1. Enzymes
By
Dr. (Mrs.) Namrata Bajpai
Guest faculty
Department of chemistry
Pt. Ravishankar shukla University
Raipur (C.G)
2. STRUCTURE OF ENZYMES
Enzyme Protein part + Non protein part
Holoenzyme (Apoenzyme) (Coenzyme)
Coenzyme : loosely bound to enzyme (non-
covalently bound).
Prosthetic group : very tightly or even
covalently bound to enzyme (covalently bound)
Cofactor
3. 2. Propertiesofenzymes
(important!)
• Catalytic efficiency – high efficiency, 103to 1017faster than
thecorrespondinguncatalyzed reactions
• Specificity - high specificity, interacting with one or a few
specific substrates and catalyzing only one type of
chemicalreaction.
• Mild reaction conditions- 37℃, physiological pH, a
m
b
i
e
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atmosphericpressure
4.
5. 4. Classification of enzymes
(1) By their composition
1) Monomeric enzyme
2) Oligomeric enzyme
3)Multienzyme complex: such as
Fatty acid synthase
6. (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)
9. What is the difference between an
enzyme and a protein?
Protein
•All enzymes are proteins except some RNAs
• not all proteins are enzymes
RN
A
Enzym
es
10. 2) Theactive site of the
enzyme
• Enzymes bind substrates to their active site and
stabilize the transition state of thereaction.
• Theactive site of the enzyme is the place where the
substrate binds and at which catalysisoccurs.
• The active site binds the substrate, forming an
enzyme-substrate(ES) complex.
Binding site
Active site
Catalytic site
11. Enzymatic reaction
steps
1. Substrate approaches active site
2. Enzyme-substrate complex forms
3. Substrate transformed into products
4. Products released
5. Enzyme recycled
12. 6. Enzymeactivity
• Enzymes are never expressed in terms of their
concentration (as mg or μg etc.), but are
expressedonly asactivities.
• Enzyme activity = moles of substrate converted
to product per unittime.
– The rate of appearance of product or the rate of
disappearance of substrate
– Testthe absorbance: spectrophotometer
13.
14. According to this model the enzyme reversibly bind with substrate to form
an ES complex that subsequently yields product.
where:
S - Substrate
E - Enzyme
ES- Enzyme substrate complex
P - product
K1,K-1 K2- Rate constants
15. ENZYME INHIBITION
Enzymes catalyze virtually every process in the cell. The
catalytic activity of certain enzymes is altered by certain inorganic
and organic molecules called modifiers. Those molecules which
increase the enzyme activity are called activators (Positive
modifiers) and those which decrease the enzyme activity are
called inhibitors ( Negative modifiers).
Compounds which convert the enzymes into inactive
substances and thus adversely affect the rate of enzyme-
catalyzed reaction are called enzyme inhibitors. Such a process
is known as enzyme inhibition. Two broad classes of enzyme
inhibitions are generally recognized : Reversible and
Irreversible , depending on whether the enzyme- inhibitor
complex dissociates rapidly or very slowly.
17. Based on the binding between enzyme andinhibitor
18. REVERSIBLE INHIBITOR
• Reversible inhibitors are bind to enzyme with noncovalent
interactions, such as hydrogen bonds, ionic bonds, and hydrophobic
interactions.
19. a) COMPETITIVE INHIBITOR
• This inhibitor has structural similarities with the substrate so, the
inhibitor competes with the substrate for the active site.
• If the inhibitor binds more tightly than the substrate, then
it is an effective competitive inhibitor.
• In competitive inhibition, the inhibitor can bind only to the free
enzyme and not with the enzyme-substrate complex.
• Hence inhibition can be overcome by increasing the concentration
of substrate in the reaction mixture.
20. Eg: The antibacterial action of sulfanilamide which is a structural
analog of PABA. Sulfanilamide inhibits the bacterial enzyme
dihydropteroate synthetase which catalyzes the incorporation of PABA
into 7,8-dihydropteroic acid.
21. b) NON COMPETITIVE INHIBITOR
• In noncompetitive inhibition, the binding of the inhibitor reduces
enzyme activity, but does not affect the binding of substrate.
22. • These inhibitors bind non covalently to sites other than the substrate
binding site.
• Inhibitor binding does not influence the availability of the binding
site for substrate.
• Binding of the substrate and the inhibitor are independent of each
other and inhibition cannot be overcome by increasing substrate
concentration.
23. c) UNCOMPETITIVE INHIBITOR
• Uncompetitive inhibitors bind only with the enzyme-substrate
complex.
• The inhibitor does not bind to the active site of the enzyme and it
does not have to resemble the substrate.
24. IRREVERSIBLE INHIBITOR
• This type of inhibition involves the covalent attachment of the
inhibitor to the enzyme.
• The catalytic activity of enzyme is completely lost.
• It can only be restored only by synthesizing molecules
25. Reversible Irreversible
1. Enzymes do follow Michaelis- Menten rate
equation [hence Lineweaver-Burk plot also] and
exhibit Rectangular hyperbolic curve when [V]is
plotted against [S].
2. A reversible inhibitor dissociates veryrapidly
from its target enzyme because it becomes very
loosely bound with the enzyme.
3. Three general types of inhibition are
distinguished depending on three factors :
(i) Whether the inhibition is or isnot
overcome by increasing the concentration of the
substrate.
(ii) Whether the inhibitor binds at
the active site or at allostericsite.
(iii) Whether the inhibitor binds with
the free enzyme only, or with the
enzyme-substrate complex only,
or with either of thetwo.
1. Enzymes usually do not follow Michaelis-Menten
rate equation [hence Lineweaver-Burk plot also]
and exhibit Sigmoidal curve when [V] is plotted
against [S].
2. An irreversible inhibitor dissociates very slowly
from its target enzyme because it becomes very
tightly bound to its active site, thus inactivating
the enzyme molecule. The bonding between the
inhibitor and enzyme may be covalent or
noncovalent in case of this type modification of
enzymes which are commonly called as
Regulatory enzymes also.
3. Two general types of inhibition/
modulation are distinguished
depending on two factors:
(i) Catalytic activity is modulated
through the noncovalent binding
of a specific metabolite at a site on
the protein other than the catalytic
site – Allostericenzyme.
(ii) Catalytic activity is interconverted
between active and inactive forms
by the action of other enzymes –
Covalently modulated enzymes.
Types of Enzyme Inhibitions
26. a) SUICIDALINHIBITOR
• They are relatively unreactive until they bind to the active site of the
enzyme.
• In the first few steps of the reaction it functions like a normal
substrate, but then it is converted into a very reactive compound that
combines with the enzyme to block its activity.
