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Biology OCR A level Enzymes Summary Notes
Biology OCR A level Enzymes Summary Notes
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1 of 4Enzymes
2.1.4
(a) The role of enzymes in catalysing reactions that affect metabolism at a cellular and whole
organism level.
• Enzymes are efficient catalysts for biochemical reactions. They speed up reactions by
providing an alternative reaction pathway of lower activation energy.
• Enzymes take part in the reaction but they don’t undergo permanent changes. They can
only alter the rate of reaction, not the position of the equilibrium.
• Anabolic and catabolic reactions can only happen as a result of the control and order
imposed by enzymes. They are related to metabolism, as sum of all reactions.
• Enzymes are usually highly selective, catalysing specific reactions only. This specificity is
due to the shapes of the enzyme molecules.
(b) The role of enzymes in catalysing both intracellular and extracellular reactions.
• Intracellular enzymes act within a cell.
• Extracellular enzymes act outside of the cell they are made in.
• Catalase is an example of an intracellular enzyme, its role is to catalyse the breakdown of
hydrogen peroxide before it can damage the cell contents.
• Amylase and trypsin are examples of extracellular enzymes.
• The digestion of scratch starts in the mouth and continues to the small intestine. Starch is
broken down into maltose by amylase. Amylase is produced by salivary glands and the
pancreas. Maltose is then broken down into glucose by maltase.
• Trypsin is a protease that catalyses digestion of proteins, produced by the pancreas.
(c) The mechanism of enzyme action.
• Enzymes are a complex tertiary structure which are complementary to the shape of a
specific substrate molecule.
• Enzymes are specific to substrates, known as the specificity of the enzyme.
• Active site of an enzyme is the area that has the complementary shape and functional
group to bind to the substrate molecule.Â
• The same way that only the correct key will fit into a lock, only a specific substrate will fit
the active site, lock and key hypothesis.
• Induced- fit hypothesis suggests that the active site of the enzyme changes shape slightly
as the substrate enters.
• When the substrate is bound to the active site an enzyme-substrate complex is formed.
The substrate then reacts and the product is formed in an enzyme-product complex. The
product is then released leaving the enzyme unchanged.
• For molecules to react they must collide with one another at the correct orientation and
with enough energy to overcome the energy barrier, called activation energy. Enzymes help
the molecules collide successfully reducing the activation energy required.
(d) (i) The effects of pH, temperature, enzyme concentration and substrate concentration on
enzyme activity.
pH
• pH is a measure of the acidity or alkalinity of a solution, linked to the number of hydrogen
ions present.
• A low pH means the solution has a lot of H+. A high pH means the solution has few H+.
• An alkali is known as a proton acceptor. An acid is known as a proton donor.
• H+ attract negative charges and interferes with the hydrogen and ionic bonds holding the
tertiary shape in place. They also interfere with charged groups of amino acids that make
up the active site, altering the shape.
• Increase/ decrease of pH beyond the optimum can prevent the formation of an enzyme
substrate complex.
• pH can change the active site altering its shape but it returns to normal, known as
renaturation.
• Enzymes in the stomach (pepsin) mix with strong acids so optimum pH is 2.
• Enzymes in the intestine (trypsin) work in neutral conditions so optimum pH is 7.
Temperature
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