from one molecule to another. NADH, NADPH, FADH2 are coenzymes they transfer electrons. ATP functions as coenzymes, it is responsible for transferring phosphate groups. Coenzyme A, is involved in the transfer of groups derived from organic acids. Most vitamins are coenzymes or components of coenzymes.
3.2 MECHANISM OF ENZYME ACTION Two models have been put forward to explain the mechanism of enzyme action: (a) Lock and key model
(b) Induced- fit model
Lock and Key Model Emile Fischer proposed it in 1894. According to this model the catalytic property of enzyme is located at specific region on the surface of their molecules. The shape and size of active site on the molecule of the specific enzyme is distinct from other enzyme molecules but precisely match with the reactive site on the surface of substrate molecule. So the enzyme reacts with the substrate molecule. Lock and key model or hypothesis assumes that like a particular key opens a particular lock, a specific enzyme acts upon a particular substrate. The notched portion of the key is equivalent to the active site on the enzyme.
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Fig: 3.3 Fisher s Lock & Key Hypothesis of Enzyme Action
The lock and key model accounts for enzyme specificity. Only specific molecules can precisely fit into the specific sites. A key (enzyme) can open several locks of the same design (substrate molecule). During complex formation substrate molecules fit exactly in the active sites of the enzyme molecules. The lock and key model explains the stereospeficity of enzyme catalyst. It however is exercised by a very small number of enzymes, for example enzymes called hydrolases like proteases, lipase, amylase and nuclease etc. The enzymes, which work on this mechanism, are called nonregulatory enzymes. Induced Fit Model Koshland proposed this model in 1959. Most of the enzymes react with the substrates only when their active sites are induced. The active site is modified as the substrate interacts with enzyme. The amino acids which makeup the active site are moulded into a precise shape which enables the enzyme to perform its catalytic function more effectively.
Fig: 3.4 Koshland's Induced Fit Hypothesis
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A suitable analogy would be that they fit like a hand in a glove. The hand corresponds to the substrate and the gloves i.e. enzyme is shaped by insertion of the hand. When the active sites are correct in position, the chemical bond is broken and the product of the reaction is released. Enzymes, which follow the induced fit mechanism, are called regulatory or allosteric enzymes. For example hexokinase.
Science Titbits How Enzymes are formed? Enzymes are proteins, so they are formed as per message or base sequence in DNA. Enzymes are synthesized by living cells but they retain their cata足 lytic action even when extracted from cells, i.e. they can act in vitro. These days enzymes are produced by recombinant DNA technology. Enzyme Catalyses Specific Reaction The ability of enzyme to catalyze one specific reaction is perhaps its most significant property. Enzymes show a broad range of specificity towards the substrate they catalyse. Some important types of enzyme specificities are: (1) absolute specificity (2) group specificity (3) relative group specificity (4) stereo specificity (5) geometric specificity. Absolute Specificity: When one enzyme can catalyze only one substrate and essentially no others it is called absolute specificity. e.g. urease
Science Titbits Enzymes are Present in Specific Organelles Enzymes are distributed in the cell as per need. The enzymes needed for photosynthesis are located in the chloroplasts. In the liver cells enzymes of glycolysis are located in the cytoplasm, whereas enzymes of the Krebs cycle are in the mitochondria. Enzyme needed for the synthesis of DNA and RNA and mitosis occur in nuclei. Q. What difference can you detect between a reaction going on when an enzyme is present and when it is not?
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Energy of Activation Molecules do not react with one another unless they are activated in someway. The energy that must be added to cause molecules to react with one another is called the energy of activation (Ea). In nonliving system we use heat as energy of activation to increase the number of effective collision between molecules. In living systems heat cannot be used as energy of activation. Why? Because all living cells and organisms are mainly composed of thermolabile protein molecules. About 1,000 chemical reactions are taking place in a cell at any time. Energy of activation required for such a large number of reactions cannot be provided by living system.
