Enzyme Enzymes are proteins that act as biological catalysts. Enzymes accelerate, or catalyze, chemical reactions. The molecules at the beginning of

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1 Enzyme

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4 Enzyme Enzymes are proteins that act as biological catalysts. Enzymes accelerate, or catalyze, chemical reactions. The molecules at the beginning of the process are called substrates and the enzyme converts these into different molecules, called products. Almost all metabolic processes in the cell need enzymes in order to occur at rates fast enough to sustain life. Enzyme are substrate specific. Enzymes are highly specific for their substrates. Each enzyme catalyzes only one kind of reaction. Because of this, enzymes are usually named after the molecules they target. The standard suffix for enzyme is ase. Many enzyme are named simply by replacing the suffix of the substrate with ase; e g. in the following reactions, Lactose becomes Lactase, maltose becomes maltase.

5 Induced-fit Model: In this model, when an enzyme binds to the appropriate substrate, subtle changes in the active site occur. This alteration of the active site is known as an induced fit. Induced fit enhances catalysis, as the enzyme converts substrate to product. Release of the products restores the enzyme to its original form. The enzyme can repeat this reaction over and over, as long as substrate molecules are present. So By binding the substrates and releasing the products, enzyme will 1. increase the rate of a reaction. 2. lowering the reaction s activation energy. 3. not change the reaction. 4. change its shape temporarily but restores its original form after release of the products.

6 Lock and Key model; In this model, the enzyme temporarily binds the substrates to its active site and forms an enzyme-substrate complex. By binding the substrates and releasing the products, enzyme will; 1. increase the rate of a reaction. 2. lowering the reaction s activation energy. 3. not change the reaction. 4. not change the shape of enzyme.

7 Enzymes are highly specific for their substrates. Each enzyme catalyzes only one kind of reaction. The efficiency of an enzyme is affected by temperature and ph; 1. The optimum temperature to maximize the rate of enzyme activity must be determined for each enzyme. Most animal enzymes will denature at above 40 degrees Celsius. Enzymes in the human body function best at body temperature, which is around 37.5 degrees Celsius. 2. Each enzyme works within quite a small ph range. There is a ph at which its activity is greatest (the optimal ph). This is because changes in ph can make and break intra- and intermolecular bonds, changing the shape of the enzyme and, therefore, its effectiveness. Most Enzymes have an optimal ph of around 7.0. They increase the rate of chemical reactions without themselves being permanently altered by the reaction. They increase reaction rates without altering the chemical equilibrium between substrates and products. The chemical equilibrium between S and P is determined by the laws of thermodynamics (as discussed further in the next section of this chapter) and is represented by the ratio of the forward and reverse reaction rates (S P and P S, respectively). In the presence of the appropriate enzyme, the conversion of S to P is accelerated, but the equilibrium between S and P is unaltered. Therefore, the enzyme must accelerate both the forward and reverse reactions equally. The reaction can be written as follows: S Enzyme In a chemical reaction, the energy necessary to break the bonds of the substrates is the activation energy, EA. Enzymes lower the EA barrier. P

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9 Enzymes speed up metabolic reactions by lowering energy barriers In a chemical reaction, the energy necessary to break the bonds of the substrates is the activation energy, EA. Enzymes lower the EA barrier:

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11 USABO2013, 1C

12 Cofactor: Coenzyme: In addition to binding their substrates, the active sites of many enzymes bind other small molecules that participate in catalysis. These molecules are called coenzymes because they work together with enzymes to enhance reaction rates. Coenzymes serve as carriers of several types of chemical groups. A example of a coenzyme is nicotinamide adenine dinucleotide (NAD + ), which functions as a carrier of electrons in oxidation-reduction reactions. NAD + can accept a hydrogen ion (H + ) and two electrons (e - ) from one substrate, forming NADH. NADH can then donate these electrons to a second substrate, re-forming NAD +. Thus, NAD + transfers electrons from the first substrate (which becomes oxidized) to the second (which becomes reduced). Some vitamins are coenzymes or components of coenzyme. Inorganic cofactors are often metal ions, like Fe ions and Mg ions.

