chapter 6 lecture slides - health science...
TRANSCRIPT
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CHAPTER 6
LECTURE
SLIDES
Prepared by
Brenda Leady University of Toledo
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2
Energy
Ability to promote change or do work
2 forms
Kinetic- associated with movement
Potential- due to structure or location
Chemical energy- energy in molecular bonds
3
4
2 Laws of thermodynamics
1. First law
Law of conservation of energy
Energy cannot be created or destroyed
Can be transformed from one type to another
2. Second law
Transfer or transformation of energy from
one form to another increases entropy or
degree of disorder of a system
5
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Highly
ordered
6
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Increase
More disordered
in entropy
Highly
ordered
7
Change in free energy determines direction
Total energy = Usable energy + Unusable energy
Energy transformations involve an increase
in entropy
Entropy - a measure of the disorder that
cannot be harnessed to do work
8
H = G + TS
H= enthalpy or total energy
G= free energy or amount of energy
for work
S= entropy or unusable energy
T= absolute temperature in Kelvin (K)
9
Spontaneous reactions?
Occur without input of additional energy
Not necessarily fast
Key factor is the free energy change
10
ΔG = Gproducts - Greactants
Exergonic
ΔG<0 or negative free energy change
Spontaneous
Endergonic
ΔG>0 or positive free energy change
Requires addition of free energy
Not spontaneous
11
Hydrolysis of ATP
ΔG = -7.3 kcal/mole
Reaction favors
formation of
products
Energy liberated can
drive a variety of
cellular processes
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Adenine (A)
Ribose
Phosphate groups
Phosphate (Pi) Adenosine diphosphate (ADP)
Adenosine triphosphate (ATP)
+ ~
H H
OH
O
OH
H H
O O
O
H2C
O–
NH2
N
N
H
N
N
P OH
O
O–
P O–
O
O–
P HO
~ ~
H H
OH
O
OH
H H
O O
O
H2C
O–
NH2
N
N
H
N
N
P O
O
O–
P
O
O–
P O–
H2O Hydrolysis
of ATP
12
Cells use ATP hydrolysis
An endergonic reaction can be coupled to
an exergonic reaction
Endergonic reaction will be spontaneous if
net free energy change for both processes
is negative
13
Glucose + phosphate → glucose-phosphate + H2O
ΔG = +3.3 Kcal/mole
endergonic
ATP + H2O → ADP + Pi
ΔG = -7.3 Kcal/mole
exergonic
Coupled reaction: Glucose + ATP → glucose-phosphate + ADP
ΔG = -4.0 Kcal/mole
exergonic
14
Enzymes and Ribozymes
A spontaneous reaction is not necessarily
a fast reaction
Catalyst- agent that speeds up the rate of
a chemical reaction without being
consumed during the reaction
Enzymes- protein catalysts in living cells
Ribozymes- RNA molecules with catalytic
properties
15
Activation energy
Initial input of energy to start reaction
Allows molecules to get close enough to
cause bond rearrangement
Can now achieve transition state where
bonds are stretched
16
Overcoming activation energy
2 common ways
Large amounts of heat
Using enzymes to lower activation energy
Small amount of heat can now push reactants to
transition state
17
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Progress of an exergonic reaction
Fre
e e
nerg
y (
G)
Transition state
Reactants
Reactant molecules
Enzyme
ATP
Glucose
Products
Activation energy (EA)
without enzyme
Activation energy (EA)
with enzyme
Change
in free
energy
(G)
18
Lowering activation energy
Straining bonds in reactants to make it
easier to achieve transition state
Positioning reactants together to facilitate
bonding
Changing local environment
Direct participation through very temporary
bonding
19
Other enzyme features
Active site- location where reaction takes
place
Substrate- reactants that bind to active site
Enzyme-substrate complex formed when
enzyme and substrate bind
20
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Glucose
Substrates
Active site ATP
21
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Glucose Substrates
Active site ATP
Enzyme-substrate complex
22
Glucose Substrates
Active site ATP
Enzyme-substrate complex
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23
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Glucose Substrates
Active site ATP
Enzyme-substrate complex
Glucose-
phosphate
ADP
24
Substrate binding
Enzymes have a high affinity or high
degree of specificity for a substrate
Example of a lock and key for substrate
and enzyme binding
Induced fit- interaction also involves
conformational changes
Enzyme reactions
Saturation- plateau where nearly all active sites
occupied by substrate
Vmax = velocity of reaction near maximal rate
Km = substrate concentration at which velocity is
half maximal value
Also called Michaelis constant
High Km enzyme needs higher substrate
concentration
Inversely related to affinity between enzyme and
substrate
25
26
Velo
