Unit 3 Science Investigation Skills Topic D: Enzymes in action Lesson 5

Lesson 5 Learning objectives To use collision theory to describe how enzymes catalyse reactions. To explain the mechanisms of enzyme action including lock and key and induced fit. To explain how enzymes catalyse reactions by lowering the activation energy needed.

The stages of an enzyme-catalysed reaction The substrate collides with its specific enzyme molecule. The enzyme and substrate form an enzyme-substrate complex and the formation of products is catalysed. At the end of the reaction, the enzyme remains unchanged and ready to receive further substrate molecules.

Collision theory This theory describes how an enzyme molecule needs to collide with a substrate molecule in order to form an enzyme-substrate complex and enable the reaction to take place. To form a complex, the substrate needs to collide with: an available active site of an enzyme sufficient energy an active site that is in the correct, complementary formation. What could affect the rate of successful collisions (and the rate of reaction)?

Activation energy Activation energy of a reaction = the energy required for a reaction to begin. In chemical reactions, this is often in the form of heat. However, catalysts reduce the activation energy of a reaction. Enzymes perform this function in metabolic reactions. Why would heat be an unsuitable source of energy to initiate a biological reaction?

Induced fit Formation of the enzyme-substrate complex causes a shape-change of the enzyme’s active site. The lock and key model has been superseded by the theory of induced fit. This describes how the active site and substrate are not wholly complementary in shape until the enzyme-substrate complex forms, when the active site is made to fit the substrate more exactly. In some reactions, this change in the shape of the enzyme molecule causes stresses on the bonds in the substrate, giving the energy required for the chemical reaction to occur.

Activation energies of reactions and enzymes Learning aim D Activity Sheet 3.1 Complete activity sheet 3.2 on: - the energy profiles of reactants and products of a reaction - the transition state - the activation energy (EA).

The effect of an enzyme on the activation energy of a metabolic reaction

Demonstration of the action of yeast zymase on glucose (fermentation) Zymase is actually a group of enzymes responsible for fermentation in yeast.

Matching game On a blank sheet of paper, draw a unique shape of substrate and its specific enzyme with its complementary active site. Label your shapes ‘substrate’ and ‘enzyme’. Cut around the shapes you have drawn. Hold on to your substrate shapes, and give the enzyme shapes to your tutor. When your tutor has redistributed the enzyme shapes randomly, try to find the enzyme for your substrate!