Fig: 3.5 Energy of Activation: Enzymes speed the rate of chemical reactions because they lower the amount of energy required to activate the reactants
The living system works in isothermic condition; the excited state of molecules or reactants is achieved by biochemical process. Enzymes (E) reacts with reactant (A) to form a AE transitional complex. The energy level of AE complex reaches to the energy level of reactant B. AE complex then reacts with reactant B to for AB and enzyme (E) is released. A + E = AE complex + B = AB + E Enzyme does decrease the energy of activation by changing energy dependent process to energy independent process. Thus the energy of activation is energy required to break the existing bonds and begin the reaction. An enzyme greatly reduces the activation energy necessary to initiate a chemical reaction.
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3.3 FACTORS AFFECTING THE RATE OF ENZYMATIC ACTION The rate of enzymatic reaction is measured by the amount of substrate changed or amount of product formed, during a period of time. The rate is determined by measuring the slope of the tangent to the curve in the initial stage of the reaction. The steeper the slope, the greater is the rate. The external conditions which affect rate of enzyme reactions are: temperature, pH, concentration of enzyme and substrate concentration. Temperature Heating increases molecular motion. Thus the molecules of the substrate and enzyme move more quickly, so probability of occurring a reaction increases. The temperature that promotes maximum activity is called an optimum temperature. If the temperature is increased above this level, then a decrease in the rate of the reaction occurs despite the increasing frequencies of collision. This is because the secondary and Fig: 3.6 Effect of Temperature on the rate of an tertiary structures of the enzyme Enzyme Controlled Reaction have been disrupted and the enzyme is said to be denatured. The enzyme unfolds and the precise structure of the active site is gradually lost. The bonds which are most sensitive to temperature change are hydrogen bonds. Most human (mammalian) enzymes o have a temperature optimum of about 37-40 C, but bacteria living in hot springs o may have an optimum temperature of 70 C or higher. Such enzymes have been used in biological washing powders for high temperature washes. If temperature is reduced to near or below freezing point, enzymes are inactivated, not denatured. They will regain their catalytic influence when higher temperatures are restored. pH Every enzyme functions most effectively over a particular pH range. Often this is a narrow range. The optimum pH is that at which the maximum rate of reaction occurs. The optimum pH is between 6 to 8. Changes in pH alter the ionic charge of the acidic and basic groups and therefore disrupt the
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Fig: 3.7 Effect of pH on the rate of an EnzymeControlled Reaction
ionic bonding that helps to maintain the specific shape of the enzyme. Thus the pH change leads to the change of enzyme shape, including its active site. If extreme of pH are encountered by an enzyme, then it will be denatured. Some enzymes like papain from green papaya act both in acidic and alkaline media. Protein digesting enzyme pepsin is active in acidic medium at pH 2 and trypsin is inactive at this pH but shows maximum activity in alkaline medium at pH 8.
Critical Thinking Industrial pollution can change the pH of a pond, lake or river to make the water more acidic. How can this affect the metabolic pathways of the plants that live in water? Enzyme Concentration Provided that the substrate concentration is maintained at a high level, and other conditions such as pH and temperature are kept constant, the rate of reaction is proportional to the enzyme concentration. Normally small quantity or concentration of enzymes will catalyze large quantity or concentration of substrates. Thus as the enzyme concentration is increased, so will be the rate of the enzymatic reaction.
Enzyme concentration Fig: 3.8 Relationship between Enzyme concentration and the rate of an Enzymecontrolled reaction
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Substrate Concentration When other conditions remain unchanged the increase in substrate concentration (S) increases the velocity (V) of the enzymatic reaction at first. The reaction ultimately reaches a maximum velocity. The rise in V decreases progressively with further increase in S. The reaction does not increase by any further rise in substrate concentration. Fig: 3.9 Effect of Substrate concentration This happens because enzyme in the rate of Enzyme-control reaction molecules are fewer than the substrate molecules. Further increase in substrate concentration will saturate all the enzyme molecules and no enzyme is left free to bind with additional molecules of the substrate. Nomenclature of Enzymes a) Enzymes are named by adding “ase” to the name of substrate they act e.g. proteases, lipases etc. b) Enzymes are named according to the types of reaction they catalyse e.g. oxidases, reductases etc. c) Enzymes are named by taking into consideration both the substrate acted upon and the type of reaction catalysed e.g. DNA- polymerase. d) Some enzymes are named as per substance synthesized e.g. rhodonase catalyses synthesis of rhodonate from hydrochloric acid and sodium thiosulphate.
