ENZYMES

LEARNING GOALS

Describe the role of enzymes and why they are essential to living organisms

List the factors that may affect the rate of an enzyme catalysed reaction

Describe the importance of the structure of an enzyme to its functioning

ENZYMES

Enzymes are molecules that act as catalysts to speed up biological reactions.

The compound on which an enzyme acts is the substrate.

Enzymes can break a single structure into smaller components or join two or more substrate molecules together.

Most enzymes are proteins.

Many fruits contain enzymes that are used in commercial processes. Pineapple (Ananas comosus, right) contains the enzyme papain which is used in meat tenderization processes and also medically as an anti-inflammatory agent.

ENZYMES

ENZYME EXAMPLES

Enzyme Enzyme RoleRole

PepsinPepsin

Stomach enzyme used to Stomach enzyme used to break protein down into break protein down into

peptides. Works at very acidic peptides. Works at very acidic pH (1.5).pH (1.5).

ProteasesProteasesDigestive enzymes which act Digestive enzymes which act on proteins in the digestive on proteins in the digestive

systemsystem

AmylasesAmylasesA family of enzymes which A family of enzymes which assist in the breakdown of assist in the breakdown of

carbohydratescarbohydrates

LipasesLipases A family of enzymes which A family of enzymes which breakdown lipidsbreakdown lipids

3D molecular structures for the enzymes pepsin (top) and hyaluronidase (bottom).

ENZYME EXAMPLES

One of the fastest enzymes in the body is catalase. Catalase breaks down hydrogen peroxide, a waste product of cell metabolism, into water and oxygen. Accumulation of hydrogen peroxide is toxic so this enzyme performs an important job in the body.

ENZYME POWER! All reactants need to have a certain energy

before they will react. This is like an energy barrier that it has to overcome before a reaction will occur. It is called the activation energy.

Enzymes are organic catalysts.

All catalysts lower the energy barrier, allowing the reactants (substrates) to react faster forming the products.

Enzymes do not participate in the reaction.

Reactant

Product

Without enzyme: The activation energy required is high.

With enzyme: The activation energy required is lower.

ENZYMES

High

Low

Start Finish

Direction of reaction

Am

ount

of

ener

gy s

tore

d in

the

ch

emic

als

Low energy

High energy

ENZYMES

Enzymes have a specific region where the substrate binds and where catalysis occurs. This is called the active site.

Enzymes are substrate-specific, although specificity varies from enzyme to enzyme.

When a substrate binds to an enzyme’s active site, an enzyme-substrate complex is formed.

Space filling model of the yeast enzyme hexokinase. Its active site lies in the groove (arrowed)

ENZYME ACTIVE SITES

This model (above) is an enzyme called Ribonuclease S, that breaks up RNA

molecules. It has three active sites (arrowed).

Active site:

The active site contains both binding

and catalytic regions. The substrate

is drawn to the enzyme’s surface and

the substrate molecule(s) are

positioned in a way to promote a

reaction: either joining two molecules

together or splitting up a larger one.Enzyme molecule:

The complexity of the

active site is what makes

each enzyme so specific

(i.e. precise in terms of the

substrate it acts on).

Substrate molecule:

Substrate molecules are the

chemicals that an enzyme

acts on. They are drawn into

the cleft of the enzyme.

LOCK AND KEY MODELThe lock and key model of enzyme action, proposed earlier this century, proposed that the substrate was simply drawn into a closely matching cleft on the enzyme molecule.

Substrate

Enzyme

Products

Symbolic representation of the lock and key model of enzyme action.1. A substrate is drawn into the active sites of the enzyme.

2. The substrate shape must be compatible with the enzymes active site in order to fit and be reacted upon.

3. The enzyme modifies the substrate. In this instance the substrate is broken down, releasing two products.

INDUCED FIT MODEL

More recent studies have revealed that the process is much more likely to involve an induced fit.

