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Nazabayev University UPCSE

Enzymes

Enzymes Pork Chop Willie11/19/2013 3:47 AM



Enzymes

Most are Globular proteins that act as

biological catalysts

Speed up chemical reactions that wouldoccur very slowly at the temperature

within cells.

Precise three dimensional shape

adopted by the enzyme includes a

depression on the surface called the

active site

2Enzymes Pork Chop Willie11/19/2013 3:47 AM



Active site

May be a relatively small part of the

large protein molecule.

Only a few amino acids may be directlyinvolved in the active site.

The remainder maintain the three

dimensional shape of the molecule




Active site




Lock and key theory

Either a single molecule with a

complementary shape or more than one

molecule that together have acomplementary shape, can fit into the

active site







Lock and Key theory

Substrate molecules form temporary

bonds with the amino acids of the active

site to produce an enzyme substratecomplex

The enzyme holds the substrate

molecules in such a way that they react

more easily.

When the reaction has taken place the

products are released




Lock and key Theory

The substrate is the ‘key’

The enzyme is the ‘lock’

Each enzyme will only catalyse onespecific reaction because only one

shape of substrate fits into its precisely-

shaped active site




Lock and Key Theory




Induced fit theory

It has been found that the active site is

often flexible

When the substrate enters the active sitethe enzyme molecule changes shape

slightly, fitting more closely around the

substrate




Induced fit theory




Activation Energy

To convert substrates into products

bonds must change both within and

between molecules. Breaking chemical bonds requires

energy whilst energy is released when

bonds are formed.

The energy required to break bonds and

start a reaction is known as the

activation energy




Activation Energy

Without an enzyme heating a substrate would

provide the energy

Heat energy agitates the atoms within themolecules and they become unstable and the

reaction can proceed.

In cells enzymes reduce the amount of energy

needed; this allows the reactions to go ahead

without raising the temperature




Activation energy

Enzymes Pork Chop Willie 1411/19/2013 3:47 AM



How do enzymes reduce the

activation energy? The specific shape of the enzymes

active site and its complementary

substrates is such that the electricallycharged groups on their surface interact.

The attraction of oppositely charged

groups may distort the shape of the

substrates and assist in the breaking ofbonds or the formation of new bonds




How important are enzymes?

Enzymes present in all organisms

Catalyse a huge range of reactions

Speed up reactions at least a milliontimes.

Most biological reactions would not

happen at all without enzymes




Metabolism

Metabolism of an organism is the sum of all

the enzyme-catalysed reactions occurring

within it. Some reactions are simple e.g. hydration of

carbon dioxide by carbonic anhydrase

CO2 + H2O H2CO3

This reaction allows CO2 to be transported in

the blood from respiring tissues to alveoli




Metabolism

Catabolic reactions

Large molecules broken down to smaller

onesHydrolysis of Starch to Maltose by Salivary

Amylase.

Anabolic Reactions

Fatty acid synthetase synthesises fatty acids

within cells




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REGULATING ENZYMES: FEEDBACK INHIBITION

Enzymes often function in

a metabolic pathway,

The products of one

reaction become the

reactants (substrate) forthe next reaction.

In feedback inhibition, the

final product of the

metabolic pathwayinhibits an earlier reaction



Finding the rates of enzyme

controlled reactions The rate of reaction is found by

determining the quantity of substrate

used or the quantity of product formed ina given time.

e.g. Catalase used to break down

Hydrogen peroxide (H2O2) to water and

oxygen

Can measure the volume of oxygen

given off in a known time




Quantity of product measured over time to determine the

progress of the enzyme reaction

Volume of Oxygen produced is measured when catalase is

added to hydrogen peroxide




How do enzyme and substrate

concentrations affect the rate

of the reaction




(A) enzyme concentration

The initial rate of reaction is directly

proportional to the enzyme concentration

The more enzyme is present the greaternumber of active sites available

Assuming that there is an excess of

substrate




(B) Substrate concentration

The enzyme concentration limits the rate

of the reaction.

Every active site is occupied Substrate molecules cannot enter an

active site until one becomes free again




Effect of temperature on

enzyme reactions





enzymes As the temperature rises, reacting

molecules have more and more kinetic

energy. This increases the chances of a

successful collision and so the rate

increases.

There is a certain temperature at whichan enzyme's catalytic activity is at its

greatest. (optimal temperature)





enzymes Above the optimum temperature the

enzyme structure begins to break down

(denature) since at higher temperaturesintra- and intermolecular bonds are

broken as the enzyme molecules gain

even more kinetic energy.




Effect of pH on enzyme

reactions




Effect of pH on enzyme

reactions Each enzyme works within quite a small

pH range.

There is a pH at which its activity isgreatest (optimal pH).

This is because changes in pH can

make and break intra- and

intermolecular bonds, changing theshape of the enzyme and, therefore, its

effectiveness.




Non-Competitive Inhibitors

An allosteric site is a location on an enzyme where

a regulating molecule can attach.

This is non competitive inhibition

(inhibitor is not competing with the substrate for

binding to the active site).

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Competitive Inhibitors

Competitive inhibition involves a molecule that is

not the substrate molecule but that can bind with

an enzyme's active site.

If this non substrate molecule occupies the active

site, then there is no room for the substrate to bindat that site.




Permanent and Non-permanent

Inhibitors

Most competitive inhibitors do not bind

permanently to the active site.

They bind to the active site then leave. They are described as reversible as

removal of the in inhibitor leaves the

enzyme unaffected




Permanent and Non-permanent

Inhibitors

Many non-competitive inhibitor bind

permanently to enzyme molecules.

The inhibitor is not reversibly (irreversible) Any enzyme molecules bound to an

inhibitor are effectively denatured

Inhibition is not always a bad thing as a

number of metabolic pathways involves

inhibition to control reaction rates





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