Transcript Cellular Energy and Enzymatic Function

Ch 8 Cellular Metabolism
How cells utilize energy
LE 8-2
On the platform,
the diver has
more potential
energy.
Diving converts
potential
energy to
kinetic energy.
Climbing up converts
kinetic energy of
muscle movement to
potential energy.
In the water, the
diver has less
potential energy.
LE 8-3
Heat
Chemical
energy
First law of thermodynamics
CO2
H2O
Second law of thermodynamics
The First Law of Thermodynamics
– Energy cannot be created or destroyed
– Energy can be transferred and transformed
Principle of conservation of energy
The Second Law of Thermodynamics
• Every energy transfer or transformation increases the
entropy (disorder) of the universe
• Because some energy is lost as heat (unusable)
• Metabolism
– an organism’s (or cell’s) total chemical reactions
Name a common cellular reaction.
Two kinds of reactions:
Catabolism (catabolic rxn)
Breakdown of a larger molecule into smaller
lower energy products
Releases of energy
Exergonic rxn
Anabolism (anabolic rxn)
Synthesis of larger high energy molecules from
lower energy reactants
Requires input of energy
Endergonic reactions
LE 8-12
ATP
Energy for cellular work
(endergonic, energyconsuming processes)
Energy from catabolism
(exergonic, energyyielding processes)
ADP +
P
i
Cellular energy used for:
transport (across membranes)
mechanical work (motility, contraction)
enzymatic activity (catalysis of reactions)
Examples
Catabolic rxn
C6H12O6 + 6O2 ----> 6CO2 + 6H2O + ATP
glucose
Exergonic
Anabolic rxn
Light
6CO2 + 6H2O ----> C6H12O6 + 6O2
glucose
Endergonic
Biological rxns
-Catalyzed by enzymes
-Often arranged in multiple steps
called pathways
LE 8-UN141
Enzymatic Pathway
Enzyme 1
A
B
Reaction 1
Starting
molecule
Enzyme 2
Enzyme 3
D
C
Reaction 2
Reaction 3
Product
Enzymes
Biological catalysts
Increase rate of reactions
by lowering activation energy (EA)
Spontaneous reactions can take a long time!
Need enzymes to speed reactions for cell survival
Activation Energy (EA)
• Needed to destabilize bonds of reactants
LE 8-14
A
B
C
D
Free energy
Transition state
A
B
C
D
Could raise temp.
to break bonds
EA
Reactants
A
B
DG < O
C
D
Products
Progress of the reaction
Why don’t cells rely on increases in
temperature to break bonds?
Denaturation of proteins and damage to the cell.
LE 8-15
Free energy
Course of
reaction
without
enzyme
EA
without
enzyme
EA with
enzyme
is lower
Reactants
Course of
reaction
with enzyme
DG is unaffected
by enzyme
Products
Progress of the reaction
LE 8-13
Example:
Sucrose
C12H22O11
Glucose
C6H12O6
Fructose
C6H12O6
Structure & Function of Enzyme DRAW
• Enzymes bind substrate molecules (the
reactant)
• Substrates bind to active site on enzyme
• Binding induces conformational change in
enzyme--better ”fit” for substrate
• Active sites are highly specific and
discriminatory i.e. sucrase does not accept
lactose
LE 8-16
Substrate
Active site
Enzyme
Enzyme-substrate
complex
How does enzyme lower activation energy of reaction?
– Orients substrates for optimal interaction
–Strains substrate bonds
–Provides a favorable microenvironment
-Covalently bonds to the substrate
LE 8-17
Substrates enter active site; enzyme
changes shape so its active site
embraces the substrates (induced fit).
Substrates held in
active site by weak
interactions, such as
hydrogen bonds and
ionic bonds.
Substrates
Enzyme-substrate
complex
Active
site is
available
for two new
substrate
molecules.
Enzyme
Products are
released.
Substrates are
converted into
products.
Products
Active site (and R groups of
its amino acids) can lower EA
and speed up a reaction by
• acting as a template for
substrate orientation,
• stressing the substrates
and stabilizing the
transition state,
• providing a favorable
microenvironment,
• participating directly in the
catalytic reaction.
