chapter 5 enzymes

Sec 3 Bio 1

Chapter 5 - Enzymes5.1 What Are Enzymes?

5.2 Classification of Enzymes

4.3 Characteristics of Enzymes

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Learning Objectives

Candidates should be able to:

• Define enzymes as proteins which function as biological catalysts.

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5.1 What Are Enzymes?Enzymes are:

• Biological catalysts,

• Protein in nature,

• Catalyze chemical reactions without being changed at the end of the reaction.

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Enzymes as catalysts• Enzymes lower the activation energy of a

reaction so that it occurs more readily.

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Activation Energy

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).

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Activation energy is the energy needed to roll the stone up the hill.

Activation Energy


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Once over the hill, the rest of the reaction occurs.

Activation Energy


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Activation Energy


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The stone rolls down and breaks into tiny pieces (products are formed).4

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The stone rolls down and breaks into tiny pieces (products are formed).

The energy needed to start a chemical reaction is called activation energy.

Activation Energy


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Digestion: An Enzyme-Catalysed 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.

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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.

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5.2 Classification of EnzymesEnzymes 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.

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Learning ObjectivesCandidates 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 .

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5.3 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.

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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.

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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.

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A synthesis reaction

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active sites

A

B

enzyme molecule (the ‘lock’)

substrate molecules ( A and B) can fit into the active sites

Lock and Key Hypothesis

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active sites

A

B


enzyme-substrate complex


Lock and Key Hypothesis

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Lock and Key Hypothesisactive sites

A

B

AB


enzyme-substrate complex


enzyme molecule is free to take part in another reaction

a new substance (product) AB leaves the active sites

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Induced fit hypothesisWhat is induced fit hypothesis?• shape of the active site adjusts to fit the

substrate.

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Induced fit hypothesisHow did induced fit hypothesis come about?

- recent imaging technology demonstrated changes in the 3-D conformation of enzymes when interacting with their substrates.

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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).

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• At 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.

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26Temperature

Rate of reaction (enzyme activity)

0 K (optimum temperature) D

At point D, the enzyme has lost its ability to catalyse the reaction. An enzyme

is less active at very low temperatures.

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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.

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The optimum temperature is reached. Enzyme is most active.3

Beyond the optimum temperature, enzyme activity decreases.

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Effect of Temperature on the Rate of Reaction

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Effect of pH • Enzymes have an optimum pH.

• Deviation from the optimum pH will decrease enzyme

activity.

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Effect of pH on Enzyme Activity

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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.

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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.

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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.

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Coenzymes• Coenzymes are altered in some way

by participating in enzyme reaction.

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Enzymes

• catalyse reversible reactions

A DB C+ +

reactants products

reactantsreactants

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CharacteristicsFunctions Mode of Action

Limiting factors

affected by

Enzymes

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Enzymes

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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.

Enzymes

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Functions

Enzymes

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Functions

• Building up or synthesising complex substances

• Breaking down food substances in cells to release energy (cellular respiration)

• Breaking down poisonous substances in cells

Enzymes

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CharacteristicsFunctions

Enzymes

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CharacteristicsFunctions

• 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

Enzymes

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Enzymes

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• Lower the activation energy of a reaction

• Interact with the substrate according to lock and key hypothesis to form an enzyme-substrate complex

Enzymes

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affected by

Enzymes

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Limiting factors

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.

affected by

Enzymes

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Limiting factors

Temperature / pH

e.g.

affected by

Enzymes

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Limiting factors

Temperature / pH

e.g.

• 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

affected by

Enzymes

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Limiting factors

Temperature / pH

e.g.

Classes

affected by

Enzymes

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Limiting factors

Temperature / pH

e.g.

Classes

based on the type of reaction catalysed e.g.

Hydrolasesaffected by

Enzymes

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Limiting factors

Temperature / pH

e.g.

Classes

based on the type of reaction catalysed e.g.

Hydrolases

Oxidation-reduction enzymes

affected by

Enzymes

chapter 5 enzymes

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