Download Unit-10-Peptides-Proteins-Enzymes

Chem 150
Unit 10 - Biological Molecules III
Peptides, Proteins and Enzymes
Proteins are the workhorses in living systems. Their many
roles include providing structure, catalyzing nearly all the
reactions that take place in a living cell, transporting and
storing materials, and controlling and defending living
systems. Like carbohydrates, proteins are polymers, but unlike
the polysaccharides, proteins are able to assume a much
wider range of 3-dimensional structures and a functions. In
this unit we will focus on one the of the most important
functions of proteins; that of biological catalysts (enzymes).
Amino Acids
α-Amino acids are the building blocks (monomers) for
polypeptides and proteins.
• Every amino acid contains,
• A carboxylic acid group
• An amino group
• A side chain (R)
2
Amino Acids
Back in Unit 7 we saw that carboxylic acids behave as acids
when dissolved in water.
3
Question (Clickers) (Unit 7)
At pH 7, which will be the predominant species?
A)
B)
Carboxylic
acid
O
CH3 C
OH
Carboxylate ion
acid
carboxylic acid
4
+
pKa ≈ 5
H2O
base
O
CH3 C
O
+
base
carboxylate ion
H3O+
acid
Carboxylic Acids & Phenols as Weak Acids
(Unit 7)
•
At pH 7, the carboxylate ion of carboxylic acids predominate
• At pH = pKa
O
•
At pH < pKa
•
At pH > pKa
CH3 C
O
OH
+
CH3 C
H2O
acid
base
pKa ≈ 5
O
CH3 C
O
OH
=
acid
O
OH
>
CH3 C
O
acid
base
O
O
CH3 C
acid
5
O
base
O
CH3 C
CH3 C
OH
<
CH3 C
O
base
O
base
pH = 7
+
H3O+
acid
Amino Acids
Back in Unit 7 we also saw that amines behave as bases
when dissolved in water.
6
Amines as Weak Bases (Unit 7)
Like ammonia, 1°, 2° and 3°, act as Brønsted-Lowry bases.
H
CH3 N
H (aq) +
H
methanamine
(base)
7
H2O (l)
CH3 N
H (aq) +
H
methylammonium ion
(acid)
OH- (aq)
Amines as Weak Bases (Unit 7)
The conjugate acids are called ammonium ions
• When placed in water, these ammonium ions will behave
like acids.
H
CH3 N
pKa - 10
H (aq)
H
methylammonium ion
(acid)
8
+
H2O (l)
CH3 N
H (aq)
H
methanamine
(base)
+
H3O+ (aq)
Amino Acids
At pH 7, amino acids are in their
zwitterionic form.
• There is no pH value at which
there are no charges on an
amino acids.
• However, there is a pH value at
which the net charge is zero.
• This pH value is called the
Net charge
+1
0
isoelectric point.
-1
9
Amino Acids
There are 20 different sidechains for the amino acids that are
used to build proteins.
•
These are classified according to their physical properties
as
•
•
•
•
10
Non-polar
Polar acidic (negatively charged at pH 7)
Polar basic (positively charged at pH 7)
Polar neutral (polar, but not charged at pH 7)
Amino Acids
Non polar
sidechains
• Most of these
sidechains
are
hydrocarbons
11
Amino Acids
Polar acidic sidechains
• Sidechains contain
carboxylic acids
• Negatively charged
at pH 7
12
Amino Acids
Polar basic sidechains
• Sidechains contain
amines
• Positively charged at pH 7
13
Amino Acids
Polar neutral sidechains
• Sidechains contain polar
groups that are capable of
hydrogen bonding
• alcohols
• phenols
• amides
• Uncharged at pH 7
14
Amino Acids
For all of the amino acids, except one (glycine), the α-carbon
is chiral.
• Fisher projection of the amino acids alanine:
• With few exceptions, only the L-amino acids are used to
make proteins.
15
Peptides, Proteins, and pH
Amino acids are joined together to form polymers of amino
acids called oligopeptides (2-10 amino acids) and
polypeptides (more than 10 amino acids).
16
•
Collectively, oligopeptides and polypeptides are called
peptides.
