UrurrA: ENZYMES

Learning Objectives:g Enzymes have several unique characteristics.

. SensitiviA b tumperature

. SensitiviA b pH

. Substrate specificity

. Required in minute quantitiesg The limitingfactor will control the rate of reaction of enzymes.

ENZYMES AS BIOLOGICAL CATALYSTS

. Enzymes are biological catalysts which speed up chemical reactions without themselves being

changed at the end of the reaction.

. They are protein in nature. They are produced when they are needed to perform specificf u nctions.

. Enzyme names end with the -.ase suffix after the adoption of the -ase naming system. Sometimes,

the -ase suffix is added to the substrate which the enzyme acts on.

For example

Protease is the enzyme that breaks down the substrate proteins.

ENZYMES LOWERING ACTIUATION ENERGY

. Activation energy is the minimum amount of energy required to initiate a chemical reaction.

. All chemical reactions need activation energy to break chemical bonds in order to trigger the

reaction.. The requirement for activation energy controls the likelihood a chemical reaction can occur and

the speed at which it occurs.

. Enzymes can lower the activation energy of a chemical reaction. As a result, in the presence of

enzymes, chemical reactions can take place at a faster rate at normal body temperature.

Energy level

activation energywithout enzymes

activation energywith enzymes

substrates(or reactants)

products

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Progress of reaction

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OREVISION I

L. (a) Define enzyme

(b) For every chemical reaction, there is a requirement of activation energy. Explain what is meantby activation energy.

(c) Explain how an enzyme changes the rate of a chemical reaction.

(d) How can enzyme be classified in enzyme-catalysed reactions?

2. Evaluate the following statements and answer each of the statements by'True/False'.

(a) Enzymes are named using a scientific system which usuaily ends with'ase'.

(b) Hydrolases uses hydrogen molecules to break down a complex molecule intosimpler molecules.

(c) Proteases are examples of hydrolases.

(d) Enzymes are inorganic catalysts that speed up the chemical reactions.

(e) All proteins, including enzymes, are denatured by high temperatures and extremechanges in acidity or alkalinity of the surroundings.

(f) The name of an enzyme indicates the type of simpler substances they produce in areaction.

(g) Enzymes are produced only when needed.

CHARACTERISTICS OF ENZYMES

Sensitivity to temperatureB Never describe enzyme being

'killed' at high temperatures.

Enzymes are inactive at low temperatures and denatured at high temperatures.

The effect of temperature on enzyme activity can be investigated using the experimental set upshown.

For example

A mixture of starch and amylase is placed at different temperatures of water bath. lodine is used

Enzymes are noldenatured below itsoptimum lemperature.

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to test for the presence of starch

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thermometer

iodine

mixture of starchand amylase

waterbath

ln this experiment:

- The substrate used is starch solution and enzyme is salivary amylase.

- Temperature of the water bath is maintained at different temperatures for eachinvestigation.

- lodine (a test reagent) is used to test for the starch. (See testing of starch in Unit 3.)

- The presence or absence of starch indicates how the activity of enzyme is affected by

temperatures of water bath.

The rate of enzyme activity at different temperatures is determined by using the formula(l/Time).

The higher the rate of enzyme action, the lesser the time taken for enzyme to digest thesubstrate.

The experimental results would show that:./ At the optimum temperature, the rate of enzyme action is at its maximum.

,/ At temperatures below the optimum temperature, the rate of enzyme action is low because

the enzyme is not active.

,/ At temperatures higher than the optimum temperature, the rate of enzyme action is low also

because the enzyme is denatured.

active site

denaturation----------------

normal enzyme denatured enzyme

At high temperatures, the weak hydrogen bonds are broken down due to vigorous vibrations ofthe atoms in the enzyme. This causes the enzyme to lose its specific shape and active site tocarry out chemical reaction. Hence, they are said to be denatured.

They have an optimum temperature at which they work best.

The optimum temperature is usually the body temperature (37'C).

Rate of activity

01020 30 40 s0

Temperature ('C)

60 70

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Sensitivity to pH

. Different enzymes have different optimum pH at which they work best. For example, pepsin inthe stomach works best al pH 2, salivary amylase at pH 7 and trypsin in the small intestineworks best at pH 9.

. Differences in pH changes their surface configuration in the same way as high temperaturedoes.

. Extreme pH may denature enzymes, as they are protein in nature.

Rate of activity

pepsin amylase trypsin

pH

0 1 2 3 4 5 6 7 I I 10 1LL2L374

The effect of pH on enzyme activity can be investigated using the experimental set up shownbelow.

