ENZYMES

These are biological catalysts implying that they are substances which speed up chemical
reactions but of biological origin. They are proteins by nature and function at temperature ranges
suitable for living cells. Enzymes function by lowering the activation energy of reactions. They
are involved in both anabolic and catabolic reactions in cells. The substance of chemical which
an enzyme works on is referred to as the substrate. Once an enzyme combines with its substrate,
an enzyme-substrate complex is formed. This formation increases the reaction rate which
subsequently leads to product formation according to the following reaction:
Properties of Enzymes
1. All enzymes are globular proteins.
2. As proteins they are coded for by the DNA.
3. They do not alter the nature or properties of the products.
4. They remain unchanged in nature at the end of a reaction.
5. They are very effective. Little amounts produce a large amount of products.
6. They are highly specific in reaction.
7. They are reversible
8. They function by reducing the activation energy of the reaction.
9. They possess active sites.
10. They are affected by pH, temperature, substrate concentration and enzyme concentration.

Activation energy
This is the energy required to make substances react or the energy required to start a reaction.
Enzymes serve to reduce the activation energy required for a chemical reaction to take place.
They speed up the overall rate without altering to any great extent, the temperature at which the
reaction occurs. According to the diagram.
Mechanism of Enzyme Reaction
Enzymes have their particular shapes which is fitted into by their respective substrates. This is
best described using the Fischer’s lock and key hypothesis. In this hypothesis, the substrate is
imagined being like a key whose shape is complementary to the enzyme or lock, the particular
site on the enzyme where the substrate binds is called the active site and it has a specific shape.

Factors affecting the rate of enzyme reactions.

The rate of an enzyme reaction is measured by the amount of substrate disappearance or the
amount of product formation during a period of time.
Factors which affect the rate of enzyme reaction include changes in temperature, pH, substrate
and enzyme concentrations and other molecules which bind to the enzyme.
Enzyme Concentration: Provided the substrate concentration is maintained at a high level and
other conditions such as pH and temperature are kept constant, the rate of reaction is directly
proportional to the enzyme concentration. Normally, reactions are catalyzed by enzymes with
concentrations much lower than the substrate concentration. Hence as enzyme concentration
increases so will the rate of reaction according to the illustration below:

Substrate Concentration: for a given enzyme concentration, the rate of reaction increases with
increasing substrate concentration. The maximum velocity (Vmax) is never quite obtained, but
there comes a point when further increase in substrate concentration produces no significant
change in reaction rate. This is because at high substrate concentration, the active sites of the
enzyme molecules at any given moment are virtually saturated with substrate. Thus any extra
substrate has to wait until the ES (Enzyme/Substrate) complex has released the products before
substrates can enter the active site of the enzyme.
Temperature: Heating increases molecular motion generally. This is because the molecules in
the substrate and enzymes are constantly colliding with each other resulting in an increase in the
rate of reaction. The temperature that promotes maximum activity is called the optimum
temperature. However, if this temperature is exceeded, a decrease in collisions occurs due to
distruption of the secondary and tertiary bonds in the enzyme which leads to denaturation of the
enzyme or loss of enzyme activity. Most mammalian enzymes function within temperature
ranges of 370C-400C. where temperature ranges drop to freezing points, the enzymes are
inactivated and only regain their ability to catalyse reactions once temperatures increase to
optimum range.
pH
Enzymes generally function at specific pH ranges under constant temperature. The optimum pH
is the pH at which maximum reaction rate occurs. Any alteration (above or below) diminishes
the rate of enzyme activity. Decreasing pH leads to an increase in H ions concentration in the
medium which leaves the medium positively charged and as such affects the ionic charge of both
acidic and basic groups in the enzyme ultimately distrupting the ionic bonds which gives the
protein its shape. This can lead to denaturation and loss of enzyme activity

ENZYME INHIBITION
Enzyme inhibitors are molecules that interact with enzymes (temporary or permanent) in some
way and reduce the rate of an enzyme-catalyzed reaction or prevent enzymes to work in a normal
manner and are hence called enzyme inhibitors. Enzyme inhibition maybe competitive, non-
competitive and uncompetitive inhibition.
Competitive Inhibition.
This form of inhibition is also called reversible or temporary inhibition. The enzyme inhibitors
possess a similar shape to that of the substrate molecule and compete with the substrate for the
active site of the enzyme. This prevents the formation of enzyme-substrate complexes.
Therefore, fewer substrate molecules can bind to the enzymes so the reaction rate is decreased.
The level of inhibition depends on the relative concentration of substrate and inhibitor. Increased
substrate concentration increases reaction rate and reverses inhibition, while increased inhibitor
concentration reduces reaction rate.

