DRM- Biology S4 2011 1

E N Z Y M E SE N Z Y M E S

What are they?What are they?What do they do?What do they do?How do they work?How do they work?What can affect how an enzyme works?What can affect how an enzyme works?

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DEFINITIONDEFINITION

An enzyme is a biological catalyst.An enzyme is a biological catalyst.

This means it is a This means it is a proteinprotein (biological (biological

molecule) molecule) that speeds up that speeds up

chemicalchemical reactionsreactions (a catalyst). (a catalyst).

A substrate is the molecule on which A substrate is the molecule on which

an enzyme acts.an enzyme acts.

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WHAT ENZYMES DOWHAT ENZYMES DO Enzymes act in chemical reactions that Enzymes act in chemical reactions that

build upbuild up (join simple molecules to make a (join simple molecules to make a bigger, complex compound) or bigger, complex compound) or break break downdown (digest) big complex molecules to (digest) big complex molecules to small simple substances.small simple substances.

ExamplesExamples:: Amino acids are joined to make proteins Amino acids are joined to make proteins

(building up or (building up or anabolic reactionsanabolic reactions)) Starch is broken down to glucose Starch is broken down to glucose (breaking (breaking

down or down or catabolic reactionscatabolic reactions))

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SOME TERMS TO REMEMBERSOME TERMS TO REMEMBER

SUBSTRATESUBSTRATE: the molecule upon which an enzyme acts.: the molecule upon which an enzyme acts.ExampleExample: The enzyme amylase breaks down starch. So starch is the : The enzyme amylase breaks down starch. So starch is the

substrate for amylase. substrate for amylase.

ACTIVE SITEACTIVE SITE (of the enzyme) part of the enzyme that binds (of the enzyme) part of the enzyme that binds to the substrate during a chemical reaction. to the substrate during a chemical reaction.

PRODUCTPRODUCT: the molecule obtained as a result of the : the molecule obtained as a result of the enzyme’s action. enzyme’s action.

RATE OF REACTIONRATE OF REACTION: how an enzymes works. It is : how an enzymes works. It is measured by recording either the disappearance of measured by recording either the disappearance of substrate or the appearance of product. In other words, it substrate or the appearance of product. In other words, it records the changes in concentration of either the substrate records the changes in concentration of either the substrate or the product as the reaction proceeds. or the product as the reaction proceeds.

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PROPERTIES OF ENZYMESPROPERTIES OF ENZYMES Enzymes are Enzymes are specificspecific. They will react only with a one . They will react only with a one

given type of substrate. given type of substrate. ExampleExample: an enzyme that : an enzyme that breaks down proteins will not break down fats.breaks down proteins will not break down fats.

Enzymes are affected by extreme temperatures and Enzymes are affected by extreme temperatures and pH. pH. Each enzyme has an optimum pH and Each enzyme has an optimum pH and temperaturetemperature at which it works best. at which it works best.

Enzymes are Enzymes are notnot affected by the chemical reaction affected by the chemical reaction they help take place. they help take place. This means that the same enzyme This means that the same enzyme can be reused again, and that a small number of enzymes can be reused again, and that a small number of enzymes can carry out a great number of chemical reactions to can carry out a great number of chemical reactions to produce a lot of products.produce a lot of products.

Enzymes are affected by poisons, such as toxins and Enzymes are affected by poisons, such as toxins and heavy metals, and by high concentrations of salts. heavy metals, and by high concentrations of salts.

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HOW ENZYMES WORKHOW ENZYMES WORK

A) lock and key modelA) lock and key model: the substrate has a similar : the substrate has a similar shape to the one in the active site – an enzyme-shape to the one in the active site – an enzyme-substrate complex forms – a product is obtained substrate complex forms – a product is obtained and the unchanged enzyme is free to start again.and the unchanged enzyme is free to start again.

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B) induced fit modelB) induced fit modelThe substrate does not have a similar shape to the The substrate does not have a similar shape to the

active site in the enzyme; instead, when the active site in the enzyme; instead, when the enzyme approaches the substrate, the enzyme’s enzyme approaches the substrate, the enzyme’s shape changes so that the substrate can now shape changes so that the substrate can now “fit” in the active site.“fit” in the active site.

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FACTORS AFFECTING ENZYME-FACTORS AFFECTING ENZYME-CONTROLLED REACTIONSCONTROLLED REACTIONS

TemperatureTemperature

pHpH

Concentration of substrate

Concentration of enzymes

Cofactors and coenzymes (these molecules increase the rate of reaction if present, they are necessary for the enzyme to work; e.g. vitamins)

Inhibitors (these molecules slow down or prevent the enzyme from working; e.g. poisons, antibiotics)

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HOW HOW TEMPERATURE TEMPERATURE

AFFECTS AFFECTS ENZYMESENZYMES

At low temperaturesAt low temperatures, enzymes , enzymes are inactive or work very slowly.are inactive or work very slowly.

As the temperature increases, As the temperature increases, enzymes become more active enzymes become more active and the rate of reaction and the rate of reaction increases until increases until the optimum the optimum temperaturetemperature, when the highest , when the highest rate of reaction is achieved.rate of reaction is achieved.

At higher temperaturesAt higher temperatures, the , the enzymes are denatured enzymes are denatured (destroyed) so they can no (destroyed) so they can no longer bind to the substrate and longer bind to the substrate and no reaction takes place.no reaction takes place.

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HOW pH HOW pH AFFECTS AFFECTS ENZYMESENZYMES

Very high or Very high or very low pH very low pH affects how affects how enzymes work, enzymes work, as extreme pH as extreme pH will denature will denature the enzyme.the enzyme.

Remember Remember each type of each type of enzyme has its enzyme has its own optimum own optimum pHpH at which it at which it works best. works best.

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pH and temperature of different pH and temperature of different enzymesenzymes