Enzyme
‫السالم عليكم ورحمة هللا وبركاته‬
Why Are Enzymes So Important?
Why are we
devoting two whole
lecture topic to a
enzyme?
Nearly all chemical
reactions in
biological cells need
enzymes to make
the reaction occur
fast enough to
support life.
From the Virtual Cell Biology Classroom on ScienceProfOnline.com
Image: Jumping rope, Meagan E. Klein
Outline
• Composition, structure and properties
of enzyme
• How Enzymes work
• Enzyme activity
• Factors affecting enzyme activity
• Regulation of enzyme activities
• Enzymes in clinical diagnosis
1. Definition of enzyme
•Enzymes are biological catalysts.
•A Catalyst is defined as "a substance
that increases the rate of a chemical
reaction without being itself changed in
the process.”
Enzymes as Biological Catalysts
• Enzymes are proteins
that increase the rate
of reaction by
lowering the energy
of activation
• They catalyze nearly
all the chemical
reactions taking place
in the cells of the
body
• Enzymes have unique
three-dimensional
shapes that fit the
shapes of reactants
(substrates)
2. Properties of enzymes (important!)
• Catalytic efficiency – high efficiency, 103 to
1017
faster
than
the
corresponding
uncatalyzed reactions
• Specificity - high specificity, interacting with
one or a few specific substrates and
catalyzing only one type of chemical reaction.
• Mild reaction conditions- 37℃, physiological
pH, ambient atmospheric pressure
3. Chemical composition of enzymes
(1) Simple protein
(2) Conjugated protein
Holoenzyme= Apoenzyme+ Cofactor
Cofactor
Coenzyme : loosely bound to enzyme (noncovalently bound).
Prosthetic group : very tightly or even
covalently bound to enzyme (covalently bound)
4. Classification of enzymes
(1). By their composition
1). Monomeric enzyme
2). Oligomeric enzyme
3). Multienzyme complex: such as
Fatty acid synthase
(2) Nomenclature
• Recommended name
•Enzymes are usually named according to the
reaction they carry out.
•To generate the name of an enzyme, the
suffix -ase is added to the name of its
substrate (e.g., lactase is the enzyme
that cleaves lactose) or the type of
reaction (e.g., DNA polymerase forms
DNA polymers).
•Systematic name (International classification)
• By the reactions they catalyze (Six
classes)
5. How enzymes work (important!)
1) Enzymes lower a reaction’s
activation energy
– All chemical reactions
have an energy barrier,
called the activation
energy, separating the
reactants and the
products.
– activation energy:
amount of energy needed
to disrupt stable
molecule so that reaction
can take place.
Enzymes
Lower a
Reaction’s
Activation
Energy
What is the difference between an
enzyme and a protein?
Protein
Enzymes
RNA
•All enzymes are proteins except some RNAs
• not all proteins are enzymes
2) The active site of the enzyme
• Enzymes bind substrates to their active site and
stabilize the transition state of the reaction.
• The active site of the enzyme is the place where the
substrate binds and at which catalysis occurs.
• The active site binds the substrate, forming an
enzyme-substrate(ES) complex.
Binding site
Active site
Catalytic site
Enzymatic reaction steps
1.
2.
3.
4.
5.
Substrate approaches active site
Enzyme-substrate complex forms
Substrate transformed into products
Products released
Enzyme recycled
6. Enzyme activity
• Enzymes are never expressed in terms of their
concentration (as mg or μg etc.), but are
expressed only as activities.
• Enzyme activity = moles of substrate converted
to product per unit time.
– The rate of appearance of product or the rate of
disappearance of substrate
– Test the absorbance: spectrophotometer
7. Factors affecting enzyme activity
•
•
•
•
•
•
Concentration of substrate
Concentration of enzyme
Temperature
pH
Activators
Inhibitors
Enzyme velocity
• Enzyme activity is commonly expressed by the
intial rate (V0) of the reaction being catalyzed.
(why?)
• Enzyme activity = moles
of substrate converted to
product per unit time.
Michaelis-Menten equation (very important!)
1. Michaelis-Menten equation describes how
reaction velocity (V) varies with substrate
concentration [S].
• The following equation is obtained
suitable algebraic manipulation.
V = Vmax
[S]
[S] + KM
Note: V means V0
Km: Michaelis constant
Km = (k2 + k3)/k1
after
(2) Effect of [E] on velocity
[S]>>[E]
V∝[E]
• The initial rate of an
enzyme-catalyzed
reaction
is
always
proportionate
to
the
concentration of enzyme.
• This property of enzyme
is
made
use
in
determining the serum
enzyme for the diagnosis
of diseases.
(3) Effect of temperature on
velocity Bell-shaped curve
(4) Effect of pH value on velocity
Bell-shaped curve
•The pH optimum varies for
different enzymes.
•Most enzyme: neutral pH (68).
• Each enzyme has
an optimal pH or pH
range (where the
enzyme has maximal
activity).
• Requirements for
the catalytic groups
in the active site in
appropriate
ionization state is a
common reason for
this phenomenon.
(5) Effect of activator on velocity
•Enzyme activators are molecules that bind to
enzymes and increase their activity.
(i). Inorganic ions
• Metal ions，such as Na+, K+, Mg2+, Ca2+, Cu2+, Zn2+,
Fe2+ et al
• Anions: such as Cl-, Br-, I-、CN- et al
(ii).
Organic
• Reducing agents, such as Cys、GSH
(iii).
Proteins
(6)
Inhibition of enzyme activities
(very important!)
• Inhibitor: any molecule which acts
directly on an enzyme to lower its
catalytic
rate
is
called
an
inhibitor.(not denaturation)
• Some enzyme inhibitors are normal
body metabolites.
• Other
may
be
foreign
substances,such as drugs or toxins.
8. REGULATION OF ENZYME ACTIVITY
1. Allosteric binding sites: Allosteric enzymes
are regulated by molecules called effectors
(modifiers) that binds nonconvalently at a site
other than the active site.
2. By Covalent Modification: Many enzymes
are regulated by covalent modification, most
frequently by the addition or removal of
‘phosphate’ group to serine, threonine or
tyrosine residue of the enzyme by kinases.
(enzyme)
3. Induction and repression of enzyme
sysnthesis: Cells can also regulate the
amount of enzymes present by altering the rate
of enzyme synthesis.
REGULATION CONT….
• 4. Zymogen Cleavage: Some enzyme are
synthesized as inactive precursor, called
zymogens, that are activated by proteolysis
(e.g., digestive enzyme, pepsinogen is inactive
and cleaved to pepsin which is active
chymotrypsin)
• 5.Location within the cell: Many enzymes are
localized in specific organelles within the cell.
This, compartmentation helps in the regulation
of the metabolic pathway.
9. Enzymes in clinical diagnosis
• An enzyme test is a blood test or urine test
that measures levels of certain enzymes to
assess how well the body’s systems are
functioning and whether there has been any
tissue damage. (why?)
• Common enzymes used for clinical diagnosis
include:
– alanine aminotransferase(ALT,also called
glutamate pyruvate transaminase,GPT)
– alkaline phosphatase
– amylase
– aspartate aminotransferase
– creatine kinase
– lactate dehydrogenase