•
The amino acids are joind together by an amide bond
called a peptide bond, which is analogous to the
glycosidic bond found in oligosaccharides and
polysaccharides.
•
Back in Unit 7 we saw how carboxylic acids can react with
ammonia and amines to form amides.
Amides (Unit 7)
•
•
17
When a carboxylic acid reacts with an amine it also
produces and ammonium salt
If the ammonium salt is then heated, an amide is
produced.
Amides (Unit 7)
Amides are important in
biochemistry.
• For example, amino
acids are connected
together to form
proteins using amide
groups.
amino acid
18
Peptides, Proteins, and pH
Peptide bond formation
Peptide Bond
H
H
O
H
H3 N
O
C
C
C
N
H
+
H
C
H
O
CH2
O
H
H
C
H
O
H3 N
N
C
C
C
O
CH2
H
Dipeptide
19
O
+
H
O
H
Peptides, Proteins, and pH
•
The amide bond that
connects the amino
acids together in a
peptide is called a
peptide bond.
•
Proteins are long
polypeptide chains,
usually with 50 or
more amino acids,
which fold into a well
defined structure. The protein
ubiquitin
20
Peptides, Proteins, and pH
Proteins are sensitive to the pH because they contain
numberous acid and base groups
• The pH affects the charge on a proteins, which in turn, can
have a marked effect on a protein’s structure and function.
21
Peptides, Proteins,
and pH
Example
• The charges on
the tripeptide
Lys-Ser-Asn
as a function of
pH.
Net Charge
+2
0
-2
22
Peptides, Proteins,
and pH
Example
• The charges on
the tripeptide
Lys-Lys-Ala
as a function of
pH.
Net Charge
+3
+2
-1
23
Peptides, Proteins, and pH
Amino acids with acid or base side chains have additional
charge groups:
•
•
•
e.g. Glutamic acid is an acid amino acid
At pH’s below the isoionic point (pI) the charge is positive
At pH’s above the isoionic point (pI), the charge is negative
H O
H3N
C
C OH
OH–
H+
H O
H3N
C
C O
H+
H3N
C
C O
H O
–
OH
+
H
H2N
C
C O
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
C OH
C OH
C O
C O
O
O
O
O
pH < 2
2 < pH < 4.4
4.4 < pH < 9
9 < pH
+1
0
-1
-2
pI = 3.2
24
OH–
H O
Peptides, Proteins, and pH
Amino acids with acid or base side chains have additional
charge groups:
•
•
•
e.g. Lysine is a basic amino acid
At pH’s below the isoionic point (pI) the charge is positive
At pH’s above the isoionic point (pI), the charge is negative
H3N
H
O
C
C
OH
OH
–
+
H
H3N
H
O
C
C
–
OH
O
+
H
H2N
H
O
C
C
O
+
H
H2N
H
O
C
C
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
NH3
NH3
NH3
NH2
pH < 2
2 < pH < 9
9 < pH < 10.3
10.3 < pH
0
-1
+2
+1
pI = 9.7
25
OH
–
O
Peptides, Proteins, and pH
We are going to simplify the determination of charge as a
function of pH by looking only at pH 1, pH 7, and pH 14:
pH 1
pH 7
pH 14
Acids
A
Include:
 -COOH
Asp sidechain
Glu sidechain
0
-1
-1
Bases
B
Includes:
 -NH
2
His sidechain
Lys sidechain
Arg sidechain
+1
+1
0
Charges at different pH values
26
Question
At pH 7, which of the following amino acids have a net
positive charge, which have a net negative charge, and which
are neutral?
Lysine
Phenylalanine
Leucine
27
Question
Lysine
B
H
O
A
H2 N
C
C
OH
Charges at different pH
values
pH 1
pH 7
pH 14
CH2
A
 -COOH
0
-1
-1
CH2
B
 -NH
2
+1
+1
0
CH2
B
Lys sidechain
+1
+1
0
+2
+1
-1
CH2
NH2
28
B
Net
Question
Draw the structure of the following tripeptide Glu-Asp-Phe at
pH 1 and high pH 14.
29
Question
Draw the structure of the following tripeptide Glu-Asp-Phe at
pH 1 and high pH 14.
