Topic 7.6 – HL Level Enzymes HL

7.6.1 State that metabolic pathways consist of chains and cycles of enzyme-catalysed reactions.

7.6.2 Describe the induced-fit model. This is an update version of the lock and key theory. It suggests that the active site of the enzyme may not exactly correspond to the shape of the substrate. The active site has a more flexible shape and is able to mould itself around the substrate. Only when it binds closely to the substrate does the active site catalyze the reaction. As in the lock and key theory, the products no longer fit the active site and are repelled. The enzyme reverts to its relaxed state and is able to attach more substrate.

7.6.3 Explain that enzymes lower the activation energy of the chemical reactions that they catalyse Chemical reactions need an initial input of energy = THE ACTIVATION ENERGY During this part of the reaction the molecules are said to be in a transition state Increasing the temperature make molecules move faster Biological systems are very sensitive to temperature changes. Enzymes can increase the rate of reactions without increasing the temperature. They do this by lowering the activation energy. They create a new reaction pathway “a short cut”

7.6.4 Explain the difference between competitive and non-competitive inhibition…

7.6.4 Competitive inhibition In the Krebs cycle, the enzyme succinate dehydrogenase removes hydrogen form its substrate succinate. But the enzyme’s activity can be inhibited if malonate is present. Malonate competes with succinate for the active site on the enzyme.

7.6.4 Non-Competitive inhibition Cytochrome oxidase is the last enzyme in the respiratory electron transport chain of mitochondria. The binding of cyanide to this cytochrome prevents transport of electrons from cytochrome c oxidase to oxygen. As a result, the electron transport chain is disrupted, meaning that the cell can no longer aerobically produce ATP for energy.

7.6.5 Explain the control of metabolic pathways by end-product inhibition… most allosteric enzymes have multiple allosteric sites; allosteric inhibition is a form of non-competitive inhibition; metabolites can act as allosteric inhibitors of enzymes earlier in a metabolic pathway to regulate metabolism; binding (of end product) to an allosteric site changes shape of enzyme;