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Enzymes An enzyme is a biological catalyst. A catalyst is a substance that speeds up a chemical reaction without being consumed in the process. It can increase the rate of biochemical reactions more than a billion times. Enzymes are proteins (amino acids polypeptides). They are present in all living cells, where they perform a vital function by controlling the metabolic processes whereby nutrients are converted into energy and fresh cell material. Enzyme Structure A substrate is the reactant that an enzyme acts on when it catalyses a chemical reaction. The active site is the location where the substrate binds to an enzyme. Enzyme-substrate complex is an enzyme with its substrate attached to the active site. During a chemical reaction, reactants need to gain energy to break bonds, so they can form new bonds. This energy is called activation energy. Enzymes work by reducing the activation energy of a reaction and thus, increase the rate of the reaction. How do enzymes decrease activation energy? When a substrate or two, comes into the enzyme, the enzyme can put strain on the substrate (helps break bonds easier or it can help break/make bonds). Two Main Theories as to how Enzymes work: 1. Lock and Key Theory Substrate fits into a rigid active site on the enzyme like a key into a lock. Forms enzyme-substrate complex. Enzyme can help change the substrate, either by splitting it apart or linking pieces together. 2. Induced Fit Theory Active site is able to change to enfold a substrate. The enzyme takes up its most effective catalytic shape after binding with the substrate. The attachment of the substrate to the enzymes’ active site creates the enzyme-substrate complex. Factors that affect the rate of enzyme-catalyzed reactions: 1. 2. Rate increases in direct proportion to substrate concentration until the reaction reaches maximum rate (the more substrate the faster the reaction) Rate is directly proportional to the concentration of the enzyme present (the more enzyme the faster the reaction) 3. Incubation time (length of time over which a reaction has taken place) increases and the reaction rate decreases. 4. The rate increases with temperature up to a maximum (optimum temperature). Most human enzymes work best at 37 C. 5. Most enzymes are effective in a narrow pH range. Substances that Reduce Enzyme Activity There are substances that reduce the activity of enzymes or prevent it completely. Some enzyme inhibitors are competitive and some are noncompetitive. Competitive Inhibition See hand-out Competitive inhibitors are substances that compete with the substrate for an enzyme’s active site. They enter the enzyme’s active site and block the normal substrate from binding. Noncompetitive Inhibition Noncompetitive inhibitors are substances that attach to a binding site on an enzyme other than the active site, causing a change in the enzyme’s shape and a loss of affinity for its substrate. This type of inhibition lowers the reaction the most. Controlling Metabolic Pathways Metabolic pathways are many chemical reactions carried out in a particular sequence An enzyme catalyses each reaction Reactions take place inside cells Some anabolic/catabolic Glycolysis consists of chain reactions Kreb’s Cycle consists of cycles of reactions In many pathways, the product of the last reaction, attaches to the enzyme that catalyses the first reaction and inhibits that enzyme This is called end-product inhibition The enzyme that is inhibited is an example of an allosteric enzyme This is a type of noncompetitive inhibition that controls reactions in the body Inorganic Catalysts What are inorganic catalysts? Many metals are inorganic catalysts because they provide a surface for molecules to hang out (Think of a dance) When molecules are hanging out on the surface of a metal, they are more likely to react than if they are floating around Inorganic catalysts vs. Enzymes Enzymes are specific, inorganic catalysts are not. 2. Enzymes function effectively only in a limited range of temperature, inorganic catalysts have a wide range of temperature, pressure and pH that they can function in. 1.