13 Regulations of enzyme activity An important feature of most enzymes is that their activities are not constant but instead can be modulated. That is, the activities of enzymes can be regulated so that they function appropriately to meet the varied physiological needs that may arise during the life of the cell. Enzyme activities can be regulated by 1. feedback inhibition 2. competitive inhibition 3. noncompetitive inhibition

14 Feedback inhibition; the product of a metabolic pathway inhibits the activity of an enzyme involved in its metabolic pathway. E.g. the cell synthesizes the necessary amount of isoleucine but avoids wasting energy on the synthesis of more isoleucine than is needed.

15 Competitive inhibition is a form of enzyme inhibition where binding of the inhibitor to the active site on the enzyme prevents binding of the substrate and vice versa. In competitive inhibition, at any given moment, the enzyme may be bound to 1. the inhibitor, 2. the substrate, 3. or neither, but it cannot bind both at the same time.

16 Non-competitive inhibition; In noncompetitive inhibition, which also is reversible, the inhibitor and substrate can bind simultaneously to an enzyme molecule at different binding sites. A noncompetitive inhibitor acts by decreasing the turnover number rather than by diminishing the proportion of enzyme molecules that are bound to substrate.

17 USABO2011, 9A

18 Q4. Binding of inhibitor Y as shown below inhibits a key catalytic enzyme by inducing a structural conformation change. Which of the following is correct: a. Y compete with substrates for binding in the active site and functions as a competitive inhibitor. b. Y compete with allosterically and functions as a competitive inhibitor. c. Y compete with substrates for binding in the active site and functions as a non-competitive inhibitor. d. Y compete with allosterically and functions as a non- competitive inhibitor. Q4.D

19 USABO2011, 10D

20 Cooperativity, enzymes or receptors have multiple binding sites where the affinity of the binding sites for a substrate (ligand) is apparently increased, binding of a substrate to one active site stabilizes favorable conformational changes at all the other subunits amplifies the response of enzymes to substrates, priming the enzyme to accept additional substrates positive cooperativity, or decreased, negative cooperativity, upon the binding of one substrate to one of these binding sites.

21 Enzymes speed up metabolic reactions by lowering energy barriers (pp ) In a chemical reaction, the energy necessary to break the bonds of the reactants is the activation energy, EA. Enzymes lower the EA barrier: Each type of enzyme has a unique active site that combines specifically with its substrate(s), the reactant molecule(s) on which it acts. The enzyme changes shape slightly when it binds the substrate(s) (induced fit). The active site can lower an EA barrier by orienting substrates correctly, straining their bonds, providing a favorable microenvironment, or even covalently bonding with the substrate. Each enzyme has an optimal temperature and ph. Inhibitors reduce enzyme function. A competitive inhibitor binds to the active site, whereas a noncompetitive inhibitor binds to a different site on the enzyme. Natural selection, acting on organisms with mutant genes encoding altered enzymes, is a major evolutionary force responsible for the diverse array of enzymes found in organisms.

22 Teacher (Enzyme)+ Parents (coenzymes) Student (Substrate) Good Student (Product)

23 81B, 82B

24 Enzyme kinetics

25 Regulation of enzyme activity helps control metabolism Many enzymes are subject to allosteric regulation: Regulatory molecules, either activators or inhibitors, bind to specific regulatory sites, affecting the shape and function of the enzyme. In cooperativity, binding of one substrate molecule can stimulate binding or activity at other active sites. In feedback inhibition, the end product of a metabolic pathway allosterically inhibits the enzyme for a previous step in the pathway. Some enzymes are grouped into complexes, some are incorporated into membranes, and some are contained inside organelles, increasing the efficiency of metabolic processes. What roles do allosteric regulation and feedback inhibition play in the metabolism of a cell?