cit
y
(pro
du
ct/
seco
nd
)
[Substrate]
A
B
Vmax
2
Vmax C
D
A
60 sec
Low
B
60 sec
Moderate
C
60 sec
High
D
60 sec
(a) Reaction velocity in the absence of inhibitors
0
Amount of
enzyme
Tube
Incubation
time
Substrate
concentration
Very
high
1 m 1 m 1 m 1 m
KM
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Inhibition
Competitive inhibition
Molecule binds to active site
Inhibits ability of substrate to bind
Apparent Km increases- more substrate
needed
Noncompetitive
Lowers Vmax without affecting Km
Inhibitor binds to allosteric site- not active site
27
28
KM with inhibitor [Substrate]
(b) Competitive inhibition
Ve
loc
ity
(pro
du
ct/
se
co
nd
)
Plus competitive inhibitor
Substrate
Inhibitor
Enzyme
Vmax
KM [Substrate]
(c) Noncompetitive inhibition
Ve
loc
ity
(pro
du
ct/
se
co
nd
)
Substrate
Inhibitor
Enzyme
Allosteric site
0
0
KM
Vmax
V max with inhibitor
Plus noncompetitive
inhibitor
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29
Other requirements for enzymes
Prosthetic groups- small molecules
permanently attached to the enzyme
Cofactor- usually inorganic ion that
temporarily binds to enzyme
Coenzyme- organic molecule that
participates in reaction but left unchanged
afterward
30
Enzymes are affected by environment
Most enzymes function maximally in a
narrow range of temperature and pH
Outside of this narrow range, enzyme
function decreases
31
0 0
10 20
Rate
of
a
ch
em
ical
reac
tio
n
30 40 50 60
High
Temperature (ºC)
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Discovery of Ribozymes
Until 1980s, scientists thought all biological catalysts were proteins
Ribonuclease P (Rnase P) found in all living organisms
Involved in processing tRNA molecules
Ribonucleoprotein- 1 RNA and 1 protein subunit
Experiments found RNA subunit alone was able to cleave substrate
True catalyst- accelerates rate without being altered
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ptRNA
tRNA
ptRNA
ptRNA
tRNA
Experimental level Conceptual level
MgCl2
Low MgCl2
(10 mM)
High MgCl2
(100 mM)
RNA
subunit
alone
RNA subunit
plus protein
subunit
Higher
mass
Lower
mass
3´
5´
RNA subunit
alone cuts here
3´
5´
5´ fragment
5´ +
5´ fragment
Catalytic function will
result in the digestion
of ptRNA into tRNA and
a smaller 5´ fragment .
5´ fragment
ptRNA
3´
5´
1 2 3 4 5
THE DATA 5
© Altman, S., (1990). Nobel Lecture: Enzymatic Cleavage of RNA by RNA. Bioscience Reports, 10, 317–337. Fig. 7
34
Overview of metabolism
Chemical reactions occur in metabolic
pathways
Each step is coordinated by a specific
enzyme
Catabolic pathways
Result in breakdown and are exergonic
Anabolic pathways
Promote synthesis and are endergonic
Must be coupled to exergonic reaction
35
Initial substrate
OH
OH OH
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36
PO42—
Enzyme 1
Initial substrate Intermediate 1
OH
OH OH OH OH
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37
Enzyme 1
Initial substrate Intermediate 1 Intermediate 2
Enzyme 2
OH
OH OH OH OH OH
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PO42— PO4
2—
PO42—
38
Enzyme 1
Initial substrate Intermediate 1 Intermediate 2 Final product
Enzyme 2 Enzyme 3
OH
OH OH OH OH OH
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PO42— PO4
2— PO42—
PO42— PO4
2— PO42—
39
Catabolic reactions
Breakdown of reactants
Used for recycling
Used to obtain energy for endergonic
reactions
Energy stored in energy intermediates
ATP, NADH
40
2 ways to make ATP
1. Substrate-level phosphorylation
Enzyme directly transfers phosphate from
one molecule to another molecule
2. Chemiosmosis
Energy stored in an electrochemical gradient
is used to make ATP from ADP and Pi
41
Redox
Oxidation
Removal of electrons
Reduction
Addition of electrons
Redox reaction
Electron removed from one molecule is added
to another
42
Ae- + B → A + Be-
A
Has been oxidized
Electron removed
B
Has been reduced
Electron added
43
Energy intermediates
Electrons removed by oxidation are used
to create energy intermediates like NADH
NAD+ Nicotinamide adenine dinucleotide
NADH…
Oxidized to make ATP
Can donate electrons during synthesis
reactions
44
Adenine
Nicotinamide
Nicotinamide
adenine
dinucleotide
(NAD+)
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+ 2e– + H+
+
45
Reduction
Oxidation
Adenine
Nicotinamide
NADH
(an electron
carrier)
Nicotinamide
adenine
dinucleotide
(NAD+)
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+ 2e– + H+
+
46
Anabolic reactions
Biosynthetic reactions
Make large macromolecules or smaller
molecules not available from food
Many proteins use ATP as a
source of energy
Each ATP undergoes 10,000 cycles of hydrolysis and resynthesis every day
Particular amino acid sequences in proteins function as ATP-binding sites
Can predict whether a newly discovered protein uses ATP or not
On average, 20% of all proteins bind ATP