How do we know when a reaction is exergonic or endergonic?
Measure the system’s ability to perform work (usable energy)
at uniform temperature and pressure.
Change in Gibbs free energy (G)
DG= DH-TDS
Where DH= change in total energy of the system,
or enthalpy
T=absolute temperature in Kelvin (oC+273)
DS =change in entropy (a measure of disorder)
Another way to think about the state of energy in a cell is before
and after a particular reaction occurs
DG = G final state - G initial state
(products)
(reactants)
If the reaction gives final products that have less energy than the
initial reactants, is DG negative or positive?
The reverse?
When DG < 0, the reaction is exergonic and spontaneous.
When DG > 0, the reaction is endergonic and not spontaneous.
LE 8-6a
Reactants
Free energy
Amount of
energy
released
(DG < 0)
Products
Energy
Progress of the reaction
Exergonic reaction: energy released
Catabolic rxn
C6H12O6 + 6O2 ----> 6CO2 + 6H2O + ATP
glucose
LE 8-6b
Free energy
Products
Energy
Amount of
energy
required
(DG > 0)
Reactants
Progress of the reaction
Endergonic reaction: energy required
Anabolic rxn
Light
6CO2 + 6H2O ----> C6H12O6 + 6O2
glucose
Relationship among Free Energy, Instability,
and Equilibrium
• Free energy:
– a measure of a system’s instability, its tendency to change to a
more stable state
• During a spontaneous change
– free energy decreases and the stability of a system increases
• Equilibrium is a state of maximum stability (DG=0)
• If the metabolism of a cell is at equilibrium, what has occurred?
RIP
LE 8-12
ATP
Energy for cellular work
(endergonic, energyconsuming processes)
Energy from catabolism
(exergonic, energyyielding processes)
ADP +
P
i
Cellular energy used for:
transport (across membranes)
mechanical work (motility, contraction)
enzymatic activity (catalysis of reactions)
ATP structure
• ATP
– adenosine triphosphate
• cellular energy carrier
LE 8-8
ATP structure
Adenosine triphosphate
Cellular energy currency
g
b
Adenine (base)
a
Phosphate groups
Ribose (sugar)
LE 8-9
P
P
P
Adenosine triphosphate (ATP)
H2O
Pi
+
Inorganic phosphate
P
P
Adenosine diphosphate (ADP)
+
Energy
• Terminal phosphate bond (ATP--> ADP + Pi)
– Hydrolysis of “high energy” phosphate bond
• Energy is released (exergonic)
• ADP lower energy than ATP
• Why?
• Is ADP more stable than ATP? Explain.
LE 8-8
Adenine
g
b
a
Phosphate groups
Ribose
• Energy from ATP hydrolysis
– drives endergonic reactions
• Overall, coupled reactions are exergonic
LE 8-10
Endergonic reaction: DG is positive, reaction
is not spontaneous
NH2
Glu
+
NH3
Ammonia
Glutamic
acid
DG = +3.4 kcal/mol
Glu
Glutamine
Exergonic reaction: DG is negative, reaction
is spontaneous
ATP
+
H2O
ADP
Coupled reactions: Overall DG is negative;
together, reactions are spontaneous
+
Pi
DG = –7.3 kcal/mol
DG = –3.9 kcal/mol
How ATP Performs Work
• Inorganic phosphate from ATP hydrolysis
– Transferred to target molecule
• Called phosphorylation
• Creates highly reactive, unstable target molecule
• More prone to do “work” or change (conformation)
– Mechanical, transport, enzymatic
LE 8-11
Pi
P
Motor protein
Protein moved
Mechanical work: ATP phosphorylates motor proteins
Membrane
protein
ADP
+
Pi
ATP
Pi
P
Solute transported
Solute
Transport work: ATP phosphorylates transport proteins
P
NH2
Glu
+
NH3
+
Pi
Glu
Reactants: Glutamic acid
and ammonia
Product (glutamine)
made
Chemical work: ATP phosphorylates key reactants
Regeneration of ATP
• ADP + P i--> ATP
– Energy for ADP phosphorylation from catabolic
reactions
LE 8-12
ATP
Energy for cellular work
(endergonic, energyconsuming processes)
Energy from catabolism
(exergonic, energyyielding processes)
ADP +
P
i
Environmental Conditions Affect
Enzyme Function
?