A B C

a mixture of substrateand enzyme

water bath at optimumtemperature

1. ln this experiment, substrate and enzyme used are the same for all testtubes A, B and C.

2. However, the pH value is different for each test tube.(This can be achieved by adding substance of specific pH value toeach test tube. For example, hydrochloric acid to test tube A, distilledwater to test tube B and sodium hydroxide solution to test tube C.)

3. Test reagent is used to test for the presence of the substrate.

4. The presence or absence of the substrate indicates how the activity ofenzyme is affected by pH.

The experimental results would show that:

At the optimum pH, the rate of enzyme action is at its maximum.

At pH below its optimum, the rate of enzyme action is low because the enzyme is notactive.

At pH higher than its optimum, the rate of enzyme action is also low because the enzyme isnot active. [Note: Enzymes are denatured below or above their optimum pH.l

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Substrate specificity. Different enzymes work on different substrates. For example, ptyalin (amylase) breaks down

starch while pepsin breaks down protein.

. Their specific action is due to their surface configuration which is called the active site. Only a

substrate that has a shape complementary to the enzyme can fit and react with it.. This gives rise to the /ock-and-key hypothesis.

/ accepts substrate of,/ specific shapeIactive site

enzyme

,. does not accept substrate

^4ffiffi- whose shape does not fitinto its active site enzyme-substrate

complex

4",/ reaction

UDproducts

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enzyme

Upon substrate binding, the enzyme-substrate complex starts a series of reactions that convertthe substrate into products.

The enzyme molecule is unchanged and free to react with more substrate molecules.

Required in minute amounts

. Because the enzymes remain unchanged after the reaction, they can be reused.

. Hence a small amount of enzymes is sufficient to catalyse many repeated reactions, eventuallyconverting a large amount of substrate molecules to products.

Catalyse reversible reactions

. Enzymes usually catalyse reversible reactions.

. A reversible chemical reaction is a reaction in which the products can change back to theoriginal reactants under suitable conditions.

Forward reaction #Reactants Products

A+B=-C+DProducts

Reverse,":;::t""tt

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Rate of reaction

enzymeconcentration

is limiting

Amount of substrate

lnitially increasing substrate concentration will lead to a corresponding increase in reactionrate i.e. maximum amount of substrate metabolised in a shorter time.

At high substrate concentrations, enzyme concentration becomes the limiting factor.

This is because all the enzyme active sites are occupied by substrate molecules.

Hence, the maximum number of enzyme-substrate complex is reached. As a result, the rateof reaction becomes constant.

By increasing the enzyme concentration, the rate of reaction will be increased further.

OREUISION 2

L. (a) Enzymes are very specific. Explain what is meant by this statement.

(b) What causes the specificity in all enzymes?

(c) The diagram below shows the action of an enzyme on a substrate.

substrate

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enzyme

prod ucts

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(i) What is meant by a substrate?

N

+

ass

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LIMITING FACTOR

. Limiting factor is a factor that directly affects the rate of a chemical reaction.

. Enzyme kinetics is the study of the rates of enzymatic reactions when certain factors, such assubstrate or enzyme concentrations, are changed.

For example

The graph shows the effects of substrate and enzyme concentration on rate of reaction

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(ii) Name regions M and explain why it is described as a lock and key hypothesis.

(iii)

(iv)

What is name of substance N? ...............

Describe how the products are produced at the end of the reaction

(v) What happens to the enzyme at the end of the reaction?

2. Besides the specificity of enzymes, state five other characteristics of enzymes.

3. The diagram below shows how temperature affects the rate of reaction of enzymes.

0

P

temperature/"C

R

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(a) Describe the rate of reaction in the regions P, Q and R.

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(b) Explain why the rate of reaction increases when temperature increases

(c) (D Name the bonds that hold the three-dimensional shape of an enzyme

(ii) Hence, explain why the rate of reaction decrease rapidly after the optimum temperature.

4. The diagram below shows how the pH affects the enzyme activity.

pH

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012345678910

(a) What is the optimum pH for this enzyme shown in this graph?

(b) Explain why there are no reactions below pH 3 and above pH 10.

Iopical Practice Questions 4

1

Section A: Multiple-Choice Questions

Which statement about enzymes is not correct?

A They are proteins.

B Each enzyme is responsible for one type of chemical reaction only.

C They are used up at the end of the chemical reaction.

D They are biological catalyst.

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