Skills: Analyzing, Interpreting and Communication Construct and interpret graphs based on data about the effect of temperature, enzyme concentration and substrate concentration on the rate of enzyme action.
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3.4 ENZYME INHIBITION The chemical substances which inhibit the activity of enzymes are called inhibitors. Inhibition may be competitive or noncompetitive. Noncompetitive inhibition may be reversible or non-reversible. The significance of inhibition is that, it is a normal part of the regulation of enzyme activity within cells. The molecules which act as inhibitors include poisons, cyanides, antibodies, anti-metabolites, penicillin, sulpha drugs etc. Competitive Inhibitors The inhibition of enzyme activity by the presence of a chemical that compete with the substrate for binding to the active site is called competitive inhibition. Usually a Penicillin blocks the active competitive inhibitor is structurally similar site of an enzyme unique to to the normal substrate and so fits into the bacteria. When penicillin is active site and combines with the enzyme. taken, bacteria die but However, it is not similar enough to human are unaffected. substitute fully for the normal substrate in the chemical reaction and the enzyme can not attack it to form reaction products. A competitive inhibitor occupies the active site only temporarily and does not permanently damage enzyme. If the concentration of the substrate is increased relative to the concentration of the inhibitor, the active site will usually be occupied by the substrate.
Science Titbits
An example of inhibitor is melonate. Succinate dehydrogenase that catalyzes the formation of fuamarate from succinate is competitively inhibited by malonate.The importance of competitive inhibitors are: (a) It supports lock and key hypothesis. (b) It shows that substances which are similar to substrate are not acted upon by enzymes. (c) Competitive inhibitors are used as drugs in the control of bacterial pathogens. Antibiotics known as sulphonamides are used to combat bacterial infection. Non-Competitive Inhibitors In noncompetitive inhibition the inhibitor molecule binds to an enzyme, but not at the active site. The other binding site is called allosteric site. The non-competitive inhibitors inactivates the enzyme by altering its shape so that the substrate can not bind to the active site. The examples of non-competitive inhibitor includes salts of heavy metals and cyanides are potent poisons of living organism because they can kill an organism by inhibiting cytochrome oxidase essential for cellular respiration. Ions of heavy
88 The distinction between competitive and non-competitive inhibition rests on whether or not the active site of the enzyme is involved. (a) In competitive inhibition, the inhibitor competes with the normal substrate for the active site of the enzyme. A competitive inhibitor occupies the active site only temporarily. (b) In non-competitive inhibition, the inhibitor binds with the enzyme at a site other than the active site, altering the shape of the enzyme and thereby inactivating it. Noncompetitive inhibition may be reversible. Allosteric regulation, used by cells to control enzyme action, is a somewhat similar process.
BIOLOGY XI: Chapter 3, ENZYMES
Fig: 3.10 (a) Competitive (b) Non competitive enzymes
Q. Suggest why substrate concentration has no effect on non-competitive inhibition? Science, Technology and Society Connections Venoms as enzyme inhibitors Snake venom is highly modified saliva that is produced by special glands of certain species of snakes. Snake venom is a combination of many toxins (proteins) and different enzymes, use for the purposes like increasing the preyÂ’s uptake of toxins. Snake venom inhibits cholinesterase to make the prey lose control of its muscles. Venom is an inhibitor for an essential enzyme cytochrome oxidase in the cells. There are three distinct type of venom that act on the body differently. (1) hemotoxic venoms act on the heart and cardiovascular system. (2) Neurotoxic venom acts on the nervous system and brain. (3) Cytotoxic venom has a localized action at the site of the bite. Venom occupies the active site of the enzyme or combining with the iron which may present in the prosthetic group or which may be required as an enzyme activator. metals such as mercury, silver and copper (Hg++, Ag+, and Cu++) combine with thiol (-SH) groups in the enzyme breaking the disulphide bridges. These bridges are important in maintaining tertiary structure. When these bridges are broken, the enzyme becomes denatured and inactive. Cyanides block the action of some enzymes by combining with iron which may be present in the prosthetic group or which may be required as an enzyme activator.
