Enzyme Action Lock and Key Model Induced Fit Model
Names of Enzymes General Effects of Enzymes Inhibitors
Temperature and Enzymes pH and Enzymes
Enzyme Concentration Substrate Concentration
Immobilised Enzymes Types of
Immobilisation
Advantages of Immobilisation Uses of Immobilised Enzymes
A catalyst is a substance
that speeds up a chemical
(metabolic) reaction. The catalyst itself is
not used up as a result of its actions. Proteins that function as biological
catalysts are called enzymes. They
are composed of C, H, O and N. Sulphur (S) may also be present
As we saw in
the nutrition and
food webpage the function of proteins is determined by their amino acid
composition as well as their shape.
Enzymes
control cellular reactions. As you remember, reactions that break down
substances and release energy are
called catabolic reactions. Examples
are respiration and digestion. The other types of reactions
are called anabolic reactions. These
reactions consume (use) energy.
These reactions build larger, more complex, molecules from smaller ones. Photosynthesis and muscle growth from amino acids are examples of anabolic reactions.
Enzymes are folded in GLOBULAR
SHAPES. The enzyme’s
shape enables it to receive only one type of molecule; that molecule that will
fit into it’s shape. The place where the substance fits into the enzyme is
called the active site and the
substance that fits into the active site is called the substrate.
Enzyme action occurs when
the enzyme and substrate collide. During the collision the substrate slots into
the active site of the enzyme. Collisions happen because of the rapid random
movement of molecules.
When the
substrate joins with the enzyme the entire structure is called the enzyme-substrate complex. The substrate
becomes changed by the enzyme’s
action and is then releases as the product.
The enzyme is then free to join another substrate.
Enzymes can
be either anabolic or catabolic. The same enzyme can be used to form smaller
molecules from a larger molecule or to do the opposite.
An example of a
catabolic enzyme is amylase. Amylase
converts starch into maltose.
An example of an anabolic enzyme is DNA polymerase. This enzyme repairs
(rebuilds) DNA.
TYPES OF ENZYME ACTION
As stated previously, the
substrate must fit into the enzyme at the active site. Some substrates fit
nicely into the active site. This situation is called the Lock and Key Model.
The enzyme is a complex protein
molecule, but there is a particular site where the reactant molecule 'docks
in' by random collision. The enzyme is sometimes referred to as the 'lock'
and the initial reactant substrate molecule as the 'key', hence this is
called the ’lock and key’ mechanism.
This is also explains why enzymes are
very specific (Enzyme Specificity). You need the right molecular
key for a particular molecular lock.
Even when
different substrate molecules are present, only those that have the specific
shape complementary to the active site are able to bind with the enzyme's
active site.
Sometimes the shape of the
active site must be slightly changed. This situation is called the Induced Fit Model.
The
enzyme’s active site has a shape closely complementary to the substrate The
substrate locks into the active site of the enzyme. The active site alters its
shape holding the substrate more tightly and straining it. An enzyme-substrate
complex is formed. The substrate undergoes a chemical change and a new
substance, product, is formed. The product is released from the active site.
The free unaltered active site is ready to receive a fresh substrate.
(press the reload button
to view this animation)
Enzymes are named by their substrate. The letters ase are added to the substrates name.
Examples are:
lactase
– breaks down lactose (milk sugars)
diastase
– digests vegetable starch
sucrase
– digests complex sugars and starches
maltase
– digests disaccharides to monosaccharides (malt sugars)
glucoamylase
– breaks down starch to glucose
protease – breaks down proteins found in meats,
nuts, eggs, and cheese
lipase – breaks down fats found in most dairy
products, nuts, oils, and meat
cellulase – breaks down cellulose, plant fibre; not
found in humans
- Enzymes lower the activation energy. This is the energy input needed to bring
about the reaction. Enzymes enable the reaction to occur with less energy
than would be needed if the enzyme were not present.
2. Regulate the thousands
of different metabolic reactions in a cell and in the organism.
- The activity of a cell is determined by which
enzymes are active in the cell at that time.
- Cell activity is altered by removing specific
enzymes and/or synthesising new enzymes.
Enzyme inhibitors are
molecules that interact in some way with the enzyme to prevent it from working
in the normal manner. Poisons and drugs are examples of enzyme inhibitors.
Inhibitors change the shape of the enzyme and make it nonuasable to a
substrate. Inhibitors can also act as a substrate and bind to the enzyme. This
prevents the enzyme from binding with its intended substrate. When this happens
the enzyme is said to be denatured.
FACTORS THAT AFFECT ENZYME ACTIVITY
TEMPERATURE
At 0°C
enzyme action is low because the movement of molecules is low. This causes the
collision frequency between enzyme and substrate to be low. Increasing the
temperature speed up the movement of molecules and thus the collision frequency
increases therefore enzyme action increases. Human bio enzymes work best at 37
degrees Celsius. As the temperature raises the shape of the enzyme changes and
the enzyme becomes denatured.
Temperature above 50 degrees Celsius will denature most human enzymes.
Most enzymes work best at
a pH of 6-8. When the pH is outside this range the enzyme will lose its shape and become denatured. The ideal (optimum) pH for most enzymes is 7.
(Press the reload button to see this animation again.)
Some enzymes work best at other pH
levels. The following graphs demonstrate this: 
As
the concentration of an enzyme increases the rate of reaction also increases,
provided that the substrate is in excess.
At low concentration of substrate an increase in
concentration will cause an increase in the rate of reaction. However, once the
concentration is such that all the active sites of the enzyme are constantly in
use then further increase in substrate concentration will have no effect on the
rate of reaction.
Immobilized enzymes are enzymes which may be
attached to each other, to insoluble materials, or enclosed in a membrane or
gel. This can provide increased resistance to changes in conditions such as pH or temperature.
It also allows enzymes to be held in place throughout the reaction, following
which they are easily separated from the products and may be used again.
Immobilised enzymes
are used in bioreactors .These procedures are used to produce many products
which used to use micro-organisms. See the bacteria
webpage for a discussion of bioreactors.
Adsorption:
In this method the enzyme is attached to a support. Supports can be ceramics,
glass, or plastics.
Membrane Enclosure: In this method the enzyme is
enclosed in a porous membrane.
Gel entrapment: The enzymes are held in a gel.
Sodium alginate is a common gel used. The gel allows the substrate to enter and
the product to leave.
Chemically
bonded to a support or to each other: See textbook page 95.
- It makes for easier purification of the
product as the separation of the enzymes from the products is easily
accomplished.
- It is easy to recover and recycle the enzymes.
This leads to a more economical process.
- The enzymes remain functional for much longer
as it is a gentler process.
USES OF IMMOBILISED ENZYMES
The following products
are derived from immobilised enzyme action:
- Fructose derived from glucose: Fructose is sweeter
than glucose and is used in soft drinks and other sweet products.
- Antibiotics: Enzymes are used to change penicillin into new, wider
used, antibiotics.
- Sewage
Treatment:
Instead of bacteria enzymes can be immobilised and used.