7.6 - Enzymes

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IB Biology notes for enzymes. Introduction to activation energy and active sites.
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   www.ibscrewed.org  7.6  –  Enzymes 7.6.1 - State that metabolic pathways consist of chains and cycles of enzyme-catalysed reactions Metabolic pathways form a series of reactions  that regulate the concentration of substances within cells by enzyme-mediated linear and circular sequences. Respiration and photosynthesis are examples of a metabolic pathway (see below). All these reactions may be classified into two types. Catabolic Reactions This is the breaking down  of larger molecules, releasing energy. This is exergonic. Enzymes in these reactions will break apart the chemical bonds, and two molecules are formed. These reactions include digestion and cellular respiration. Anabolic Reactions This is when smaller molecules form bonds  and become larger molecules. This process requires energy, therefore it is endergonic. Enzymes will draw the smaller molecules in and help the new bonds to form. Examples include protein synthesis (build up of polypeptides from peptides) and cellular respiration.   www.ibscrewed.org  Chain Pathways  move from one reaction to the next. Each substrate has its own enzyme. The final product is called the end product. Cyclic Pathways  are when the initial substrate is fed into the cycle. The final product is reacted with the initial substrate. From here, the products are converted. The only difference in this pathway is the regeneration of the final intermediate. Examples of this type of pathway include Krebs cycle and the Calvin cycle. 7.6.2 - Describe the induced fit model In the more accurate, induced fit model , there is modification. Enzymes are fairly flexible, and will reshape the active site by interactions with the substrate. Hence, the substrate does simply bind to a rigid active site, but the amino acid side chains will mould into positions for the enzyme to perform its function.   www.ibscrewed.org  This change in shape is critical to momentarily raise the substrate molecule to the transitional state, when it can react. This model also accounts for the range of substrates that some enzymes can bind to. 7.6.3 - Explain that enzymes lower the activation energy of the chemical reactions that they catalyse The activation energy is the minimum amount of energy  required to raise substrate molecules to their transition state. The reaction cannot happen until this energy barrier has been overcome. The use of an enzyme reduces the amount of this energy that is needed. The transition state is when the bonds in the reactant molecules break and begin to form the bonds of the products. Since breaking bonds is an endothermic process, the reactants require some energy to be added before they can start making the new bonds and begin the reaction. This amount of energy is the activation energy . Enzymes work by providing an alternative reaction pathway that requires a lower activation energy. The frequency of collision between molecules increases, speeding up the reaction.   www.ibscrewed.org  7.6.4 - Explain the difference between competitive and non-competitive inhibition, with reference to one example of each Enzyme inhibitors deactivate enzymes. They come in two types: Reversible Inhibitors are used to control enzyme activity. Competitive inhibition  is when the inhibitor and substrate must compete for the active site. The inhibitor is structurally similar to the substrate, and it prevents the substrate from binding. Examples are:      O₂ competing with CO₂ for the active site of RuBisCo      Malonate competing with succinate for the active site of succinate dehydrgenase Non-competitive inhibitors  slow down the rate of reaction because they distort the shape of the active site. When there is a build-up of the end-product or lack of substrate, the enzyme may become deactivated. Examples are:    Cyanide ions blocking cytochrome oxidase in terminal oxidation in cell aerobic respiration    Nerve gas Sarin blocking acetyl cholinesterase in synapse transmission
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