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05 Enzymes - Enzymes Enzymes are catalysts Almost all...

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Enzymes Enzymes are catalysts. Almost all enzymes are proteins (though some ribosomal reactions are catalysed by RNA, and evolutionary theorists suggest that the first enzymes were RNA molecules). As catalysts, enzymes do not change the equilibrium of a reaction, but speed up the attainment of equilibrium. Chemical catalysts usually speed up reactions 100 to 10,000 fold. Some enzymes can speed up reactions by as much as 10 12 times. Many reactions which they catalyse would effectively not take place without them. Why are enzymes so much more effective as catalysts than chemical catalysts?
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The Catalytic Process Chemical catalysts usually work in one of two ways: They may provide a surface on which reactants can be orientated, so that they come into contact with each other more easily. Alternatively, they may take part in the reaction, providing an alternative reaction mechanism with a lower activation energy. Enzymes usually do both these things, though because they bind substrates extremely specifically they orientate the reacting substrates much more exactly than a chemical catalyst does.
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The Active (or Catalytic) Site The conversion of substrate to product occurs on an area of the enzyme’s surface called the active site. This is often in a cleft in the protein’s surface. A more correct name is the catalytic site , to distinguish it from other ligand binding sites which may be found on the protein surface, and involved in control. Substrates bind to the active site by interacting with the side chains of amino acid residues which form the protein surface in this area. Amino acids which are involved in binding the substrate to the active site are called contact amino acids. There may be several or many contact residues at the active site. At the actual site of the enzyme reaction some amino acid residues may take part in the catalytic reaction of the enzyme. These are called catalytic amino acids. There are unlikely to be more than one, two, or a few such residues.
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Co-factors Many (but by no means all) enzymes have a non-protein molecule bound to the active site region, which take part in the catalytic process. These may take the place of the catalytic residues described in the last slide, or may work alongside catalytic residues. These non protein components are essential to the activity of the enzymes which have them. They are called co-factors or co-enzymes. (The protein portion of these enzymes is called the apo-enzyme.) There are a small number of molecules which commonly act as co-factors in enzyme reactions. Many of these are water soluble vitamins , most of which are enzyme co- factors. Metal ions also often act as co-factors.
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The ‘Lock and Key’ theory The earliest idea about how enzyme and substrates fit together was put forward by a chemist called Emil Fischer. It suggests that the substrate molecule or molecules fit onto the enzyme’s active site like a key fitting into a lock.
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