Draw the backbone
H2 N
H
O
C
C
R
30
H
O
N
C
C
H
R
H
O
N
C
C
H
R
OH
Question
Draw the structure of the following tripeptide Glu-Asp-Phe at
pH 1 and high pH 14.
Add the sidechains and identify the acids (A) and bases (B)
B
H
O
H2 N
C
C
CH2
CH2
A
N
C
C
H
CH2
A
H
O
N
C
C
H
CH2
C
OH
OH
Glu tam ic Acid
(Glu )
31
O
O
C
O
H
Aspartic acid
(Asp)
Phenylalanine
(Phe)
A
OH
Question
Draw the structure of the following tripeptide Glu-Asp-Phe at
pH 1 and high pH 14.
At pH 1, Acids (A) are 0 and Bases (B) are +1
B
H
O
H3 N
C
C
CH2
CH2
A
N
C
C
H
CH2
A
H
O
N
C
C
H
CH2
C
Aspartic acid
(Asp)
A
OH
Net Charge = +1
at pH 1
OH
OH
Glu tam ic Acid
(Glu )
32
O
O
C
O
H
Phenylalanine
(Phe)
Question
Draw the structure of the following tripeptide Glu-Asp-Phe at
pH 1 and high pH 14.
At pH 14, Acids (A) are -1 and Bases (B) are 0
B
H2 N
H
O
C
C
CH2
CH2
A
N
C
C
H
CH2
A
H
O
A
N
C
C
O
H
CH2
C
Net Charge = -3
at pH 14
O
O
Glu ta m ic Acid
(Glu )
33
O
O
C
O
H
Aspartic acid
(Asp)
Phenylalanine
(Phe)
Protein Structure
Proteins are polypeptides that fold to adopt a well-defined,
three-dimensional structure.
There two general classifications of proteins
34
•
Fibrous proteins exist as long fibers that are usually
tough and insoluble in water; examples include
• collagen (skin and bones)
• Keratin (hair)
•
Globular proteins are spherical, highly folded, and usually
soluble in water; examples include
• enzymes
• antibodies
• transport proteins like hemoglobin and myoglobin
Protein Structure
Fibrous versus globular
35
Protein Structure
Proteins display up to four levels of structure
•
•
Primary structure
• This is the amino acid sequence, which is unique for each protein
• This defines the covalent structure of a protein
Secondary structure
• Regular, periodic structures, that involve hydrogen bonding between
the backbone amides
hydrogen bonds
between amides
H
O
N
C
C
H
R
O
C
36
H
O
N
C
C
H
R
H
O
N
C
C
H
R
N
H
H
O
N
C
C
H
R
Protein Structure
Proteins display up to four
levels of structure
•
Tertiary structure
•
The 3-dimensional fold of the
the polypeptide in which the
backbone twists and turns its
way through the folded structure
of the protein.
•
It involves interactions between
the sidechains of the the amino
acids and is highly influenced by
the amino acid sequence.
The protein
ubiquitin
37
Protein Structure
Primary Structure
38
•
A protein’s amino acid sequence is referred to as its
primary structure.
•
Every protein has a unique primary structure that is
determined by the gene for that protein.
•
The primary structure defines the covalent structure
of a protein.
