Chapter 3 Enzymes

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Other example: washing powder with protease to digest & remove protein dirts

• Enzymes are globular molecules with catalytic properties.

• Until recently it was thought that all biological catalysts were enzymes.

Properties of enzymes: (81-I-1)

1) Enzymes are proteins, thus activity easily affected by temperature & pH

2) Enzymes increases the rate of reaction by lowering the activation energy barrier, thus allowing reactions to proceed without an input of energy

3)Mechanism: forms enzyme-substrate complex to facilitate their interaction and reaction

4)An enzyme only changes the rate at which equilibrium is reached; it does not affect the position of of the equilibrium

5)A small quantity of the enzyme is needed for the reaction because they are unchanged at the end of the reaction

6)Enzymes catalyze reactions which are specific to themselves

Properties of enzymes:

• Abzymes are antibodies with catalytic properties and

• Ribozymes are molecules of RNA which act catalytically on themselves.

3.1 Enzyme structure and function• Enzymes are complex three-dimensional

globular proteins, some might have other associated molecules. Only a small part (called the active site) of the enzyme molecule actually come in contact with the substrate.

3.1.1 Enzymes and activation energy

• Enzymes can lower the activation energy, thus reactions take place at a relatively lower temperature

3.1.2 Mechanism of enzyme action -

a lock and key mechanism:

enzyme + substrate

enzyme-substrate complex

enzyme + products active site may change in order to suit the

substrate's shape for reaction

3.2 Properties of enzymes

3.2.1 Specificity

• All enzymes operate only on specific substrate:

• some enzymes will act only on one particular substrate;

• others act on similar molecules;

• many will break a particular linkage, e.g.

hydrogen bonds

3.2.2 Reversibility

• Enzymes do not alter the equilibrium of a reaction but the speed at which it is reached,

e.g. carbonic anhydrase

CO2 + H2O ———— H2CO3

————

carbonic anhydrase

3.2.3 Enzyme concentration

active sites of an enzyme can be used again & again,

therefore only a low concentration of the enzyme is needed

Turnover number:The number of molecules which an enzyme

can act upon in a given time

3.2.4 Substrate concentration:

The rate increases with an increase in substrate concentration -up to a point,

Then the rate is saturated.

At low substrate concentration, the active sites of the enzyme molecules are not all used. As the substrate concentration is increased, more and more sites come into use and eventually all sites are fully occupied.

Increasing the substrate concentration cannot increase the rate of reaction because substrate concentration has now become a limiting factor.

3.2.5 Temperature

It is the combined effect of kinetic energy (substrate and enzyme molecules) and the denaturation of enzyme molecules.

Optimum temperature for an enzyme varies considerably.

Normally:

optimum at 40°C and denaturation at 60 °C

3.2.6 pH

3.2.6 pH

• The precise three-dimensional molecular shape which is vital to the functioning of enzymes is partly the result of hydrogen bonding;

• H+ ions may break these bonding and change the shape of the molecule.

3.2.7 Inhibition: Competitive inhibition

Competitive inhibitors: inhibitor have structure which can combine with the active site of the enzyme molecule, thus prevent the substrate molecules from combining with the active site;

• They reduces the rate of reaction;• Less inhibition occurs with substrate concentration

increases because the greater the proportion of substrate molecules, the greater their chance of finding the active sites, leaving the fewer to be occupied by the inhibitor.

• e.g. malonic acid competes with succinate for the active site of succinic dehydrogenase.

3.2.7 Inhibition

Non-competitive inhibitors:

Inhibitors attach on the enzyme (not active site) and cause a change in the structure of the active site which now cannot attach to the substrate molecule, e.g. cyanide

increase in substrate concentration cannot reduce the effect of the inhibitor.

Non-reversible inhibitors: • cause the enzyme permanently damaged &

unable to carry out its function, e.g. heavy metallic ions such as Hg2+ and Ag+ cause disulphide bonds to break, thus denaturing the enzymes.

Non-competitive inhibitors:

Inhibitors attach on the enzyme (not active site) and cause a change in the structure of the active site which now cannot attach to the substrate molecule, e.g. cyanide

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3.3 Enzyme cofactors –

activators, coenzymes and prosthetic groups a non-protein substance which is essential for

some enzymes to function efficiently

3.3.1 Activators

assist in forming the enzyme-substrate complex by moulding either the enzyme or the substrate into a more suitable shape,

e.g. salivary amylase needs the presence of Cl- ions before it can catalyse starch into maltose

3.3.2 Coenzymes

• non-protein organic substance essential for the functioning of some enzymes but the molecule is not bound to the enzyme,

e.g. NAD (nicotinamide adenine dinucleotide), a member of vitamin B complex and

acts as a hydrogen acceptor to dehydrogenase in the electron transport system of the respiratory enzymes .

3.3.3 Prosthetic groups

A non-protein organic molecules bound to the enzyme molecule, e.g. haem group of haemoglobin

3.4 The range of enzyme activities

Enzymes are classified according to the type of reaction they catalyse:

1. Oxidoreductase

2. Transferases

3. Hydrolases

4. Lyases

5. Isomerases

6. Ligases

3.4 The range of enzyme activities

Enzymes are classified according to the type of reaction they catalyse:

1. Oxidoreductase:

Transfer O and H atoms between substances in redox reactions, e.g. dehydrogenases and oxidases

3.4 The range of enzyme activities

Enzymes are classified according to the type of reaction they catalyse:

1. Oxidoreductase

2. Transferases:Transfer a chemical group from one substance to another, e.g. transaminases, phosphorylases.

3.4 The range of enzyme activities

Enzymes are classified according to the type of reaction they catalyse:

1. Oxidoreductase

2. Transferases

3. Hydrolases:

Hydrolysis reactions, e.g. peptidases, lipases, amylase, etc.

3.4 The range of enzyme activities

Enzymes are classified according to the type of reaction they catalyse:

1. Oxidoreductase

2. Transferases

3. Hydrolases

4. Lyases:

Addition or removal of a chemical group other than by hydrolysis, e.g. decarboxylases

3.4 The range of enzyme activities

Enzymes are classified according to the type of reaction they catalyse:

1. Oxidoreductase

2. Transferases

3. Hydrolases

4. Lyases

5. Isomerases:Rearrange groups within a molecule, e.g. isomerases

3.4 The range of enzyme activities

Enzymes are classified according to the type of reaction they catalyse:

1. Oxidoreductase

2. Transferases

3. Hydrolases

4. Lyases

5. Isomerases

6. Ligases:

Form bonds between 2 molecules with energy from ATP, e.g. synthetases

3.5 Control of metabolic pathwaysCells contain organelles which have

enzymes bind to their inner membranes;

• Organelles have varying conditions to suit the enzymes they contain

By controlling these conditions, the enzymes available, the cell can control the metabolic pathway within it

Cells can make use of the enzyme’s own properties to control metabolic pathways:

enzyme a enzyme b enzyme c enzyme d

A B C D E

Negative feedback: a high concentration of E reduces its own production rate