Chapter 5 - Enzymes What Are Enzymes? Classification of Enzymes Characteristics of Enzymes Sec 3 Bio

Learning Objectives Candidates should be able to: Define enzymes as proteins which function as biological catalysts.

What Are Enzymes? Enzymes are: Biological catalysts, Protein in nature, Catalyze chemical reactions without being changed at the end of the reaction.

Enzymes as catalysts Enzymes lower the activation energy of a reaction so that it occurs more readily.

Activation Energy 1 Imagine a chemical reaction as the process of rolling a huge stone (reactant) up a hill so that it rolls down and breaks into tiny pieces (products). Figure 5.1 page 73

Activation Energy 1 Imagine a chemical reaction as the process of rolling a huge stone (reactant) up a hill so that it rolls down and breaks into tiny pieces (products). 2 Activation energy is the energy needed to roll the stone up the hill. Figure 5.1 page 73

Activation Energy 1 Imagine a chemical reaction as the process of rolling a huge stone (reactant) up a hill so that it rolls down and breaks into tiny pieces (products). 3 Once over the hill, the rest of the reaction occurs. 2 Activation energy is the energy needed to roll the stone up the hill. Figure 5.1 page 73

Activation Energy 1 Imagine a chemical reaction as the process of rolling a huge stone (reactant) up a hill so that it rolls down and breaks into tiny pieces (products). 3 Once over the hill, the rest of the reaction occurs. 2 Activation energy is the energy needed to roll the stone up the hill. Figure 5.1 page 73 4 The stone rolls down and breaks into tiny pieces (products are formed).

Activation Energy 1 Imagine a chemical reaction as the process of rolling a huge stone (reactant) up a hill so that it rolls down and breaks into tiny pieces (products). 3 Once over the hill, the rest of the reaction occurs. 2 Activation energy is the energy needed to roll the stone up the hill. Figure 5.1 page 73 4 The stone rolls down and breaks into tiny pieces (products are formed). 5 The energy needed to start a chemical reaction is called activation energy.

Digestion: An Enzyme Process Why do we need to digest our food? Starch, proteins and fats are very large. They cannot diffuse across cell membranes for absorption. Therefore, they must be digested into Simpler, smaller and soluble substances. Diffusible across cell membranes.

Other applications of Enzymes Anabolic processes Eg. Synthesis of proteins from amino acids. Catabolic processes Eg. Oxidation of glucose (tissue respiration) Catalase production Catalase catalyses the breakdown of toxic hydrogen peroxide into harmless water and oxygen. Catalase is abundant in liver and blood.

Classification of Enzymes Enzymes are classified according to the chemical reaction involved in: Enzymes that catalyse hydrolysis reactions are called hydrolases. Example of hydrolases: Carbohydrases, proteases, lipases. Enzymes involved in oxidation of food as called oxidation-reduction enzymes.

Learning Objectives Candidates should be able to: Explain enzyme action in terms of the ‘lock and key’ hypothesis. Investigate and explain the effects of temperature and of pH on the rate of enzyme catalyzed reactions .

Characteristics of Enzymes Enzymes alter or speed up the rates of chemical reaction that occur in a cell. Enzymes are required in minute amounts. Since enzymes are not altered in a chemical reaction, a small amount can catalyse a huge reaction.

Enzymes are specific Specificity of enzyme is due to its shape (or surface configuration). The substrate will fit into an enzyme, forming an enzyme-substrate complex. The product will then be released.

Lock and key hypothesis What is the ‘lock and key’ hypothesis? It is the old view of enzyme specificity, that there was an exact match between the active site and the substrate.

A synthesis/ dehydration Rx.

Lock and Key Hypothesis active sites enzyme molecule (the ‘lock’) B A substrate molecules ( A and B) can fit into the active sites Figure 5.6 page 77

Lock and Key Hypothesis active sites enzyme molecule (the ‘lock’) B A substrate molecules ( A and B) can fit into the active sites enzyme-substrate complex Figure 5.6 page 77

Lock and Key Hypothesis active sites enzyme molecule (the ‘lock’) B A enzyme molecule is free to take part in another reaction substrate molecules ( A and B) can fit into the active sites enzyme-substrate complex Figure 5.6 page 77 AB a new substance (product) AB leaves the active sites

Induced fit hypothesis What is induced fit hypothesis? shape of the active site adjusts to fit the substrate.