Protein Structure
Primary Structure
Glycine
Gly
Alanine
Ala
Serine
Ser
Aspartic Acid
Asp
O
OH
H
H
O
H
C
H3 N
N
C
C
C
CH3
H
C
N
N
H
O
H
C
C
C
H
O
CH2
N-Terminus
H
H
C
N
O
H
C
C
C
CH2
HC
H
CH3
CH3
CH2
H
C
N
N
H
O
C-Terminus
C
CH2
N
O
O
H
C
C
C
O
O
CH2
H
O
CH2
H
CH2
CH2
CH2
C
CH2
O
NH2
NH3
Phenylalanine
Phe
39
Leucine
Leu
Lysine
Lys
Gluctamine
Gln
Protein Structure
Primary Structure
O
OH
H
H
O
H
C
H3 N
N
C
C
C
CH3
H
C
N
N
H
O
H
C
C
C
H
O
N-Terminus
CH2
H
H
C
N
O
H
C
C
C
CH2
HC
CH3
H
CH3
CH2
H
C
N
N
H
O
C-Terminus
C
CH2
N
O
O
H
C
C
C
O
O
CH2
H
O
CH2
H
CH2
CH2
CH2
C
CH2
O
NH2
NH3
Glycylphenylalanylalanylleucylseryllysylaspartylglutamine
H2H-Gly-Phe-Ala-Leu-Ser-Lys-Asp-Gln-COOH
40
Gly-Phe-Ala-Leu-Ser-Lys-Asp-Gln
GFALSKDQ
Protein Structure
Primary structure
• The Central Dogma
• DNA → mRNA → Polypeptide
•The genetic code is
used to match up the
DNA/mRNA sequence
to the sequence of
amino acids in a protein
•All living organisms use
the same code
41
Protein Structure
•
•
The functional diversity of proteins results from the large
number of possible proteins that can be built using the 20
different amino acids
Question: How much mass would it take to construct one
molecule each of all of the possible polypeptides
containing 100 amino acids residues?
o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o
•
42
View the polypeptides as beads on a string, with one of 20 possible
types of beads at each position.
Protein Structure
27
The Earth weighs 6.0 x 10
take?
43
g, how many Earths would it
Protein Structure
33
The Sun weighs 2.0 x 10
44
g, how many Suns would it take?
Protein Structure
45
The Milky Way galaxy weighs 1.2 x 10 times the mass of
the sun
• (1.2 x 1045 suns)(2.0 x 1033 g/sun) = 2.4 x 1078 g
• How may galaxies would it take?
45
Protein Structure
The Coma galaxy cluster contains several thousand galaxies,
how many ...?
46
Protein Structure
Number of polypeptides
(20100)
Avg. Mass of each polypeptide
Total mass needed
47
130
1.26 x 10
-22
1.83 x 10
108
2.32 x 10
80
Number of Earths
3.9 x 10
Number of Suns
1.2 x 10
Number of Galaxies
9.7 x 10
75
29
g
g
Protein Structure
Secondary structure
• The polypeptide backbone can take on regular shapes that
allow the backbone amides to hydrogen bond to one
another.
• The primary forms of secondary structure include
• α-helix
• β-sheet
48
Protein Structure
Secondary structure
α-Helix
49
Protein Structure
Secondary Structure
β-Sheet
50
Protein Secondary Structure
Secondary Structure
Antiparallel β-Sheet
51
Protein Structure
Secondary Structure
Parallel β-Sheet
52
Protein Structure
Tertiary Structure
53
•
The different elements of
secondary structure come
together to create the
overall 3-dimensional
structure of the the protein
•
The structure is stabilized
primarily by sidechain
interactions and is highly
influenced by the amino
acid sequence (primary
structure).
The protein
ubiquitin
Protein Structure
Tertiary Structure
54
•
This is usually the native,
or biologically active, form
of the protein.
•
When placed in water, the
polypeptide folds to
maximize the number of
nonpolar (hydrophobic)
residues that are buried
on the inside away from
exposure to water.
The protein
ubiquitin
Protein Structure
Tertiary Structure
Explore the
tertiary structure
and the
secondary structure
of the protein ubiquitin
The protein
ubiquitin
55
Protein Structure
Tertiary Structure
• The tertiary structure is stabilized by the same noncovalent interactions that we looked at in determining
boiling points and solubilites
• Charge/Charge interactions (Salt bridges)
• Ion/Dipole interactions
• Dipole/Dipole interactions
• Hydrogen bonding
• Hydrophobic interactions (nonpolor/water)
• There is one covalent interactions that stabilizes the
tertiary structure of some proteins.
• Disufide bond
56
Protein Structure
Tertiary Structure
• The interactions that
stabilize the tertiary
structure.
