1. Title: Lesson 4 B.2 Enzymes Learning Objectives: – Describe the structure and the function of an enzyme – Identify and explain the factors that affect enzyme activity

2. Main Menu Enzymes are globular proteins  Complexity of metabolism demands a highly sensitive control system  Enzymes can respond to the changing needs of the cell  Enzymes are biological catalysts  Enzymes exist in compact spherical shapes when in aqueous solution in cells  Well defined tertiary structure gives a specific 3D shape  Large molecules – usually containing several hundred amino acids, with some having a quaternary structure  In addition, some enzymes requires non-protein molecules to be bound for activity, known as co-factors  Organic – coenzymes  Inorganic – metal ions

3. Enzymes form a complex with the substrate They make chemical reactions go faster but don’t actually take part in the reaction. They do this by reducing the activation energy required to initiate the reaction. The reactant that is catalysed by the enzyme is known as the substrate What is activation energy? Before any chemical reaction can proceed it must initially be activated, i.e. its energy must be increased. The energy required is called activation energy. Once provided, the activation energy allows the products to be formed.

4. How do enzymes lower activation energy? Heat is often the source of activation energy and enzymes often dispense with the need for this heat and so allow reactions to take place at lower temperatures. Enzymes provide a platform for the reactants of a reaction to come together at the active site. Without enzymes, more energy would be required in order for reactants molecules to collide with one another. When a chemical reaction involves two or more reactants, the enzyme provides a site where the reactants are positioned very close to each other and in an orientation that facilitates the formation of new covalent bonds.

5. How do enzymes work? Each type of enzyme catalyses a specific reaction. The enzyme has the ability to forma temporary binding to the substrate, held by relatively weak forces of attraction. The important part of the enzyme is the active site. The substrate (usually smaller) fits into this part, and an enzyme-substrate complex is formed. Computer representation of the enzyme lysozyme, which breaks down polysaccharides. Protein = blue Backbone = magenta Substrate = yellow

6. Main Menu Formation of substrate complex depends on…  ‘Chemical fit’ or compatibility between substrate and the side chains of the amino acids at the active site  This involves non covalent interactions such as hydrophobic attractions, dipole-dipole attractions, hydrogen bonds, and ionic attractions  Binding puts a strain on the substrate molecule, facilitating the breaking and forming of bonds  Substrate will react, and the product will no longer fit the active site so detaches  Enzyme is released unchanged and can catalyse further reactions Summary of enzyme action: Equilibrium signs show reactions are reversible under the right conditions

7. The lock and key model of enzyme action Enzymes are highly specific in the reactions catalysed. The specificity of an enzyme for its substrates results from its conformation, as it determines the arrangement of the side chains at the active site and therefore the ability to bind precisely with the substrate Some enzymes catalyse the transformation of one particular type of substrate molecule or, at most, a very restricted group of substrate molecules. Some catalyse only one type of chemical change. The specificity of enzymes is due to the configuration of the active site. In the lock and key model, the substrate is the ‘key’ that fits exactly into the enzyme ‘lock’

8. The induced fit model of enzyme action Evidence from protein chemistry suggests that a small rearrangement of chemical groups occurs in both the enzyme and the substrate molecules when the enzyme-substrate complex is formed. This is called induced fit. The induced fit theory can be compared to a hand entering a glove. The hand will move slightly to fit into the glove, while the gloves’ shape will change to accommodate the hand.

9. Main Menu Enzyme kinetics indicate that saturation occurs in the formation of the complex  When substrate concentration against rate of reaction are plotted, curve shows the distinctive shape of saturation  Following points can be deduced from the graph:  (a) At low [substrate], rate (v) is proportional to [substrate]  (b) As [substrate] increases, the rate (v) decreases, and is no longer proportional to [substrate]. Some enzymes have active sites that are already occupied  (c) At high [substrate], rate (v) is constant and independent of [substrate]. This is saturation. NOTE: V (velocity) is used to describe the rate of enzyme reactions All enzymes show this saturation effect, but vary widely with the [substrate] needed for saturation.

10. Main Menu Michaelis-Menten Equation  1 The maximum velocity V max  Has the units of rate  Varies greatly from one enzyme to another  Varies with pH and temperature  Can be expressed as the turnover number  Turnover number – defined as the number of molecules of substrate that can be processed into products per enzyme molecule per unit of time  E.g. catalase is very fast with turnover rate of up to 100 000 molecules of its substrate H 2 O 2 per second

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12.  2 The Michaelis constant K m  Substrate concentration at which reaction rate is equal to one half its maximum value  [S] = K m when rate is V max /2  K m has units of concentration  Give information about the affinity of the enzyme for the substrate.  Inverse relationship – low K m means reaction is going quickly even at low [S].  Higher K m means enzyme has a lower affinity for the substrate…

13. Main Menu Some K m values…  The values for K m determines how responsive an enzyme will be to changes in substrate concentration  Low K m – the enzyme will be saturated under most cell conditions, so will have a more constant rate regardless of variations in [substrate]  High K m – the enzyme will not normally be saturated so its activity is more sensitive to variations in [substrate] Saturated Low Higher

14. Main Menu Enzyme activity is influenced by its physical and chemical environment  Factors that influence enzyme activity:  Temperature  pH  Heavy metal ions  Inhibitors (competitive and non-competitive)  TASK: Look at the posters around the room and complete the worksheet ‘Factors influencing enzyme activity’

15. Plenary: Explain how inhibitors affect the rate of enzyme- controlled reactions. (6 marks) 1 Statement about two types, competitive and non-competitive; Note. Award points 2 –5 only in context of competitive and non- competitive inhibition Competitive 2 Similarity of shape of inhibitor and substrate; 3 Inhibitor can enter/bind with active site (of enzyme); Non-competitive 4 Affect/bind to enzyme other than at active site; 5 Distorts shape of active site; Inhibitors 6 Prevent entry of/binding of substrate to active site; 7 Therefore fewer/no enzyme-substrate complexes formed; max 6

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