Temperature: cold-->decreased chance of bumping into substrate
hot--> good chance of substrate interaction but
chance of denaturation at some point
pH->change in charge (H+ or OH-) can denature proteins
and substrate
Examples of pH sensitive enzymes?
LE 8-18
Optimal temperature for
typical human enzyme
What is
your
normal
body
temp.?
0
Optimal temperature for
enzyme of thermophilic
(heat-tolerant
bacteria)
40
60
Temperature (°C)
20
80
100
Optimal temperature for two enzymes
Optimal pH for pepsin
(stomach enzyme)
0
1
2
3
4
Optimal pH
for trypsin
(intestinal
enzyme)
5
pH
Optimal pH for two enzymes
6
7
8
9
10
Cofactors
• Non-protein enzyme helpers (like metals
(Fe))
•Coenzymes
•organic cofactors
•Vitamins
•e.g. Vitamin K: required for blood clotting &
Required in certain carboxylation reactions
Regulation of Enzymes
Enzyme Inhibitors
• Competitive inhibitor
– binds to active site of enzyme
– blocks substrate binding by competition
•Noncompetitive inhibitor
– binds to another part of enzyme
– causes enzyme to change shape
– prevents active site from binding substrate
–Allosteric effect
DRAW
LE 8-19
A substrate can
bind normally to the
active site of an
enzyme.
Substrate
Active site
Enzyme
Normal binding
A competitive
inhibitor mimics the
substrate, competing
for the active site.
Competitive
inhibitor
Competitive inhibition
A noncompetitive
inhibitor binds to the
enzyme away from the
active site, altering the
conformation of the
enzyme so that its
active site no longer
functions.
Noncompetitive inhibitor
Noncompetitive inhibition
Allosteric Regulation of Enzymes
• Where protein function at one site is affected
by binding of a regulatory molecule at another
site
• May inhibit or stimulate enzyme activity
Allosteric Activation and Inhibition
• Most allosterically regulated enzymes are made from
polypeptide subunits
• active and inactive forms
• binding of activator stabilizes active form of enzyme
• binding of inhibitor stabilizes inactive form of enzyme
LE 8-20a
Allosteric enzyme
with four subunits
Regulatory
site (one
of four)
Active site
(one of four)
Activator
Active form
Oscillation
Nonfunctional
active site
Allosteric activator
stabilizes active form.
Inactive form
Stabilized active form
Allosteric inhibitor
stabilizes inactive form.
Inhibitor
Allosteric activators and inhibitors
Stabilized inactive
form
• Cooperativity
– form of allosteric regulation that can amplify enzyme
activity
• binding of substrate to one active site stabilizes
favorable conformational changes at all other
subunits
LE 8-20b
Binding of one substrate molecule to
active site of one subunit locks all
subunits in active conformation.
Substrate
Inactive form
Stabilized active form
Cooperativity another type of allosteric activation
Feedback Inhibition
• End product of a metabolic pathway shuts
down the pathway
• Prevents over-production of unneeded
molecules
LE 8-21
Initial substrate
(threonine)
Active site
available
Isoleucine
used up by
cell
Threonine
in active site
Enzyme 1
(threonine
deaminase)
Intermediate A
Feedback
inhibition
Enzyme 2
Active site of
enzyme 1 can’t
bind
Intermediate B
theonine
pathway off
Enzyme 3
Isoleucine
binds to
allosteric
site
Intermediate C
Enzyme 4
Intermediate D
Enzyme 5
End product
(isoleucine)
Metabolic regulation influenced by cellular localization
• Cellular structures organize and concentrate
enzymes in pathways
– Membranes, organelles (mitochondria, chloroplast)
LE 8-22
Mitochondria,
sites of cellular respiration
1 µm
LE 8-22
It’s nice to get so much attention!