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Feedback Inhibition The activity of almost every enzyme in a cell can be regulated by its product. When a product is in abundance it binds competitively with its enzymeÂ’s active site. It is called feedback inhibition. The amino acid aspartate becomes the amino acid threonine by a sequence of five enzymatic reactions. When threonine, the end product of this pathway, is present in excess, it binds to an allosteric site on enzyme 1 and then the active site is no longer able to bind aspartate.
Fig: 3.11 Feedback Inhibition
Skills: Analyzing Identify the competitive and non-competitive inhibitors from the given list of chemical (consult any book of Biochemistry or Enzymology). List: Antibodies, antimetabolites, penicillin, iodoacetate, malonate, Acetaldehyde, CoA, (high concentration), Disopropylfluorophosphate (DFP- nerve gas), mercury, silver, copper, cyanide.
3.5 CLASSIFICATION OF ENZYMES A systematic nomenclature and classification of enzymes based on reaction types and reaction mechanism was given by International Union of Biochemistry in 1961. On that basis all the enzymes have been classified into six groups: oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases. Oxidoreductases These enzymes catalyse oxidation/reduction of their substrate and act by removing or adding electron or H+ ions from or to the substrate. For example cytochrome oxidase oxidizes cytochrome.
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Transferases These enzymes catalyse the transfer of specific functional group other than hydrogen from one substrate to another. The chemical group transferred in the process is not in a free state. For example hexokinase transfers a phosphate group from ATP to glucose. Hydrolases These enzymes bring about the break down of large complex organic molecules into smaller ones by adding water (hydrolysis) and breaking the specific covalent bonds. For example proteolytic enzymes which breakdown proteins into peptones and peptides, e.g. pepsin, rennin (chymosin), trypsin and erepsin. Nucleases break down nucleic acids into nucleotides which are again hydrolysed by nucleotidases into nucleosides. Lyases These enzymes catalyse the breakdown of specific covalent bonds and removal of groups without hydrolysis. For example histidine decarboxylase breaks the covalent bonds between carbon atoms in histidine forming carbon dioxide and histamine. Isomerases These enzymes bring about intramolecular rearrangement of atoms in the molecules and thus forming one isomer from another. For example Phosphohexose isomerase changes glucose 6- phosphate to fructose 6phosphate. Ligases (Synthetases) These enzymes bring about joining together of two different molecules. The energy is derived by hydrolysis of ATP. For example polymerase are responsible for linking monomers into a polymer such as DNA or RNA. Enzymes can be classified on the basis of substrates they use. Some of the examples are: Proteases break up proteins into amino acids, lipases hydrolyse lipids, sucrase hydrolyses sucrose, nucleases act on nucleic acids, urease acts on urea, maltase acts upon maltose, diastase acts on starch, amylase acts on amylose (carbohydrate-starch).
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Uses of Enzymes Proteases is used in washing powders for dissolving stains from proteins e.g. egg, milk and blood; removing hair form animal hides, cheese manu facture; tenderising meat. Lipases are used flavour enhancer in cheese; in washing powders for removal of fatty stains. Pectinases are used in clari fication of fruit juices; maximising juice extraction. The industrial amy lases are used to convert starch to glucose and fructose for sweeteners, or to destarch fabrics in the course of processing. Sugar-fermenting en zymes obtained from living yeast cells and are used in brewing and bak ing industry. Rennin is extracted from calves stomach. It is now pro duced by genetically engineered bacteria and the product is called chymosin. It clots milk in the first stage of making cheese. Carbohy drases are used in the making of chocolates, syrups and other food prod ucts. Glucose oxidase and peroxidase are used to detect glucose in liquid e.g. urine by the diabetics. Catalase is obtained from bacteria and animal livers. It turns latex into foam rubber by producing gas. Science, Technology and Society Connections List the diagnostic uses of enzymes.