Likely underestimated because there may be other types of ATP-binding sites
Enormous importance of ATP as energy source
Synthesis
Hydrolysis
ADP + Pi
ATP + H2O Energy release
(Exergonic)
Energy input
(Endergonic)
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49
Regulation of metabolic pathways
1. Gene regulation
Turn on or off genes
2. Cellular regulation
Cell-signaling pathways like hormones
3. Biochemical regulation
Feedback inhibition- product of pathway
inhibits early steps to prevent
overaccumulation of product
50
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Enzyme 1
Initial substrate
Allosteric site
Active site
51
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Enzyme 1
Initial substrate
Allosteric site
Intermediate 1
Active site
Final product
Conformational
change
52
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Enzyme 1
Initial substrate
Allosteric site
Intermediate 1
Enzyme 2
Active site
Final product
Conformational
change
53
Enzyme 1
Initial substrate
Allosteric site
Intermediate 1 Intermediate 2
Enzyme 2
Active site
Final product
Conformational
change
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54
Enzyme 1
Initial substrate
Allosteric site
Intermediate 1 Intermediate 2
Enzyme 2
Enzyme 3
Active site
Final product
Conformational
change
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55
Enzyme 1
Initial substrate
Conformational
change
Allosteric site
Intermediate 1 Intermediate 2 Final product
Enzyme 2
Enzyme 3
Active site
Final product
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56
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Enzyme 1
Initial substrate
Conformational
change
Allosteric site
Intermediate 1 Intermediate 2 Final product
Enzyme 2
Enzyme 3
Active site
Final product
57
Recycling
Most large molecules exist for a relatively
short period of time
Half-life- time it takes for 50% of the
molecules to be broken down and recycled
All living organisms must efficiently use
and recycle organic molecules
Expression of genome allows cells to
respond to changes in their environment
RNA and proteins made when needed
Broken down when they are not
mRNA degradation important
Conserve energy by degrading mRNAs for
proteins no longer required
Remove faulty copies of mRNA
58
mRNA degradation
Exonucleases
Enzyme cleaves off nucleotides from end
Exosome
Multiprotein complex uses exonucleases
59
60
A Cap mRNA
Poly A tail 5´
3´
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61
A Cap mRNA
Poly A tail
Poly A tail is shortened.
5´
5´
3´
3´
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62
A Cap mRNA
Poly A tail
Poly A tail is shortened.
5´
5´ cap is removed.
5´ 3´
5´
3´
3´
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63
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A
Exonuclease
Cap mRNA
Poly A tail
Poly A tail is shortened.
5´
5´ cap is removed.
Nucleotides
are recycled.
5´ 3´
RNA is degraded in
the 5´ to 3´ direction
via an exonuclease.
5´
3´
3´
3´
64
A
Exonuclease
Cap mRNA
Poly A tail
Poly A tail is shortened.
RNA is degraded in
the 3´ to 5´ direction
via the exosome.
5´
5´ cap is removed.
Nucleotides
are recycled.
5´ 3´
RNA is degraded in
the 5´ to 3´ direction
via an exonuclease.
5´
3´
Nucleotides
are recycled.
3´
5´
3´ Exosome
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65
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A
Exonuclease
Cap
Exosome
mRNA
Poly A tail
Poly A tail is shortened.
RNA is degraded in
the 3´ to 5´ direction
via the exosome.
5´
5´ cap is removed.
Nucleotides
are recycled.
5´ 3´
RNA is degraded in
the 5´ to 3´ direction
via an exonuclease.
5´
3´
(a) 5´ (b) 3´ 5´ degradation by exosome
Nucleotides
are recycled.
3´ degradation by exonuclease
3´
5´
3´
© Liu, Q., Greimann, J.C., and Lima, C.D., (2006). Reconstitution, activities, and structure of the eukaryotic exosome. Cell, 127, 1223-1237.
Graphic generated using DeLano, W.L. (2002). The PyMOL Molecular Graphics System (San Carlos, CA, USA, DeLano Scientific)
Lysosomes contain hydrolases to break
down proteins, carbohydrates, nucleic
acids, and lipids
Digest substances taken up by endocytosis
Autophagy- recycling worn out organelles
Autophagosome
Proteosome digests proteins targeted for
destruction with ubiquitin.
66
67
Ubiquitin
Target
protein
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68
Ubiquitin
Target
protein
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69
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Ubiquitin
Target
protein
70
Cap
1
2
3
4
Cap
(a) Structure of the eukaryotic proteasome
Core
proteasome
(4 rings)
(b) Steps of protein degradation in eukaryotic cells
Ubiquitin
Target
protein
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71
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