Enzymes are biological catalyst, which speed up chemical reaction

1. Enzymes
BY: Amanuel Tobe (Msc in Clinical Biochemistry)
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2. Objectives
 At the end of the sessions students should be able to:
• Define enzymes and identify their functions
• List the biological locations of enzymes
• List the IUBMB classes of enzymes
• Indicate mechanism of action of enzymes
• Know about factors affecting enzyme activity

3. Enzymes
• An enzyme is a biological catalyst. It speeds up the rate of a
specific chemical reaction in the cell. The enzyme is not
destroyed during the reaction and is used over and over. A cell
contains thousands of different types of enzyme molecules,
each specific to a particular chemical reaction.
• Acting in organized sequences, they catalyze the hundreds of
stepwise reactions that degrade nutrient molecules, conserve
and transform chemical energy, and make biological
macromolecules from simple precursors.
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4. Cont.
• Enzyme-catalyzed reactions are highly efficient, proceeding
from 103–1012 times faster than uncatalyzed reactions. The
number of molecules of substrate converted to product per
enzyme molecule per second is called the turnover number.
• Lack of enzymes will lead to block in metabolic pathways
causing inborn errors of metabolism.
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5. Functions
• Catalysis
• Diagnostic markers
• Regulation
• Drug design
• Therapy
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6. Cont.
• The vast majority of enzymes are proteins (exceptions are
some kinds of RNA molecules called ribozymes). Apart from
protein component, enzymes can also contain non-
protein part.
• Enzymes do not invent new reactions; they simply make
reactions occur faster.
• An enzyme may catalyze both forward and reverse reaction
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7. Types of enzymes based on location of action
• Intracellular (endo-enzymes)- Most enzymes
• Plasma membrane (ecto-enzymes)- Alkaline phosphatase,
Nucleotide phosphatase
• Extracellular (exo-enzymes)- Enzymes of blood clotting
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8. Cont.
 Enzyme-catalyzed reactions have three basic steps:
• (1) binding of substrate: E + S ↔ES
• (2) conversion of bound substrate to bound product: E+S ↔
EP
• (3) release of product : EP→E + P
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9. Cont.

10. Characteristics of Enzymes
• Almost all enzymes are proteins except ribozymes. Enzymes
follow the physical and chemical reactions of proteins.
• They are heat labile.
• They are water-soluble.
• They can be precipitated by protein precipitating reagents
(ammonium sulfate or trichloroacetic acid).
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11. Enzyme Specificity
• Absolute specificity
• E.g Sucrase
• Group specificity
• E.g Hexokinase
• Bond specificity
• E.g trypsin, pepsin, alpha amylase
• Sterospecificity
• E.g Fumerase
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12. Cont.
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13. IUBMB system of classification of enzymes
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14. Active site
• The region of the enzyme where substrate binding and
catalysis occurs is referred to as active site or active center.
• Although all parts are required for keeping the exact three
dimensional structure of the enzyme, the reaction is taking
place at the active site. The active site occupies only a small
portion of the whole enzyme.
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15. Cont.
• The amino acids or groups that directly participate in making
or breaking the bonds (present at the active site) are called
catalytic residues or catalytic groups.
• The active site contains substrate binding site and catalytic
site; sometimes these two may be separate.
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16. Models of enzyme action
 The Lock and Key Model
• According to the lock and key model, the enzyme’s active site
complements the substrate precisely
• The substrate fits a particular active site like a key fits into a
particular lock
• This theory of enzyme-substrate interaction explains how
enzymes exhibit specificity for a particular substrate
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17. Cont.
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18. Cont.
 The Induced Fit Model
• Complementarity between the substrate and the binding site is
only part of the picture. As the substrate binds, enzymes
undergo a conformational change (“induced fit”) that
repositions the side chains of the amino acids in the active site
and increases the number of binding interactions
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19. CO-ENZYMES
• Enzymes may be simple proteins, or complex enzymes,
containing a non-protein part, called the prosthetic group.
The prosthetic group is called the co-enzyme. It is heat stable.
• The protein part of the enzyme is then named the apo-enzyme.
It is heat labile.
• These two portions combined together is called the holo-
enzyme.
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20. Cont.
• Co-enzymes may be divided into two groups
• Those taking part in reactions catalyzed by oxidoreductases
by donating or accepting hydrogen atoms or electrons.
• Those co-enzymes taking part in reactions transferring groups
other than hydrogen.
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21. Cont.
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22. Cont.
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23. Metallo-enzymes
• These are enzymes which require certain metal ions for their
activity.
• In certain cases, e.g. copper in Tyrosinase, the metal is tightly
bound with the enzyme.
• In other cases, even without the metal ion, enzyme may be
active; but when the metal ion is added, the activity is
enhanced. They are called ion-activated enzymes, e.g.
calcium ions will activate pancreatic lipase.
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24. Cont.
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25. MODE OF ACTION OF ENZYMES
• The mode of enzyme action depends upon the nature of the
enzyme and the substrate molecule, and it can be understood
by the following:
• 1 Formation of Enzyme Substrate complex (ESC):
• 2 Lowering of Activation energy.
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26. Lowering of Activation energy by enzymes
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27. Factors Affecting Enzyme Activity
• 1. Enzyme concentration
• 2. Substrate concentration
• 3. Product concentration
• 4. Temperature
• 5. pH
• 6. Presence of activators
• 7. Presence of inhibitors
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28. Cont.
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29. Enzyme regulation
• Enzyme activity can be regulated, that is, increased or
decreased, so that the rate of product formation responds to
cellular need.
• Physiologic regulation of a metabolic pathway depends on the
ability to alter flux through the pathway by activating the
enzyme catalyzing the rate-limiting step in the pathway
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30. Reversible Inhibition within the Active Site
• One of the ways of altering enzyme activity is through
compounds binding in the active site. If these compounds are
not part of the normal reaction, they inhibit the enzyme. An
inhibitor of an enzyme is defined as a compound that
decreases the velocity of the reaction by binding to the
enzyme.
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31. COMPETITIVE INHIBITION
• A competitive inhibitor “competes” with a substrate for
binding at the enzyme’s substrate recognition site and
therefore is usually a close structural analog of the substrate.
• The effect of a competitive inhibitor is reversed by
increasing [S]. At a sufficiently high substrate concentration,
the reaction velocity reaches the Vmax observed in the
absence of inhibitor.
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32. Cont.
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33. Non competitive
• Noncompetitive inhibition occurs when the inhibitor and
substrate bind at different sites on the enzyme. The
noncompetitive inhibitor can bind either free enzyme or the ES
complex, thereby preventing the reaction from occurring.
• The inhibitor usually binds to a different domain on the
enzyme, other than the substrate binding site. Since these
inhibitors have no structural resemblance to the substrate, an
increase in the substrate concentration generally does not
relieve this inhibition.
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34. Cont.
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35. Uncompetitive
• Here inhibitor does not have any affinity for free enzyme.
Inhibitor binds to enzyme–substrate complex; but not to the
free enzyme. In such cases both Vmax and Km are decreased
Inhibition of placental alkaline phosphatase (Regan
isoenzyme) by phenylalanine is an example of uncompetitive
inhibition.
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36. 36
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