57
Protein Structure
Quaternary Structure
• Some proteins contain multiple
polypeptides
• Each peptide is called a subunit
• The polypeptides are held
together by the same types of
interactions that stabilize the
Explore the
tertiary structure.
quaternary structure
of the protein ubiquitin
58
Protein Denaturation
Because the secondary, tertiary and quaternary structures of
proteins are stabilized by weak, non-covalent interactions,
these structures are easily disrupted by agents that disrupt
theses interactions, including:
• Changes in temperature
• Changes in pH
• Mechanical stress (agitation)
• Soaps and detergents
These agents typically cause the protein to unfold
• Only the primary structure remains
• The protein loses it function
The process is called protein denaturation.
59
Protein Denaturation
Christain Anfinsen won a Nobel Prize for showing that protein
denaturation can be reversed.
• The experiment demonstrated that the information necessary
to obtain the correctly folded protein structure is contained
within the protein’s amino acid sequence (primary structure).
Active
Protein
60
Inactive
Protein
Enzymes
Nearly every reaction that takes place in a living cell has an
enzyme associated with it.
•
Enzymes are biological catalysts
•
Most enzymes are proteins
Many human diseases involve enzymes misbehaving
•
61
Many treatments for diseases involve drugs that target
enzymes.
Enzymes
The common names for enzymes often describe the
substrate (reactant) for the reaction and a description of the
reaction that is being carried out on that substrate.
•
•
The names usually end with -ase.
Example: alcohol dehydrogenase
alcohol
aldehyde
NAD+
CH3
CH2
ethanol
62
NADH + H+
OH
O
CH3
alcohol dehydrogenase
enzyme
C
H
ethanal
(acetaldehyde)
Question (Clicker)
What class of reaction is the alcohol dehydrogenase
reaction?
alcohol
aldehyde
NAD+
CH3
CH2
OH
ethanol
63
NADH + H+
O
CH3
alcohol dehydrogenase
enzyme
A)
Hydrolysis
B)
Decarboxylation
C)
Oxidation/reduction
D)
Acid/base
E)
Hydration
C
H
ethanal
(acetaldehyde)
Reactions of Alcohols and Thiols (Unit 8)
64
Enzymes
The common names for enzymes often describe the
substrate (reactant) for the reaction and a description of the
reaction that is being carried out on that substrate.
•
•
The names usually end with -ase.
Example: pyruvate decarboxylase
-keto acid
O
O
CH3 C
C
aldehyde
O
OH
pyruvic acid
65
pyruvate
decarboxylase
CH3
C
H
ethanal
(acetaldehyde)
+
CO2
Question (Clicker)
What class of reaction is the pyruvate decarboxylase
reaction?
-keto acid
O
O
CH3 C
C
aldehyde
O
OH
pyruvic acid
66
pyruvate
decarboxylase
A)
Hydrolysis
B)
Decarboxylation
C)
Oxidation/reduction
D)
Acid/base
E)
Hydration
CH3
C
H
ethanal
(acetaldehyde)
+
CO2
Carboxylic Acids & Phenols, Other Reactions
(Unit 7)
The decarboxylation of β-keto acids produces ketones
The decarboxylation of α-keto acids produces aldehydes
67
Enzymes
The common names for enzymes often describe the
substrate (reactant) for the reaction and a description of the
reaction that is being carried out on that substrate.
•
•
The names usually end with -ase.
Example: succinate dehydrogenase
O
HO
FAD
O
C CH2 CH2 C
FADH2
68
HO
H
C
OH
C
succinate
dehydrogenase
succinic acid
O
H
C
O
C
fumaric acid
OH
Question (Clicker)
What class of reaction is the alchohol dehydrogenase
reaction?
O
HO
FAD
O
C CH2 CH2 C
FADH2
HO
H
C
OH
C
succinate
dehydrogenase
succinic acid
69
O
A)
Hydrolysis
B)
Decarboxylation
C)
Oxidation/reduction
D)
Acid/base
E)
Hydration
H
C
O
C
fumaric acid
OH
Oxidation and Reduction (Unit 4)
The reaction equation on the previous slide also illustrates
another shorthand method of writing equations, which used
multiple reaction arrows.
