Chapter 5: Enzymes

Enzymes are Biological Catalysts

What are enzymes?

Biological catalysts made up of protein which alter rate of chemical reaction without themselves being

chemically changed at the end of reactions

Catalysts : Substance that increase rate of reaction without being used up.

Enzymes are Biological Catalysts Enzymes are made up of protein Denatures (enzyme loses it shape and

function) if exposed to high temperature or extreme changes in pH

Speeds up reaction without the need to increase temperature.

Mode of actions of enzymes

Body cannot withstand high temperatures Energy is required to start breaking down

fats – activation energy Enzymes lower activation energy >

Required to start chemical reaction.

Enzymes-catalysed Reactions1. Digestion Process whereby large insoluble food

substances are broken down into small soluble molecules

Carbohydrates MonosaccharidesProtein Amino Acids

Fats Fatty Acids + Glycerol

Enzymes-catalysed Reactions2. Synthesis of complex substances from

simpler ones. Synthesis of proteins Synthesis of glycogen for storage in the

body Synthesis of fats

Enzymes-catalysed Reactions3. Oxidation of Glucose Release of Energy to do work Involves a series of enzyme catalysed

reactions

4. Breakdown of toxic materials Hydrogen peroxide to water and oxygen

catalysed by catalase.Enzymes catalyse almost all reactions in body.

There are many different types of enzymes and eachis specific and will be produced only when required.

Classifying Enzyme• According to chemical reaction they catalyse• Usually ends up with –ase• Example:

• Breaking down of starch = amylase• Breaking down of Protein = Protease• Breaking down of Lipids/Fats = Lipase

• Enzyme combine with molecules it acts upon (substrate) to form an enzyme-substrate complex.

• Enzyme controlled reaction can be either• Anabolic : building up larger molecules• Catabolic: breaking down large molecules into smaller molecule

Characteristics of Enzymes• Speeds up rate of chemical reactions• Required in minute amount (small) – (itself not

breakdown)• Specific (Lock & Key hypothesis)

Lock & Key Hypothesis• Substrate which the enzyme acts on is called substrate• Example: Breaking down of starch, amylase is the

enzyme and starch is substrate. End-product = Maltose

• Enzyme molecules are usually larger than substrate• Small part of enzyme, called active site comes into

contact with substrate• Enzyme (lock) and substrate(key)

Enzyme(lock) Substrate (Key) Product

Amylase Starch Maltose

Lipase Lipid Fatty acids + glycerol

Protease Protein Amino acids

amylase

Active site

starch

Enzyme substrate complex

amylase 2 maltose

units

Enzyme are affected by temperature

• Enzyme and substrate in constant motion• Increase temperature results in increased kinetic

energy• Molecules move faster• Rate of enzyme colliding with substrate is higher• Rate of enzyme-substrate complex formed higher• Only up to an optimum temperature• Low temperature = enzymes are inactive

Enzyme are affected by temperature

• Around 37 ℃ in mammals• Too high a temperature will result in protein being

denatured• Active sites change shape and activity gradually decline• Different enzymes can withstand different temperature

range• Most enzymes will be completely denatured above 60

℃• Molecules move slower and rate of collision lower

Q: How does temperature affect Enzyme activity

What happens at low temperature?Enzyme are inactive at low temperature

What happens as temperature increase?As temperature increase, enzymes gain more kinetic energy causing them to move faster. When they move faster, chances of colliding into a substrate is higher. Hence, rate of forming enzyme-substrate complex is higher. Rate of reaction increases

What happens at optimum temperature?At optimum temperature, enzyme are most active

What happens above optimum temperature?Enzymes loses active sites, denatured.

0 ℃ 50 ℃ 100 ℃

Activeness of enzymes

Enzymes are affected by temperature:

Enzyme are affected by pH

• Enzyme usually work efficiently within a narrow pH range.

• Some enzymes works best in slightly acidic conditions (pepsin and rennin) while others in slightly alkaline conditions.

• Extreme pH will result in denaturation of enzymes

LowLow HighHigh

TemperatureTemperature InactiveInactive DenaturedDenatured

pH valuepH value DenaturedDenatured denatureddenatured

pH 1 pH 7 pH 14

Activeness of enzymes

Enzymes are affected by pH:

Enzyme are affected by Enzyme concentration

• Increase enzyme concentration, increased rate of reaction

• Temperature and pH are at optimum values• Substrate level must be high.

Enzyme are affected by substrate concentration

• Concentration of enzyme remains the same• Increased substrate concentration doesn’t necessarily

increase rate of reaction• Active sites become saturated, therefore could not work

any faster

5

4

3

2

1

42 86 10

No. of patties(Rate ofReaction)

Number of buns(substrate)

For example, you are making burgers with buns and the number of patties you have is limited.

Buns = Substrate, Patties = Rate of reaction

Enzyme are affected by substrate concentration

• How do you know that substance is not enough?• When I increase number of substance, I can produce

more burgers (Also: When I increase number of that substance, there is an increase of rate of reaction)

Limiting Factors

• Any factor that directly affects the rate of reaction if its quantity is changed

• Value of this factor has to be increased in order to increase rate of reaction

Reversible Reactions

• Most reactions in living cells are reversible.

• Enzyme can catalyse reversible reactions• Usually not allowed as products formed are used up or

transported away.

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