SECTION I : MULTIPLE CHOICE QUESTIONS Select the correct answer 1. The catalytic activity of an enzymes is restricted to its small portion called A)
active site
B)
passive site
C)
regulation site
D)
allosteric site
2. Which of the following has a coenzyme activity? A)
nicotinamide
B)
purine
C)
pyrimidine
D)
alcohol
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3. Enzymes exist in the cells as A)
solid
B)
crystals
C)
solution
D)
colloids
4. Combination of apoenzyme and coenzyme produces A)
prosthetic group
B)
holoenzyme
C)
apoenzyme
D)
isoenzyme
5. The specificity of enzyme is due to their A)
surface configuration
B)
pH
C)
hydrogen bonding
D)
high molecular weight
6. An essential feature of a competitive inhibitor is its ability to A)
activate an operator gene B)
combine with prosthetic group
C)
modify a substrate
occupy an active site
D)
7. The reaction rate of salivary amylase with starch decreases as the concentration of chloride ions is reduced. Which of the following describe the role of the chloride ions? A)
allosteric inhibitors
B)
cofactors
C)
coenzyme
D)
competitive inhibitor
8. How does an enzyme increase the rate of a reaction? A)
by bringing the reacting molecules into precise orientation
B)
by increasing the rate of random collisions of molecules
C)
by shifting the point of equilibrium of the reaction
D)
by supplying the energy required to start the reaction
9. Many enzymes are secreted in inactive form to protect A)
cell proteins
B)
mitochondria
C)
cell membrane
D)
cell DNA
10. Which graph shows the expected relationship between enzyme activity and substrate concentration?
BIOLOGY XI: Chapter 3, ENZYMES
11. The graph shows the effect of an enzyme on a reaction.
Which combination identifies X, Y and Z?
SECTION II : SHORT QUESTIONS 1. 2.
How are enzymes named? At what temperature enzymes act the best?
3.
What is the role of free energy of activation in a chemical reaction?
4.
What are cofactors, coenzymes and prosthetic group?
5.
What is the function of active site of an enzyme?
6.
Name three properties that are common to all enzymes.
7.
What is the role of co-enzymes in enzyme action?
8.
How enzymes were discovered?
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9.
Write the main characteristics of enzymes.
10.
List the factors affecting enzyme activity.
11.
Differentiate between apoenzyme and co-enzyme.
12.
Explain the term: holoenzyme, competitive inhibition, feedback inhibition, non-competitive inhibition.
SECTION III : EXTENSIVE QUESTIONS 1
Define enzyme. Write the chemical nature of enzymes.
2.
Write a note on enzyme specificity.
3.
Explain Lock and Key Model an Induced Fit Model of enzyme action.
4.
Discuss four factors that influence how an enzyme functions?
5.
Give an account of enzyme inhibition.
6.
Write the classification of enzyme.
7.
The actions of enzymes allow the cell to maintain itself and grow. Explain why most reactions do not occur in a cell unless a specific enzyme is present.
ANSWER MCQS 1. A 10. B
2. A 11. C
3. D
4. B
5. A
6. D
7. B
8. A
9. A
SUPPLEMENTARY READING MATERIAL 1. Campbell N.A. Mhchell, L.G. & Reece J.B., Biology Concepts and connections, 2nd edition Benjamin/Cummings Company California, 2003 2. Madar, S.S. Biology, 6ht edition, WCB, McGraw-Hill, USA, 1998.
USEFUL WEBSITES 1. www.chem.qmul.ac.uk/iubmb/enzyme 2. www.biochem.ucl.ac.uk/bsm/enzymes 3. www.elmhurst.edu/~chm/vchembook/571lockkey.html