Chapter 19
Goals:





To understand the induced fit theory of enzyme activity.
To learn about cofactors and coenzymes, and to understand the oxidation and reduction of NAD+ and FAD which
will be integral in the study of catabolism.
To understand the different classes of enzymes.
To understand the class of drugs that work by enzyme inhibition: enzyme inhibitors.
To understand the class of drugs that target receptors: receptor agonists and receptor antagonists.
1
Enzymes and the Induced Fit Theory
•
Enzymes are biological catalysts. Recall that a catalyst speeds up the rate of a reaction by decreasing the
activation energy needed for the reaction to occur. A catalyst remains unchanged in a reaction and does not
affect the amount of product formed, but only the rate at which the product forms.
•
Almost all enzymes are large globular proteins with an active site. The active site is the region where the reaction
takes place.
•
All enzyme names end in –ase (sucrase, lactase, etc…).
•
The substrate is the reactant that fits into the active site of an enzyme and undergoes a change.
•
The substrate approaches the active site, interactions between the substrate and enzyme hold the complex
together and induce a structural change in the enzymes shape. Bonds break/form in the substrate. The product
is then released and the enzyme resumes its original conformation.
E + S  E- - -S  E—S  E + P
2
Cofactors and Coenzymes
•
Cofactors & Coenzymes: Metal ions (cofactors) and organic molecules (coenzymes) are essential to the catalytic
activity of many enzymes.
•
Two coenzymes that we will encounter later are below:
3
Classes of Enzymes
1.
Oxidoreductases: catalyze oxidation-reduction reactions of substrate molecules. Recall that oxidation and
reduction occur together, so these enzymes have coenzymes that are oxidized or reduced as the substrate is
reduced or oxidized.
Example:
CH3CH2OH (reducer) + NAD+ (oxidizer)
CH3CHO (oxidized) + NADH (reduced) + H+
Subcategories of Oxidoreductases:
•
Oxidases: Oxidation of a substrate
•
Reductases: Reduction of a substrate
•
Dehydrogenases: Removal of hydrogen from a substrate to form a double bond
2.
Transferases: catalyzes the transfer of a group from one molecule to another.
Example:
A + B-C  A-B + C
CH OPO
CH2OH
O
O
HO
23
2
phosphofructokinase
H
HO
H
+ ADP
+ ATP
H
OH
H
OH
H
OH
H
OH
CH2OPO32-
fructose-6-phosphate
CH2OPO32-
fructose 1,6-biphosphate
Subcategories of Transferases:
•
Transaminases: Transfer of an amino group
•
Kinases: Transfer of a phosphoryl group
4
Classes of Enzymes
3.
Hydrolases: catalyzes the hydrolysis of substrates; recall that this is the breakage of bonds by the addition of
water.
Example: The enzymes that hydrolyze food molecules during digestion.
Subcategories of hydrolases:
•
Lipases: Hydrolyze the ester group of lipids
•
Proteases: Hydrolyze the peptide bonds of proteins
•
Nucleases: Hydrolyze the phosphate ester bonds in nucleic acids
4. Lyases: catalyze the addition of a molecule to a double bond (or the reverse).
Example:
O
-O
C
O
C
H
C
H
O
fumarase
C
O-
+ H2O
-O
C
fumarate
C
H2
OH
O
C
H
C
O-
L-malate
Subcategories of lyases:
•
Dehydrases: Removal of water
•
Decarboxylases: Removal of carboxyl group
•
Synthases: Addition of molecule to a double bond
5.
Isomerases: catalyzes the isomerization of a substrate (recall that an isomer is just a different arrangement of
atoms, but the same molecular formula)
Example:
H
CH2OH
C
triose phosphate isomerase
O
CH2OPO32-
dihydroxyacetone
phosphate
C
HC
O
OH
CH2OPO32-
D-glyceraldehyde
3-phosphate
5
Classes of Enzymes
6.  Ligases: catalyze the bonding together of two substrates using energy from ATP.
Example:
A + B + ATP  A-B + ADP + HPO42- + H+
O
O
C
C
pyruvate carboxylase
H3C
+ CO2 + ATP
O-
pyruvate
O
H3C
C
oxaloacetate
O
C
H2
C
C
O-
+ ADP + HOPO32- + H+
O
Subcategories of ligases:
•
Synthetases: General synthesis reaction
•
Carboxylases: Synthesis reaction where carbon dioxide is added to substrate to form carbonyl group
6
Enzyme Inhibitors
•
Compounds that cause Enzyme Inhibition:
–  Enzyme inhibitions is a natural method used for feedback regulation in the body
–  Enzyme inhibition is also common in drug design
–  Enzyme inhibitors can be reversible or irreversible
7
Receptor Agonists and Antagonists
•
•
•
Not all inhibitors are enzyme inhibitors. Some inhibit other functions such as hormones or neurotransmitters that
act to bring a chemical message to a target via interaction with a receptor.
Agonists: Substances that act to produce or prolong the normal biological response of a receptor.
–  Morphine and other opium derivatives are agonists of the opiate receptors. They bind in place of
neurotransmitters known as enkaphalins and produce the normal euphoric response.
Antagonists: Substances that inhibit the normal biological response of a receptor.
–  Anti-histamines are antagonists of histamine receptors. They bind in place of the neurotransmitter histamine
and block the normal allergic response.
8
Problems
1.
The first step of alcohol metabolism requires alcohol dehydrogenase to convert ethanol into ethanal. This
reaction will not take place without NAD+ turning into NADH. Answer the following questions regarding this
reaction:
a.
What is the enzyme?
b.
What is the substrate?
c.
What is the product?
d.
What is the coenzyme?
e.
Is the coenzyme oxidized or reduced in this reaction?
2.
Protease inhibitors are one class of drugs used to treat HIV. Protease inhibitors work by blocking a viral
enzyme that is critical to viral replication. Drugs that act as protease inhibitors are best classified as ________.
a.
Receptor agonists
b.
Receptor antagonists
c.
Enzyme inhibitors
d.
Receptor inhibitors
e.
All of the above
9