The enzyme or the reactants (substrate) change their shape slightly.

The reactants become bound to enzymes by weak chemical bonds.

This binding can weaken bonds within the reactants themselves, allowing the reaction to proceed more readily.

The enzyme changes shape, forcing the substrate molecules to combine.

Two substrate molecules are drawn into the cleft of the enzyme.

The resulting end product is released by the enzyme which returns to its normal shape, ready to undergo more reactions.

CHANGING THE ACTIVE SITE Changes to the shape of the active site will result

in a loss of function. Enzymes are sensitive to various factors such as temperature & pH.

When an enzyme has lost its characteristic 3D shape, it is said to be denatured. Some enzymes can regain their shape while in others, the changes are irreversible.

THE EFFECT OF TEMPERATURE ON ENZYME ACTION

Speeds up all reactions, but the rate of denaturation of enzymes also increases at higher temperatures.

High temperatures break the disulphide bonds holding the tertiary structure of the enzyme together thus changing the shape of the enzyme.

This destroys the active sites & therefore makes the enzyme non – functional.

Too cold for

Enzyme to work

Too hot for

Enzyme to work

Optimum Temperatur

e for enzyme

THE EFFECT OF TEMPERATURE ON ENZYME ACTION

The curve in the blue represents an enzyme isolated from an organism living in the artic. These cold dwelling organisms are called psychrophiles.

The curve in red represents an enzyme isolated from the digestive tract of humans.

The curve in green represents an enzyme isolated from a thermophile bacteria found growing in geothermal sea beds.

THE EFFECT OF PH ON ENZYME ACTION

Like all proteins, enzymes are denatured by extremes of pH (acidity/alkalinity).

The green curve is for pepsin that digests proteins in the stomach.

The red curve represents the activity of arginase that breaks down arginine to ornithine & urea in the liver.

THE EFFECT OF ENZYME CONCENTRATION ON ENZYME ACTION

Assuming that the amount of substrate is not limiting, an increase in enzyme concentration causes an increase in the reaction rate.

THE EFFECT OF SUBSTRATE CONCENTRATION ON ENZYME ACTION

Assuming that the amount of enzyme is constant, an increase in substrate concentration causes a diminishing increase in the reaction rate.

A maximum rate is obtained at a certain concentration of substrate when all enzymes are occupied substrate (the rate cannot increase any further).

THE EFFECT OF COFACTORS ON ENZYME ACTION

Cofactors are substances that are essential to the catalytic activity of some enzymes.

Cofactors may alter the shape of enzymes slightly to make the active sites functional or to complete the reactive site.

Enzyme cofactors include coenzymes (organic molecules) or activating ions (eg. Na+, K+..)

Vitamins are often coenzymes (eg. Vit B1, Vit B6…)

THE NATURE OF ENZYME INHIBITORS

Enzyme inhibitors may or may not act reversibly:

Reversible: the inhibitor is temporarily bound to the enzyme, thereby preventing its function (used as a mechanism to control enzyme activity).

Irreversible: the inhibitor may bind permanently to the enzyme causing it to be permanently deactivated.

THE NATURE OF ENZYME INHIBITORS

Reversible Enzymes work in one of two ways:

Competitive inhibitors: the inhibitor competes with the substrate for the active site, thereby blocking it and preventing attachment of the substrate.

Non-competitive: the inhibitor binds to the enzyme (but not at the active site) and alters its shape. It markedly slows down the reaction rate by making the enzyme less able to perform its function (allosteric inhibition).

SUMMARY: ENZYMES

1. Enzymes work very rapidly and help to speed up biological reactions.

2. Enzymes can be used multiple times (however they do degrade eventually).

3. Enzymes can work in both directions of a chemical reaction.

4. Enzymes have optimal temperatures and pH that they will operate. Beyond these optimum ranges they will either not work or become denatured (unfolded/breakdown).

5. Enzymes are usually specific to one particular substrate.