FAD
FADH2 equation is
• The longhandO form
H H of
O this reaction
O
H O
HO C
C
C
H
H
C
OH
HO C
H
H
O
HO C
C
C
C
H
H
succinic acid
(saturated)
70
C
OH
fumaric acid
(unsaturated)
O
OH
C
H
succinic acid
(saturated)
O
C
+
FAD
HO C
C
H
O
C
C
H
fumaric acid
(unsaturated)
OH + FADH2
Enzymes
The common names for enzymes often describe the
substrate (reactant) for the reaction and a description of the
reaction that is being carried out on that substrate.
•
•
The names usually end with -ase.
Example: succinate dehydrogenase
O
HO
O
H
C
C
H
C
O
C
fumaric acid
71
+
OH
HO
H2O
fumarase
OH O
C CH2 C
C
H
L-malic acid
OH
Question (Clicker)
What class of reaction is the fumarase reaction? (Unit 8)
A)
B)
C)
Hydrolysis
O
HO
C
Decarboxylation
O
+
C C
H
C
OH
D)
Acid/base
E)
Hydration
HO
H2O
Oxidation/reduction
fumaric acid
72
O
H
fumarase
OH O
C CH2 C
C
H
L-malic acid
OH
Reactions Involving Water (Unit 4)
Hydration
• In the hydration reaction water is also split, but instead of
being used to split another molecule, it is added to another
molecule to produce a single product.
•
•
The water it is added to either an alkene or alkyne:
H OH
The hydration of an alkene produces an alcohol.
H
C
C
H
H
ethene
(an alkene)
73
H
+
H
OH
acid
catalyst
H
C
C
H
H
ethanol
(an alcohol)
H
Enzymes
Specificity
• Absolute spectificity - enzyme only accepts one specific
substrate.
• Relative specificity - enzyme accepts a range of related
substrates.
alcohol
aldehyde
NAD+
R
CH2
ethanol
74
NADH + H+
OH
O
CH3
alcohol dehydrogenase
enzyme
C
H
ethanal
(acetaldehyde)
Enzymes
Specificity
• Stereospecific specificity - enzyme only reacts with or
produces one specific stereoisomer
cis
trans
O
HO
FAD
O
C CH2 CH2 C
FADH2
O
HO
H
C
OH
C
succinate
dehydrogenase
succinic acid
C
H
+
O
C
HO
O
O
C
C
C
H
OH
fumaric acid
OH
C
H
maleic acid
O
HO
O
H
C
C
H
C
O
C
fumaric acid
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+
OH
HO
H2O
fumarase
OH O
C CH2 C
C
H
L-malic acid
O
OH
+
HO
H
O
C CH2 C
C
OH
D-malic acid
OH
Enzymes
Specificity
• Absolute
• Relative
• Stereospecific
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Enzymes
Catalysis
• As catalysis, enzyme have no effect on the change in free
energy, ΔG, for a reaction
• Ezymes speed up reactions by decreasing the activation
energy, Eact.
• Enzymes do this by binding the substrates and by directing the
reaction
77
Free Energy and Reaction Rates (Unit 4)
There is a third way to speed up the reaction rate and that is
to lower the height of the hill.
•
This is done using catalysts, which provide an alternative
pathway over the hill for the reactants.
Α → B
Free
Energy
(G)
Eact > 0
without catalyst
with catalyst
A
ΔG < 0
Β
Progress of
reaction
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spontaneous
Enzymes
Catalysis
• The location on the enzyme where the substrate binds and
the reaction occurs is called the active site.
• Back in Unit 4 we saw a specific example of this with the
hexokinase reaction
Explore the enzyme
hexokinase
79
Enzymes
Cofactors and Coenzymes
• Sometimes enzymes need some help with catalzying the
reactions.
• Cofactors are non-protein components of an enzyme
• Metal ions
• Organic molecules (Coenzymes)
• Many of the coenzymes are derived from the vitamins that
we take in in our diet.
80
Enzymes
81
Enzymes
pH and Temperature
• Enyzme activity is often critically dependent on the pH and
temperature.
82
Control of Enzyme-Catalyzed Reactions
Michaelis-Meten enzymes behave according to a model
proposed in the early 1900’s by Michaelis and Menten.
• E = enyzme
• S = substrate
• ES = enzyme-substrate complex
• P = product
83
Control of Enzyme-Catalyzed Reactions
Raymond describes an analogy of reaching into a box for an
orgrange, pulling one out, and peeling it.