26 The second law of thermodynamics If energy cannot be destroyed, why can t organisms simply recycle their energy over and over again? It turns out that during every energy transfer or transformation, some energy becomes unavailable to do work. In most energy transformations, more usable forms of energy are at least partly converted to heat, which is the energy associated with the random motion of atoms or molecules. Only a small fraction of the chemical energy from the food in pictures is transformed into the motion of the brown bear shown in most is lost as heat, which dissipates rapidly through the surroundings. A logical consequence of the loss of usable energy during energy transfer or transformation is that each such event makes the universe more disordered. Scientists use a quantity called entropy as a measure of disorder, or randomness.

27 Energy basics How energy does work follows two laws of thermodynamics: The first law of thermodynamics and the second law of thermodynamics. Energy conversions are usually discussed within the context of a system. The energy in a system that is available for conversions is called its Gibbs free energy, and the change in free energy that occurs as result of a conversion is represented by ΔG. The value of ΔG can be negative or positive; Exergonic reaction, there is a net release of free energy when ΔG is negative for a reaction. Endergonic reaction, free energy must be added to the reaction for it to occur when ΔG is positive for a reaction.

28 potential energy Q1. All of the following statements are correct about enzymes Except: a. They raise the energy of activation of all reaction b. They enable reactions to occur at a relatively low temperature. c. They remain unchanged during a reaction d. They are often located within the plasma membrane of the cell Q2. Which of the following can be used to determine the rate of an enzyme catalyzed reaction? a. The rate of substrate formed b. The decrease in temperature in the system c. The rate of enzyme used up d. The rate of substrate used up Q3. Which of the following best describes the reaction shown below? A+B----AB + energy a. Hydrolysis b. An exergonic reaction c. An endergonic reaction d. Catabolism Q4. Which letter shows the energy of activation for enzyme-catalyzed reaction? Which letter shows the potential energy of the product? a. A b. B c. C d. D A B C D

29 Q1. All of the following statements are correct about enzymes Except: a. They raise the energy of activation of all reaction b. They enable reactions to occur at a relatively low temperature. c. They remain unchanged during a reaction d. The are often located within the plasma membrane of the cell Q2. Which of the following can be used to determine the rate of an enzyme catalyzed reaction? a. The rate of substrate formed b. The decrease in temperature in the system c. The rate of enzyme used up d. The rate of substrate used up Q3. Which of the following best describes the reaction shown below? A+B----AB + energy a. Hydrolysis b. An exergonic reaction c. An endergonic reaction d. Catabolism Q4. Which letter shows the energy of activation for enzyme-catalyzed reaction?c Which letter shows the potential energy of the product? D a. A b. B c. C d. D

30 Q4. Binding of inhibitor Y as shown below inhibits a key catalytic enzyme by inducing a structural conformation change. Which of the following is correct: a. Y compete with substrates for binding in the active site and functions as a competitive inhibitor. b. Y compete with allosterically and functions as a competitive inhibitor. c. Y compete with substrates for binding in the active site and functions as a non-competitive inhibitor. d. Y compete with allosterically and functions as a non- competitive inhibitor. Q5. The second law of thermodynamics states that entropy, or disorder, is constantly increasing in the universe for spontaneous processes. Therefore, how is it possible that organisms exist in very ordered states? a. The second law of thermodynamics does not apply to biological life b. Energy can be created or destroyed c. The catabolic reactions in metabolism balance the effect of anabolic reactions d. Biological life creates an increase in entropy through dissemination of heat and waste

31 Q4. Binding of inhibitor Y as shown below inhibits a key catalytic enzyme by inducing a structural conformation change. Which of the following is correct: a. Y compete with substrates for binding in the active site and functions as a competitive inhibitor. b. Y compete with allosterically and functions as a competitive inhibitor. c. Y compete with substrates for binding in the active site and functions as a non-competitive inhibitor. d. Y compete with allosterically and functions as a non- competitive inhibitor. Q5. The second law of thermodynamics states that entropy, or disorder, is constantly increasing in the universe for spontaneous processes. Therefore, how is it possible that organisms exist in very ordered states? a. The second law of thermodynamics does not apply to biological life b. Energy can be created or destroyed c. The catabolic reactions in metabolism balance the effect of anabolic reactions d. Biological life creates an increase in entropy through dissemination of heat and waste