ENZYMES AHL Topic 7.6 IB Biology Miss Werba

AHL TOPIC 7 – NUCLEIC ACIDS AND PROTEINS 7.1 DNA STRUCTURE 7.2 DNA REPLICATION 7.3 TRANSCRIPTION 7.4 TRANSLATION 7.5 PROTEINS 7.6 ENZYMES J WERBA – IB BIOLOGY 2

THINGS TO COVER Enzymes in metabolic pathways Induced-fit model Activation energy & catalysis Competitive & non-competitive inhibition End-product inhibition J WERBA – IB BIOLOGY 3

METABOLIC PATHWAYS Command term = STATE 7.6.1 METABOLIC PATHWAYS Command term = STATE Metabolic pathways consist of chains and cycles of enzyme-catalysed reactions Metabolic pathways are also known as biochemical pathways All the reactions occur inside cells. An enzyme catalyses each reaction. The product of one reaction is the substrate for the next J WERBA – IB BIOLOGY 4

METABOLIC PATHWAYS Command term = STATE 7.6.1 METABOLIC PATHWAYS Command term = STATE Some pathways consist of chains of reactions eg. glycolysis a chain of 10 enzyme controlled reactions Some pathways consist of cycles of reactions eg. the Krebs Cycle (respiration) eg. the Calvin Cycle (photosynthesis) Ref: Biology for the IB Diploma, Allott J WERBA – IB BIOLOGY 5

METABOLIC PATHWAYS Command term = STATE 7.6.1 METABOLIC PATHWAYS Command term = STATE J WERBA – IB BIOLOGY 6

THE INDUCED-FIT MODEL Command term = DESCRIBE 7.6.2 THE INDUCED-FIT MODEL Command term = DESCRIBE The lock and key model of enzyme action does not fully explain enzyme activity. If the lock and key model were true, then one enzyme would only catalyse one reaction. However, there are some enzymes that are capable of catalysing multiple reactions. The Induced Fit Model proposes that the active site does not fit the substrate precisely until the substrate binds. J WERBA – IB BIOLOGY 7

THE INDUCED-FIT MODEL Command term = DESCRIBE 7.6.2 THE INDUCED-FIT MODEL Command term = DESCRIBE As the substrate binds to the active site, the active site changes shape to better fit the substrate. This weakens the bonds in the substrate, thus reducing the activation energy required for the reaction. J WERBA – IB BIOLOGY 8

THE INDUCED-FIT MODEL Command term = DESCRIBE 7.6.2 THE INDUCED-FIT MODEL Command term = DESCRIBE eg. some proteases have quite a broad specificity The induced fit model explains this better than the lock and key. J WERBA – IB BIOLOGY 9

ACTIVATION ENERGY Command term = EXPLAIN 7.6.3 ACTIVATION ENERGY Command term = EXPLAIN Activation energy: the amount of energy needed to start off a reaction Enzymes work by lowering the activation energy. When the enzyme-substrate complex is formed, the bonds of the substrate are stressed/weakened. This means that less energy is required to break them. J WERBA – IB BIOLOGY 10

ACTIVATION ENERGY Command term = EXPLAIN 7.6.3 ACTIVATION ENERGY Command term = EXPLAIN Exothermic / exergonic / catabolic reactions: These reactions involve the breakdown of complex compounds into simpler ones. eg. respiration, digestion This is where energy is released. The products have less energy than the reactants. J WERBA – IB BIOLOGY 11 Ref: Biology for the IB Diploma, Allott

ACTIVATION ENERGY Command term = EXPLAIN 7.6.3 ACTIVATION ENERGY Command term = EXPLAIN Endothermic / endergonic / anabolic reactions: These reactions involve the building up of complex compounds from simpler ones. eg. photosynthesis, protein synthesis, production of glycogen This is where energy is taken in from the surroundings. The products have more energy than the reactants. J WERBA – IB BIOLOGY 12 Ref: Biology for the IB Diploma, Allott

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ENZYME INHIBITION Command term = EXPLAIN 7.6.4 ENZYME INHIBITION Command term = EXPLAIN Some substances reduce the activity of enzymes or even prevent it completely. These substances are called enzyme inhibitors. There are two main classes of inhibitors: Competitive Non-competitive J WERBA – IB BIOLOGY 14

ENZYME INHIBITION Command term = EXPLAIN 7.6.4 ENZYME INHIBITION Command term = EXPLAIN Competitive inhibitors: inhibitor and substrate are structurally similar inhibitor binds to the active site substrate cannot bind Examples: the inhibition of folic acid synthesis in bacteria by sulfonamide Prontosil™ (an antibiotic) the inhibition of enzyme-catalysed synthesis of fumerate by oxaloacetate J WERBA – IB BIOLOGY 15

ENZYME INHIBITION Command term = EXPLAIN 7.6.4 ENZYME INHIBITION Command term = EXPLAIN Non-competitive inhibitors: inhibitor and substrate are not structurally similar inhibitor binds at another site, other than the active site changes the shape of the active site substrate cannot bind Examples: cyanide inhibition of cytochrome oxidase (an enzyme in cellular respiration). J WERBA – IB BIOLOGY 16

ENZYME INHIBITION Command term = EXPLAIN 7.6.4 ENZYME INHIBITION Command term = EXPLAIN J WERBA – IB BIOLOGY 17

ENZYME INHIBITION Command term = EXPLAIN 7.6.4 ENZYME INHIBITION Command term = EXPLAIN J WERBA – IB BIOLOGY 18

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END-PRODUCT INHIBITION Command term = EXPLAIN 7.6.5 END-PRODUCT INHIBITION Command term = EXPLAIN End-product inhibition prevents the build up of products It is an example of negative feedback The site where the end-product inhibitor binds is called the allosteric site. Binding of the product to the allosteric site changes the shape of the active site of the enzyme. This prevents binding of the substrate and a build up of the end product. Allostery is a therefore a form of non-competitive inhibition. J WERBA – IB BIOLOGY 20

END-PRODUCT INHIBITION Command term = EXPLAIN 7.6.5 END-PRODUCT INHIBITION Command term = EXPLAIN The advantage of this method is that if there is an excess of the end-product, the whole pathway can be switched off and intermediates do not build up. J WERBA – IB BIOLOGY 21

END-PRODUCT INHIBITION Command term = EXPLAIN 7.6.5 END-PRODUCT INHIBITION Command term = EXPLAIN Ref: Biology for the IB Diploma, Allott J WERBA – IB BIOLOGY 22

Sample questions Q1. Which is correct for the non-competitive inhibition of enzymes? J WERBA – IB BIOLOGY 23

Sample questions Q2. Consider the metabolic pathway shown below. If there is end-product inhibition, which product would inhibit which enzyme? J WERBA – IB BIOLOGY 24

Sample questions Q3. Outline how enzymes catalyse reactions. [7] J WERBA – IB BIOLOGY 25

Sample questions A1. D A2. D J WERBA – IB BIOLOGY 26

Sample questions they increase rate of (chemical) reaction; remains unused/unchanged at the end of the reaction; lower activation energy; activation energy is energy needed to overcome energy barrier that prevents reaction; annotated graph showing reaction with and without enzyme; substrate joins with enzyme at active site; to form enzyme-substrate complex; active site/enzyme (usually) specific for a particular substrate; enzyme binding with substrate brings reactants closer together to facilitate chemical reactions (such as electron transfer); induced fit model / change in enzyme conformation (when enzyme- substrate/ES complex forms); making the substrate more reactive; 7 max J WERBA – IB BIOLOGY 27