3. Why Are Enzymes So Important?
Why are we
devoting two whole
lecture topic to
enzymes?
Nearly all chemical
reactions in
biological cells need
enzymes to make
the reaction occur
fast enough to
support life.
Image: Jumping rope, Meagan E. Klein
From the Virtual Cell Biology Classroom on ScienceProfOnline.com
4. • Composition, structure and properties
of enzyme
• How Enzymes work
• Enzyme activity
• Factors affecting enzyme activity
• Regulation of enzyme activities
• Enzymes in clinical diagnosis
Outline
5. 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.”
6. 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)
7. 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
8. 3. Chemical composition of enzymes
(1) Simple protein
(2) Conjugated protein
Holoenzyme= Apoenzyme+ Cofactor
Coenzyme : loosely bound to enzyme (non-
covalently bound).
Prosthetic group : very tightly or even
covalently bound to enzyme (covalently bound)
Cofactor
9.
10. 4. Classification of enzymes
1). Monomeric enzyme
2). Oligomeric enzyme
3). Multienzyme complex: such as
Fatty acid synthase
(1). By their composition
11. (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)
12.
13.
14. 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.
16. What is the difference between an
enzyme and a protein?
Protein
•All enzymes are proteins except some RNAs
• not all proteins are enzymes
RNA
Enzymes
17. 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.
Active site
Binding site
Catalytic site
18. Enzymatic reaction steps
1. Substrate approaches active site
2. Enzyme-substrate complex forms
3. Substrate transformed into products
4. Products released
5. Enzyme recycled
19. 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
20. Factors affecting enzyme activity
• Concentration of substrate
• Concentration of enzyme
• Temperature
• pH
• Activators
• Inhibitors
21. 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.
22. Note: V means V0
[S]
[S] + KM
V = Vmax
Km: Michaelis constant
Km = (k2 + k3)/k1
Michaelis-Menten equation (very important!)
1. Michaelis-Menten equation describes how
reaction velocity (V) varies with substrate
concentration [S].
• The following equation is obtained after
suitable algebraic manipulation.
23. [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.
(2) Effect of [E] on velocity
24. (3) Effect of temperature on
velocity Bell-shaped curve
25. (4) Effect of pH value on velocity
• 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.
•The pH optimum varies for
different enzymes.
•Most enzyme: neutral pH (6-
8).
Bell-shaped curve
26. (5) Effect of activator on velocity
(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
(iii). Proteins
• Reducing agents, such as Cys、GSH
•Enzyme activators are molecules that bind to
enzymes and increase their activity.
27. (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.
28. 7. REGULATION OF ENZYME ACTIVITY
1. Allosteric binding sites: Allosteric
enzymes are regulated by molecules
called effectors (modifiers) that binds
non-convalently 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)
29. REGULATION OF ENZYME ACTIVITY Cont’d
3. Induction and repression of enzyme
sysnthesis: Cells can also regulate the
amount of enzymes present by altering the
rate of enzyme synthesis.
4. Location within the cell: Many enzymes are
localized in specific organelles within the cell.
This, compartmentation helps in the regulation
of the metabolic pathway.
30. REGULATION CONT….
5. 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)
31. 8. 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?)
32. • 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
