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SICM Tuition Biology AS Enzymes OK….so now we’ve done all of that Chemistry stuff that you all love so much...let’s get down to the real stuff… Having just learnt about proteins, let’s now look at one of the fundamental substances within the body. Enzymes are used for almost all chemical reactions in the body. But what are enzymes and how do they work? - - Enzymes are catalysts. They “react” with substrates to form products o What is a catalyst? (We will look at how some work later) A catalyst is a substance that speeds up the rate of a reaction (by lowering the activation energy) but it remains unchanged at the end of the reaction. Without catalysts, 37˚C would be too slow to sustain life. We will look at how different conditions affects enzymes later An organism’s metabolism consists of thousands of different reactions and each one has a different catalyst or enzyme Metabolism consists of hundreds of reactions linked together where the product from one reaction is the substrate of the next So what is an enzyme? Enzymes are proteins Proteins are made up of amino acids From this, we know that there is a wide range of possible structures that could form (depending on the primary, secondary, tertiary structures) There is, therefore always going to be an area that is only specific for certain substances This area is called the active site It is where the enzyme comes into contact with the substrate o The substrate is the substance that will react with the enzyme to for the product The way in which this works is still being debated, but there are two possible hypotheses that you need to know about: Lock and Key Hypothesis during catalysis, the substrate molecule fits into the active site and interacts with the amino acids by ionic and hydrogen bonding forming an ENZYMESUBSTRATE COMPLEX (yes…that’s in capitals and bold for a reason! Learn it!) A reaction occurs and the product leaves the active site The precise shape of the active site must be complementary to the shape of the substrate. Active site Enzyme Enzyme-Substrate complex Substrate Page 1 SICM Tuition Biology AS Induced Fit hypothesis This is a more recent version of the mechanism It states that the enzyme’s active site does not “fit” the substrate until the substrate actually enters the site The shape of the enzyme then moulds around the substrate This then forms an ENZYME-SUBSTRATE COMPLEX (yes yes…that’s the second time…) Active site Enzyme-Substrate complex The change in the active site brings the amino acids into their correct positions in the active site so a reaction can occur. The active site then returns to its original shape Enzyme Substrate Enzyme reactions are reversible The enzyme will catalyse the reaction equally well in either direction This refers to catabolic and anabolic reactions o What do these words mean? Catabolic reactions are the breaking down of molecules into smaller units. Anabolic reactions are the building up of structural units from smaller units. - The direction depends on the concentration of the substrate and product and will go in the direction of the low concentration An enzyme has no effect on the point of equilibrium: it just affects the time taken to reach the equilibrium Page 2