Biology Form 4 Chapter 4.5 Enzymes

7/16/2019 Biology Form 4 Chapter 4.5 Enzymes

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Biology4.5 ENZYMES



Enzymes are proteins which act as biological catalysts. They speed up

biochemical reactions in the cell.

The substance whose reactivity is increased by an enzyme is known

as substrate.

Example:

substrate —enzyme –> products

sucrose + water —sucrase –> glucose + fructose

Thousands of simultaneous biochemical reactions occur in living

cells. Without enzymes, these biochemical reactions would be too

slow to sustain life.



General characteristics of enzymes

1. Enzymes work very rapidly

One molecule of enzyme can turn thousands or millions of substrate

molecules into products per minute. For example, catalyse can

transform approximately six million hydrogen peroxide molecules

into oxygen and water molecules per minute.

2. Enzymes are not destroyed by the reactions which that catalyse

Since enzymes are not altered by the reactions they catalysed, they

can be used again. A smaill concentration of enzymes can bring

about a large amount of biochemical reactions

3. Enzyme-catalysed reactions are reversible

lactose + water —lactase –> glucose + galactose

lactose + water < –lactase— glucose + galactose

The enzyme which catalyses a reaction works in such a way that the

reaction can proceed from left to right or from right to left,

depending on circumstances. Note the two way arrows.



4. Enzymes are extremely specific

Most enzymes are specific to one particular substrate molecule.Thus, a given enzyme will catalyse only one reaction or one type of

reaction. Maltase, for example, acts only on maltose.

5. Enzymes are denatured by high temperature

An enzyme inactive at very low temperature. As temperature rises,its activity increases until the optimum temperature is reached. The

optimum temperature is around 40′ C. Above the optimum

temperature, the rate of reaction decline rapidly, ceasing altogether

at about 60′ C. This is because enzymes are made of protein, so they

are denatured at high temperature. When an enzyme becomes

denatured, the bonds are broken and the polypeptide chains open

up. The enzyme loses its normal shape and becomes inactive.

6. Enzymes are sensitive to pH

Every enzymes has its own optimum pH in which it functions best.

Small changes in the pH of the medium will denature the enzyme

and render its activity. Alterations in the ionic charges of the acidicand basic groups of the enzyme change the shape of the enzyme.



Naming of enzymes based on the substrate

An enzyme is named by attaching the suffix -ase to the name of

the substrate on which it acts. For example, maltase acts on

maltose, sucrase on sucrose and cellulase on cellulose.

The ‘-ase‘ rule does not apply to enzymes discovered before

the ‘-ase‘ idea was introduced. For example, pepsin rennin,

ptyalin and trypsin.

Intracellular and extracellular enzymes

Enzymes can be divided into two groups: intracellular and

extracellular.

Enzymes formed and retained in the cell are known as

intracellular enzymes, and occur in the cytoplasm, organelles or

the nucleus. Examples of intracellular enzyme are DNA

polymerase, RNA polymerase and ATP synthetase.

Extracellular enzymes are produced in the cell then packed and

secreted from the cell, Extracellular enzymes caralyse their

reactions outside the cell. Most digestive enzymes are

extracellular enzymes. For example, amylase, cellulase and

zymase.



Site of Enzyme Synthesis

Since enzymes are made of proteins, they are synthesised by

ribosomes.

Intracellular enzymes are synthesised on ‘free’ ribosomes.

Extracellular enzymes are synthesised on ribosomes attached

to the endoplasmic reticulum.



Formation and secretion of extracellular enzymes:

1. The instruction for making the extracellular enzyme is

transcribes from deoxyribonucleic acid (DNA) to ribonucleic

acid (RNA) in the nucleus.

2. The RNA then leaves the nucleus through the nuclear pore and

attaches itself to the ribosome located on the endoplasmic

reticulum.

3. When the enzyme synthesis has completed, it is extruded into

the interior of the endoplasmic reticulum.

4. The enzyme is then encapsulated in a transport vesicle.

5. The transport vesicle fuses with the Golgi apparatus, releasing

the enzyme into the Golgi apparatus.

6. In the Golgi apparatus the enzyme is further modified before

packing the enzyme in a secretory vesicle.

7. The secretory vesicle transports the enzyme to the plasma

membrane.

8. The secretory vesicle membrane fuses with the plasma

membrane and the enzyme is release outside the cell.



Mechanism of enzyme action

Each enzyme molecule has a region with very precise shape

called the active site.

The substrate molecule fits into the active site of the enzyme

like a key into a lock.

Various types of bonds including hydrogen bonds and ionic

bonds hold the substrate(s) in the active site to form a enzyme-

substrate complex.

The enzyme then changes the substrate(s) either by splitting it

apart (for example, hydrolysis) or linking them together (for

example, condensation)

Once formed, the products no longer fit into the active site and

escape into the surrounding medium, leaving the active site

free to receive further substrate molecules.

enzyme+substrate —enzyme-substrate complex –> enzyme+product

The explanation of enzyme action is known as the ‘lock and key

hypothesis’, where the substrate is like a key whose shape is

complementary to the enzyme or lock.

The ‘lock and key’ hypothesis is able to explain why enzymes

are specific and why any change in enzyme shape alters itseffectiveness.



Factors afftecting enzymes

1. pH

Most enzymes are effective in only a narrow pH range.

The optimum pH is the particular pH at which the rate of

reaction is the highest.

Deviations from the optimum pH decrease the rate of reaction

because bonds maintaining the tertiary shape of the enzyme

are broken.

The active site loses its shape and the enzyme-substrate

complex can no longer be formed. The enzyme is denatured.

2. Temperature

Initially an increase in temperature leads to an increase in the

rate of reaction because the kinetic energy of the enzyme and

substrate molecules produce more collisions, and therefore

more enzyme-substrate complexes are formed.

The rate of reaction will increase up to a maximum, known as

the optimum temperature.

After the optimum temperature, the rate of reaction falls

quickly because the bonds maintaining the structure of the

enzyme start to break and the active site loses its shape.

The enzyme-substrate complexes can no longer form and the

enzyme is denatured.



Substrate Concentration

Initially an increase in substrate concentration increases the

chance of enzyme-substrate collisions, and the rate of reaction

increases.

Eventually all the active sites are filled at any one time and the

rate remains constant The reaction has reached its maximum

rate, Vmax.

Further addition of substrate will not increase the rate of

reaction anymore because the constant enzyme concentrationbecomes the limiting factor.



4. Enzyme Concentration

As the concentration of the enzyme increases there are more

chances of enzyme-substrate collisions. The rate of reaction

increases linearly as long as no other factors are limiting.

As more active sites are available, more substrates can be

converted to products.

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Biology Form 4 Chapter 4.5 Enzymes