Induced fit hypothesis How did induced fit hypothesis come about? - recent imaging technology demonstrated changes in the 3-D conformation of enzymes when interacting with their substrates.

Effect of temperature At low temp: Rate of reaction is slow. Enzymes are inactive at low temp. Every 10oc rise in temp, rate of reaction increases by double (till it reaches optimum temp).

Effect of temperature At optimum temp: Beyond optimum temp: Rate of reaction is the highest. Enzymes are most active. Beyond optimum temp: Rate of enzyme activity decreases sharply. Enzymes are being denatured. Hydrogen bonds are easily disrupted by increasing temperature.

Effect of temperature

Effect of Temperature on the Rate of Reaction 3 The optimum temperature is reached. Enzyme is most active. Rate of reaction (enzyme activity) 4 Beyond the optimum temperature, enzyme activity decreases. 2 As the temperature rises, enzyme activity increases as indicated by the increase in the rate of reaction it catalyses. Usually the enzyme is twice as active for every 10°C rise in temperature until the optimum temperature is reached. 5 At point D, the enzyme has lost its ability to catalyse the reaction. 1 An enzyme is less active at very low temperatures. Figure 5.7 page 80 K (optimum temperature) D Temperature

Effect of pH Enzymes have an optimum pH. Deviation from the optimum pH will decrease enzyme activity.

Effect of pH on Enzyme Activity

Effects of substrate and enzyme concentration on rate of reaction Increasing substrate concentration will increase rate of reaction until a certain limit. Cause: Enzyme molecules are saturated. Enzyme concentration is now the limiting factor.

What is a limiting factor? Any factor that directly affects the rate of a process if its quantity is changed The value of the limiting factor has to be increased in order to increase the rate of the process.

Coenzymes What are coenzymes? Some enzymes require a coenzyme to be bound to them before they can catalyse reactions. Usually, coenzymes are non-protein organic compounds. Eg. Vitamins, especially the B complex vitamins.

Coenzymes Coenzymes are altered in some way by participating in enzyme reaction.

Enzymes catalyse reversible reactions D A B C + + products reactants

Enzymes Functions Characteristics Mode of Action Limiting factors affected by Limiting factors Concept Map page 87

Enzymes Concept Map page 87

Enzymes Biological catalysts, which are mainly made of proteins. They speed up the rate of chemical reactions without themselves being chemically changed at the end of the reactions. Concept Map page 87

Enzymes Functions Concept Map page 87

Enzymes Functions Building up or synthesising complex substances Breaking down food substances in cells to release energy (cellular respiration) Breaking down poisonous substances in cells Concept Map page 87

Enzymes Functions Characteristics Concept Map page 87

Enzymes Functions Characteristics Speed up chemical reactions Required in small amounts Highly specific Work best at an optimum temperature and pH May need coenzymes for activity Some catalayse reversible reactions Concept Map page 87

Enzymes Functions Characteristics Mode of Action Concept Map page 87

Enzymes Functions Characteristics Mode of Action Lower the activation energy of a reaction Interact with the substrate according to lock and key hypothesis to form an enzyme-substrate complex Concept Map page 87

Enzymes Functions Characteristics Mode of Action affected by Concept Map page 87

Enzymes Functions Characteristics Mode of Action Limiting factors affected by Limiting factors Concept Map page 87 Factors that directly affect the rate at which a chemical reaction occurs if their quantity is changed. The value of a limiting factor must be increased in order to increase the rate of reaction.

Enzymes Functions Characteristics Mode of Action Limiting factors affected by Limiting factors Concept Map page 87 e.g. Temperature / pH

Enzymes Functions Characteristics Mode of Action Limiting factors affected by Limiting factors Increase in temperature increases the rate of enzyme reaction until optimum temperature is reached Increase in pH increases the rate of enzyme reaction until optimum pH is reached Concept Map page 87 e.g. Temperature / pH

Enzymes Functions Characteristics Mode of Action Classes affected by Limiting factors Concept Map page 87 e.g. Temperature / pH

Enzymes Functions Characteristics Mode of Action Classes based on the type of reaction catalysed e.g. affected by Hydrolases Limiting factors Concept Map page 87 e.g. Temperature / pH

Enzymes Functions Characteristics Mode of Action Classes based on the type of reaction catalysed e.g. affected by Hydrolases Limiting factors Oxidation-reduction enzymes Concept Map page 87 e.g. Temperature / pH