Download Enzymes I - eCurriculum

M1 – Biochemistry
Enzymes I – III
Dr. Sato-Bigbee
1
Enzymes can increase the rate of a chemical
2
reaction by a factor of 1011 or higher
E+S
Enzymes increase the speed at which
reactions reach equilibrium but they
do not change the equilibrium constant
ES
P+E
Ea: Energy of activation
Enzymes speed up reactions by decreasing Ea
3
Why do we need enzymes???
4
Why do we need enzymes???
Speed
Speed:
Specificity
Enzymes make the speed of
reactions compatible with life
Control
5
6
Why do we need enzymes???
Speed
Specificity:
B
D (toxic product or
molecule without function)
A
C
E (beneficial molecule)
7
8
Why do we need enzymes???
Speed
Specificity
Control:
Inhibited by D
End product
A
E1
B
E2
C
E3
D
9
Enzyme unit:
amount of activity that catalyzes the
transformation of 1 umol of substrate / minute
10
active site
- binds the substrate
-has the amino acid groups
that participate in the reaction
Specific activity:
number of enzyme units/mg of protein
(umoles substrate /min /mg protein)
Reaction velocity:
amount of substrate modified per minute
(umoles substrate / min)
11
12
Glucokinase specificity
13
14
15
16
Cofactors
Molecules used by enzymes to increase the number
of functional groups that participate in the reaction
1) Metal ions
2) Organic coenzymes
Thiamine pyrophosphate
Important in decarboxylation
reactions
17
18
Michaelis-Menten kinetics
E + S ↔ [ES] → E + P
19
Km: reflects the affinity of an enzyme for a particular
20
substrate
The higher the Km , the lower the affinity for the substrate
A
E1
B
E2
C
E3
D
E4
E
Km for E2 >>> Km for E1, E3, or E4
E2 is the rate limiting enzyme
For a particular enzyme and substrate,
Km is a constant
21
23
a limiting rate enzyme in a pathway has a high Km
22
24
-1/Km
competitive inhibitor
likehaving
havingless
less substrate)
(is(is
like
substrate)
Enzyme inhibitors
25
Vmax remains the same
Km (apparent Km) increases
noncompetitive inhibitor
havingless
lessenzyme)
enzyme)
(is (is
likelike
having
Vmax is decreased 26
Km remains the same
Irreversible
inhibitors
Combine with or destroy a functional group
that is essential for enzyme activity
Several are pharmacologically
very important
Uncompetitive inhibition
(like having less S and less E)
27
28
acetylcholine
acetylcholinesterase
choline + acetate
(Aricept)
Penicillin is an irreversible inhibitor of the 29
bacterial enzyme glycopeptide transpeptidase
30
Allopurinol: suicide inhibitor
of xanthine oxidase
Inhibition of xanthine oxidase by allopurinol
31
for the treatment of gout
32
- Genetic control:
regulates the “amount” of enzyme
- Induction: activation of the gene encoding
an enzyme
- Repression: inhibition of the gene encoding
an enzyme
33
35
Regulation of glycogen phosphorylase
by phosphorylation
34
36
Zymogens or proenzymes: inactive enzyme precursors
Cleavage of fibrinogen and
clot formation
37
Triggered by
interaction of plasma
proteins with damaged
blood vessel
Triggered by
tissue factor
Exposed on damaged
tissue
38
Regulatory Enzymes
Allosteric Enzymes:
Do not follow Michaelis-Menten kinetics (hyperbolic),
they show sigmoideal plots
Have two or more subunits
The substrate binding sites exhibit co-operativity
Are modified by reversible non-covalent binding of
regulators
Allosteric activators lock the enzyme in a conformation
that has high affinity for the substrate
Blood coagulation cascade:
sequential activation of proenzymes
39
Allosteric inhibitors shift the enzyme to a conformation
40
that has less affinity for the substrate
Regulatory Enzymes
Allosteric Enzymes:
Do not follow Michaelis-Menten kinetics (hyperbolic),
they show sigmoideal plots
Have two or more subunits
The substrate binding sites exhibit co-operativity
Are modified by reversible non-covalent binding of
regulators
Allosteric activators lock the enzyme in a conformation
that has high affinity for the substrate
41
Allosteric inhibitors shift the enzyme to a conformation
42
that has less affinity for the substrate
43
44
Regulatory Enzymes
Allosteric Enzymes:
Do not follow Michaelis-Menten kinetics (hyperbolic),
they show sigmoideal plots
Have two or more subunits
The substrate binding sites exhibit co-operativity
Are modified by reversible non-covalent binding of
regulators
Allosteric activators lock the enzyme in a conformation
that has high affinity for the substrate
Aspartate transcarbamylase
45
Allosteric inhibitors shift the enzyme to a conformation
46
that has less affinity for the substrate
Isoenzymes
they catalyze the same reaction
they have different amino acid
sequence and properties
feedback inhibition
their characteristics may be related
to their function in specific tissues
clinically important diagnostic tool
47
48
Isoforms of lactate dehydrogenase
Isoenzymes of creatine kinase
are used to diagnose
myocardial infarction
LDH 1 (H4) : heart, kidney, red cells
LDH 2 (H3 M): heart, kidney, red cells
LDH3 (H2 M2): brain, pancreas
creatine kinase:
LDH 4 (H M3): lung, spleen
CK-1 (BB): brain
CK-2 (MB): heart
CK-3 (MM): skeletal muscle
and heart
49
LDH 5 (M4): skeletal muscle, liver
50