Biochemistry
Instructor:
Dr. Areej Eskandrani
E-mail:
a.eskandrani@gmail.com
aeskandrani@taibau.edu.sa
Enzymes
Definition
• Enzymes are specific
biological
catalysts, that accelerate the rate of
chemical reactions without being
changed and without change in the end
products( reaction equilibrium).
• Substrate is the substance upon which
the enzyme acts.
Properties of enzymes
Specificity :
• Enzymes are specific in their action.
• Specificity may be :
• Group specificity: Enzyme acts on a number of
compounds having the same group or bonds.
Example; Pepsin acts on proteins but not
carbohydrates.
• Absolute specificity:The enzyme acts only on one
substrate.
• Example; Lactate dehydrogenase which removes
hydrogen only from lactic acid.
Properties of enzymes
Enzyme structure:
• All enzymes are proteins (with the exception of a small
group of catalytic RNA molecules).
• Some enzymes are simple proteins.
• Other
enzymes
are
conjugated
proteins
(called
holoenzymes) formed of protein part (called apoenzyme)
and a non protein part.
• The non protein part is called prosthetic group if firmly
attached to the protein and called coenzyme if loosely
attached to the protein or attached to the enzyme only
during action.
• Example of coenzymes, NAD (NicotinamideAdenine
Dinuceotide) and FAD (FlavinAdenine Dinucleotide) which
act as a hydrogen carriers for dehydrogenases.
• Vitamins are important constituents of many coenzymes.
Properties of enzymes
Enzyme nomenclature:
• Enzymes are named by adding suffix –
ase to the substrate (as maltase and
sucrase) or to the type of the reaction (
as dehydrogenase).
• Some exceptions are present as pepsin
and trypsin.
Properties of enzymes
Classification of enzymes
• Enzymes are classified into six major classes on the
basis of the reaction they catalyze.
• Oxidoreductases: oxidation-reduction reactions
• Transferases: transfer of functional groups
• Hydrolases: hydrolysis
introduction of water)
reactions
(cleavage
and
• Lyases: group elimination to form double bond
• Isomerases:
rearrangements
isomerization
(intramolecular
• Ligases (synthases): bond formation coupled with ATP
hydrolysis
Mechanism of enzyme action
Energy of activation:
• The substrate must be energized
(absorb energy) to reach an activated
or transition state. This energy is called
activation energy.
• At the transition state there is a high
probability that the chemical bond will
be made or broken to form the product.
• The activation energy is the amount of
energy needed to raise all the
molecules of one mole of the substrate
to the transition state.
• The enzyme is important to decrease
the activation energy.
Active sites
• During enzyme action, there is a temporary
combination between the enzyme and its substrate
forming enzyme – substrate complex. This occurs at
active sites of enzyme.
• The substrate reaches the transition state followed by
dissociation of this complex into enzyme and product.
Factors affecting enzyme activity
Concentration of enzyme:
• The initial velocity of
the
reaction
is
directly proportional
to the amount of
enzyme
available
provided that all other
conditions
are
constant.
Factors affecting enzyme activity
Concentration of substrate
• The initial velocity of the
reaction
is
directly
proportional to the amount
of substrate present till
reaches a maximum point
known
as
maximum
velocity where any further
increase in the substrate
causes no increase in the
velocity of the reaction.
Factors affecting enzyme activity
Effect of temperature
• The
optimal
temperature
for
enzymatic activity in
human body is 37 °C .
• Below and above the
optimal temperature
the enzymatic activity
decreases. At 55°C 60°C most enzymes
are denatured and
become permanently
inactive.
Factors affecting enzyme activity
Effect of pH
• The optimal pH for enzyme activity is that pH at which
the enzyme acts maximally.
• Above and below this pH the rate of reaction will
decrease.
• The optimal pH of most human enzymes is 7.4.
• The optimal pH of pepsin Is 2.
• Extreme pH may lead to denaturation.
Factors affecting enzyme activity
• Enzyme activators
• Certain substances may be needed to activate the
enzyme.
Examples: chloride ions activate salivary amylase and
calcium ions activate blood clotting enzymes.
Factors affecting enzyme activity
Enzyme Inhibitors
• Any substance that can decreases the velocity of
enzymatic reaction is called inhibitor.
• Inhibition can be classified into:
1. Competitive inhibitors:
• The inhibitor binds to the active sites and
competes with substrate .
• The inhibition is reversible.
• The inhibitor is similar to substrate in structure
• The inhibition depends on:
1.Concentration of substrate
2.Concentration of inhibitor
3. Affinity of active sites to substrate and inhibitor
Factors affecting enzyme activity
• Example:
Malonic acid inhibits succinatedehydrogenase by
competing with succinic acid (substrate).
2. Noncompetitive inhibitors:
• The inhibitor is not similar to the substrate.
• The inhibitor binds to a site on the enzyme other
than the active site.
• The inhibition is irreversible.
• Examples: heavy metals as mercury and oxidizing
agents.
Regulation of enzyme activity
• Enzyme activity is regulated by many
mechanisms.
• This regulation helps to maintain a constant
intracellular environment .
• Mechanisms of regulating enzyme activity
include:
1. Amount of enzyme present in the cell
2. Allosteric regulation:
Allostericmodulators are molecules that can bind
non- covalently at sites on the enzyme other than
active sites.
3. Feed back regulation:
It means the end product of a series of metabolic
reactions inhibits the gene controlling the
synthesis of the first enzyme