ch 8 cellular metabolism how cells utilize energy
Post on 22-Dec-2015
217 views
TRANSCRIPT
Ch 8 Cellular MetabolismHow cells utilize energy
LE 8-2On the platform,the diver hasmore potentialenergy.
Diving convertspotentialenergy to kinetic energy.
Climbing up convertskinetic energy ofmuscle movement topotential energy.
In the water, the diver has lesspotential energy.
LE 8-3
Chemical energy
Heat CO2
First law of thermodynamics Second law of thermodynamics
H2O
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 productsReleases of energyExergonic rxn
Anabolism (anabolic rxn)Synthesis of larger high energy molecules fromlower energy reactantsRequires input of energyEndergonic reactions
LE 8-12
Pi
ADP
Energy for cellular work
(endergonic, energy-
consuming processes)
Energy from catabolism
(exergonic, energy-
yielding processes)
ATP
+
Cellular energy used for:transport (across membranes)mechanical work (motility, contraction)enzymatic activity (catalysis of reactions)
Catabolic rxn
C6H12O6 + 6O2 ----> 6CO2 + 6H2O + ATPglucose
Exergonic
Anabolic rxn
6CO2 + 6H2O ----> C6H12O6 + 6O2
glucose
Endergonic
Light
Examples
Biological rxns
-Catalyzed by enzymes
-Often arranged in multiple steps called pathways
LE 8-UN141
Enzyme 1
A B
Reaction 1
Enzyme 2
C
Reaction 2
Enzyme 3
D
Reaction 3
ProductStarting
molecule
Enzymatic Pathway
Enzymes
Biological catalysts
Increase rate of reactionsby 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
Transition state
C D
A B
EA
Products
C D
A B
G < O
Progress of the reaction
Reactants
C D
A B
Fre
e en
erg
y
Could raise temp.to break bonds
Why don’t cells rely on increases in temperature to break bonds?
Denaturation of proteins and damage to the cell.
LE 8-15
Course ofreactionwithoutenzyme
EA
without enzyme
G is unaffectedby enzyme
Progress of the reaction
Fre
e en
erg
y
EA withenzymeis lower
Course ofreactionwith enzyme
Reactants
Products
LE 8-13
SucroseC12H22O11
GlucoseC6H12O6
FructoseC6H12O6
Example:
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-substratecomplex
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
Enzyme-substratecomplex
Substrates
Enzyme
Products
Substrates enter active site; enzymechanges shape so its active siteembraces the substrates (induced fit).
Substrates held inactive site by weakinteractions, such ashydrogen bonds andionic bonds.
Active site (and R groups ofits 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.
Substrates areconverted intoproducts.
Products arereleased.
Activesite is
availablefor two new
substratemolecules.
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)
G= H-TS
Where H= change in total energy of the system, or enthalpy
T=absolute temperature in Kelvin (oC+273)
S =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
G = G final state - G initial state
If the reaction gives final products that have less energy than the initial reactants, is G negative or positive?
The reverse?
When G < 0, the reaction is exergonic and spontaneous.
When G > 0, the reaction is endergonic and not spontaneous.
(products) (reactants)
LE 8-6a
Reactants
Energy
Products
Progress of the reaction
Amount ofenergy
released(G < 0)
Fre
e en
erg
y
Exergonic reaction: energy released
Catabolic rxn
C6H12O6 + 6O2 ----> 6CO2 + 6H2O + ATPglucose
LE 8-6b
ReactantsEnergy
Products
Progress of the reaction
Amount ofenergy
required(G > 0)
Fre
e en
erg
y
Endergonic reaction: energy required
Anabolic rxn
6CO2 + 6H2O ----> C6H12O6 + 6O2
glucose
Light
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 (G=0)
• If the metabolism of a cell is at equilibrium, what has occurred?