• The Michaelis-Menten model is characterized by two
KM
Vmax
parameters.
• KM (The Michaelis-Menten constant), which is related to the strength
•
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of the substrate binding.
Vmax (The maximum velocity), which corresponds to the maximum
rate that the enzyme-substrate complex forms product.
Control of Enzyme-Catalyzed Reactions
Enzyme Inhibition
• Can be a normal
event used by the cell
to control enzyme
activity.
• Can also be exploited
in the design of drugs.
• Example, the
irreversible
inhibition of COX
enzymes by aspirin
85
Control of Enzyme-Catalyzed Reactions
Enzyme Inhibition can also be Reversible
• Competitive inhibition
• The inhibitor competes with the substrate for the active site
KM
Vmax
Competitive inhibition
affect
KM but not Vmax
86
Control of Enzyme-Catalyzed Reactions
Some drugs are competitive inhibitors of enzymes
• Example: the anti-HIV drug AZT
• The AZT inhibits the enzyme reverse trascriptase enzyme,
which the HIV virus uses to converts it RNA to DNA. The
drug mimics the normal substrate for this enzyme, dTTP.
• This drug targets the activity of the HIV virus because
humans do not use a reverse transcriptase enzyme.
87
Control of Enzyme-Catalyzed Reactions
Some drugs are competitive inhibitors of enzymes
• Example: the bacterial drug sufanilamide (a sulfa drug).
• The sufanilamide inhibits the an enzyme that bacteria use
to synthesize the coenzyme folate. The drug mimics the
normal substrate for this enzyme, p-aminobenoate.
88
Control of Enzyme-Catalyzed Reactions
•
This drug targets the activity of bacteria because humans
do not synthesize their own folate, getting it instead from
their diet.
Folate
89
Control of Enzyme-Catalyzed Reactions
Enzyme Inhibition can also be Reversible
• Noncompetitive inhibition
• The inhibitor binds at a different site thatn the substrate.
KM
Vmax
Noncompetitive inhibition
affects
Vmax but not KM
90
Control of Enzyme-Catalyzed Reactions
Noncompetitive inhibition is often used to regulate
biosynthetic pathways by a mechanism called feedback
inhibition.
• The end product of the pathway binds to a site on an
enzyme used earlier in the pathway, and turns it off.
91
Control of Enzyme-Catalyzed Reactions
Enzymes that are inhibited by a substance binding to a site
other than the active site are called allosteric enzymes.
• The enzymes that are regulated by noncompetive inhibition
in feedback inhibition are examples of allosteric enzyme.
• The substance that inhibits the enzyme in this way is called
a negative effector.
Some allosteric enzymes are activated instead of inhibited by
as substance binding at their allosteric site.
• These substances are called positive effectors.
92
Control of Enzyme-Catalyzed Reactions
Some enzymes are controlled by covalent modifications to
their structure.
• In some cases the modifications are reversible, such as
placing a phosphate on an enzyme to turn it on, and then
taking the phosphate off to turn it off again.
93
Control of Enzyme-Catalyzed Reactions
Some enzymes are controlled by covalent modifications to
their structure.
• Example: Glycogen phosphorylase, which is the enzyme
that breaks down the polysaccharide glycogen.
• The phosphorylation/dephosphorylation of this enzyme is
under hormonal control.
94
Control of Enzyme-Catalyzed Reactions
Some enzymes are controlled by covalent modifications to
their structure
• In other cases the covalent modification is irreversible.
95
Control of Enzyme-Catalyzed Reactions
Some enzymes are controlled by covalent modifications to
their structure
• Example: the digestive enzymes trypsin and chymotrypsin,
which are used to break down proteins in the small
intestine are synthesized in the pancreas in an inactive
form called trypsinogen and chymotrypsinogen,
respectively.
• They are transported in this inactive form to the small
intestine, there are activated by removing parts of their
amino acids sequence.
96
Control of Enzyme-Catalyzed Reactions
In a clinical setting, the presence of enzymes in blood is often
used to diagnose tissue damage that is related to various
diseases.
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The End