Noncompetitive inhibition
Noncompetitive enzyme inhibitors bind to a site other than the active site of the enzyme, called
an allosteric site. Due to this binding, it deforms the structure of the enzyme so that it does not
form the ES complex at its normal rate, and it prevents the formation of enzyme-product
complexes, which leads to fewer product formations. Because they do not compete with
substrate molecules, noncompetitive inhibitors are not affected by substrate concentration. It is
also a reversible inhibition because once the inhibitor is unbound to the enzyme, catalysis may
proceed.

Allosteric enzymes are designed to change shape so that their activities can be regulated by non-
competitive inhibitors also called allosteric inhibitors. They bind to enzymes at allosteric sites
and this binding cause the change of the binding site of the enzyme hence catalysis is prevented.
This form of inhibition is regulatory as seen in feedback inhibition of enzymes or end-product
inhibition of enzymes according to the diagram below.
Uncompetitve inhibition
Uncompetitive inhibitors bind permanently to enzymes either in their active sites or elsewhere
but alter enzyme structure and affect catalysis. This type of inhibition is irreversible (permanent).
Some enzyme inhibitors can be used as a medicine or as metabolic poison in the treatment of a
particular disease. Examples are Heavy metal ions such as Hg2+, Ag+ and arsenic which bind
permanently to sulphydryl groups and cause the protein to precipitate.

Enzyme cofactors
Enzyme co factors are non-protein components which are required for efficiency in enzyme
activity. They range from simple inorganic molecules to complex organic molecule. The y either
remain unchanged at the end if the catalysis or are regenerated by another process, groups of
enzyme cofactors are: Inorganic ions, prosthetic groups and coenzymes.
Inorganic ions: are thought to increase catalysis by assisting in conforming the enzyme to shape
specific for its substrate. Eg. Salivary amylase shows increased activity in the presence of
chloride ions.

Prosthetic groups: these are organic molecules known to be tightly bound to enzymes on
permanent basis and assist in the catalysis of reactions. Eg is FAD (Flavin Adenine
Dinucleotide) which is bound to respiratory chain and responsible for the transfer of hydrogen
ions for the production of ATP during respiration. Another example is heam.

Coenzymes: these are also of organic origin but they differ from the prosthetic groups in that
they are not permanently attached to enzymes between reactions. They are only attached during
catalysis after which they detach and they are all derived from vitamins. Eg: NAD (Nicotinamide
Adenosine Dinucleotide), ATP (Adenosine Triphosphate), Coenzyme A etc.
Classification of enzymes

Types Biochemical Property

The enzyme Oxidoreductase catalyzes the oxidation reaction where the

Oxidoreductases electrons tend to travel from one form of a molecule to the other.eg
Pyruvate dehydrogenase

The Transferases enzymes help in the transportation of the functional

Transferases
group among acceptors and donor molecules. Eg transaminase

Hydrolases are enzymes which catalyze the hydrolysis reaction. This is

achieved by breaking chemical bonds of large molecules by adding
Hydrolases
water to cleave the bond. Eg: lipases, peptidases, glycosidases,
nucleosidases, phosphatases among others.

The catalyze the breaking up of a chemical bond between two parts of a

molecule through biochemical means. They also add water, carbon
Lyases
dioxide or ammonia across double bonds or eliminate these to create
double bonds. Examples are aldolases, decarboxylases, dehydratases

The Isomerases enzymes catalyze the structural shifts present in a

Isomerases molecule, thus causing the change in the shape of the molecule. Eg;
phophoglucomutase
 The Ligases are enzymes known to charge the catalysis of a ligation
reaction. Ligases link two substrate molecules together. They may be
referred to as biochemical glues which join pieces or fragments of an
Ligases
organic molecule together. Eg: DNA ligase join fragments of DNA to
form a more complete DNA molecule. Other examples are Acetyl CoA
Synthetase, etc.