RIP
LE 8-12
Pi
ADP
Energy for cellular work
(endergonic, energy-
consuming processes)
Energy from catabolism
(exergonic, energy-
yielding processes)
ATP
+
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
Phosphate groups
Ribose (sugar)
Adenine (base)
ATP structure Adenosine triphosphate
Cellular energy currency
LE 8-9
Adenosine triphosphate (ATP)
Energy
P P P
PPP i
Adenosine diphosphate (ADP)Inorganic phosphate
H2O
+ +
• 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
Phosphate groups
Ribose
Adenine
• Energy from ATP hydrolysis – drives endergonic reactions
• Overall, coupled reactions are exergonic
LE 8-10
Endergonic reaction: G is positive, reactionis not spontaneous
Exergonic reaction: G is negative, reactionis spontaneous
G = +3.4 kcal/mol
G = –7.3 kcal/mol
G = –3.9 kcal/mol
NH2
NH3Glu Glu
Glutamicacid
Coupled reactions: Overall G is negative;together, reactions are spontaneous
Ammonia Glutamine
ATP H2O ADP P i
+
+ +
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
NH2
Glu
P i
P i
P i
P i
Glu NH3
P
P
P
ATPADP
Motor protein
Mechanical work: ATP phosphorylates motor proteins
Protein moved
Membraneprotein
Solute
Transport work: ATP phosphorylates transport proteins
Solute transported
Chemical work: ATP phosphorylates key reactants
Reactants: Glutamic acidand ammonia
Product (glutamine)made
+ +
+
Regeneration of ATP
• ADP + P i--> ATP
– Energy for ADP phosphorylation from catabolic reactions
LE 8-12
Pi
ADP
Energy for cellular work
(endergonic, energy-
consuming processes)
Energy from catabolism
(exergonic, energy-
yielding processes)
ATP
+
Environmental Conditions Affect Enzyme Function
?
Temperature: cold-->decreased chance of bumping into substratehot--> 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 fortypical human enzyme
Optimal temperature forenzyme of thermophilic (heat-tolerant bacteria)
Temperature (°C)
Optimal temperature for two enzymes
0 20 40 60 80 100
Ra
te o
f re
ac
tio
n
Optimal pH for pepsin(stomach enzyme)
Optimal pHfor trypsin(intestinalenzyme)
pH
Optimal pH for two enzymes
0
Ra
te o
f re
ac
tio
n
1 2 3 4 5 6 7 8 9 10
What isyournormalbodytemp.?
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 EnzymesEnzyme 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 8-19Substrate
Active site
Enzyme
Competitiveinhibitor
Normal binding
Competitive inhibition
Noncompetitive inhibitor
Noncompetitive inhibition
A substrate canbind normally to the
active site of anenzyme.
A competitiveinhibitor mimics the
substrate, competingfor the active site.
A noncompetitiveinhibitor binds to the
enzyme away from theactive site, altering the
conformation of theenzyme so that its
active site no longerfunctions.
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 enzymewith four subunits
Regulatorysite (oneof four) Active form
Activator
Stabilized active form
Active site(one of four)
Allosteric activatorstabilizes active form.
Non-functionalactive site
Inactive formInhibitor
Stabilized inactive form
Allosteric inhibitorstabilizes inactive form.
Oscillation
Allosteric activators and inhibitors
• 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
Substrate
Binding of one substrate molecule toactive site of one subunit locks allsubunits in active conformation.
Cooperativity another type of allosteric activation
Stabilized active formInactive form
Feedback Inhibition
• End product of a metabolic pathway shuts down the pathway
• Prevents over-production of unneededmolecules
LE 8-21
Active siteavailable
Initial substrate(threonine)
Threoninein active site
Enzyme 1(threoninedeaminase)
Enzyme 2
Intermediate A
Isoleucineused up bycell
Feedbackinhibition Active site of
enzyme 1 can’tbindtheoninepathway off
Isoleucinebinds toallostericsite
Enzyme 3
Intermediate B
Enzyme 4
Intermediate C
Enzyme 5
Intermediate D
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!