09/01/2009biology 401: thermodynamics ii1 thermodynamics, concluded andy howard biochemistry, fall...
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09/01/2009Biology 401: Thermodynamics IIp. 3 of 27 Thermodynamics Special topics in Thermodynamics –Free energy –Equilibrium –Work –Coupled reactions –ATP: energy currency –Other high-energy compounds –Dependence on concentrationTRANSCRIPT
09/01/2009 Biology 401: Thermodynamics II 1
Thermodynamics, concludedThermodynamics, concluded
Andy HowardBiochemistry, Fall 2009
IIT
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Thermodynamics matters!Thermodynamics matters!
•Thermodynamics tells us which reactions will go forward and which ones won’t.
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ThermodynamicsThermodynamics
•Special topics in Thermodynamics– Free energy– Equilibrium– Work– Coupled reactions– ATP: energy currency– Other high-energy compounds– Dependence on concentration
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Entropy matters a lot!Entropy matters a lot!
•Most biochemical reactions involve very small ( < 10 kJ/mol) changes in enthalpy
•Driving force is often entropic• Increases in solute entropy often is
at war with decreases in solvent entropy.
•The winner tends to take the prize.
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Apolar molecules in waterApolar molecules in water
•Water molecules tend to form ordered structure surrounding apolar molecule
• Entropy decreases because they’re so ordered
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Binding to surfacesBinding to surfaces
•Happens a lot in biology, e.g.binding of small molecules to relatively immobile protein surfaces
•Bound molecules suffer a decrease in entropy because they’re trapped
•Solvent molecules are displaced and liberated from the protein surface
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Free EnergyFree Energy
•Gibbs: Free Energy EquationG = H - TS
•So if isothermal, G = H - TS•Gibbs showed that a reaction will
be spontaneous (proceed to right) if and only if G < 0
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Standard free energy of Standard free energy of formation, formation, GGoo
ff
•Difference between compound’s free energy & sum of free energy of the elements from which it is composed
Substance Gof, kJ/mol Substance Go
f, kJ/molLactate -516 Pyruvate -474Succinate -690 Glycerol -488Acetate -369 Oxaloacetate -797
HCO3- -394
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Free energy and equilibriumFree energy and equilibrium
•Gibbs: Go = -RT ln Keq
•Rewrite: Keq = exp(-Go/RT)
• Keq is equilibrium constant;formula depends on reaction type
•For aA + bB cC + dD,Keq = ([C]c[D]d)/([A]a[B]b)
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Spontaneity and free energySpontaneity and free energy
• Thus if reaction is just spontaneous, i.e. Go = 0, then Keq = 1
• If Go < 0, then Keq > 1: Exergonic
• If Go > 0, then Keq < 1: Endergonic• You may catch me saying “exoergic”
and “endoergic” from time to time:these mean the same things.
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Free energy as a source of workFree energy as a source of work
•Change in free energy indicates that the reaction could be used to perform useful work
• If Go < 0, we can do work• If Go > 0, we need to do work to
make the reaction occur
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What kind of work?What kind of work?
•Movement (flagella, muscles)•Chemical work:
– Transport molecules against concentration gradients
– Transport ions against potential gradients•To drive otherwise endergonic reactions
– by direct coupling of reactions– by depletion of products
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Coupled reactionsCoupled reactions
•Often a single enzyme catalyzes 2 reactions, shoving them together:reaction 1, A B: Go
1 < 0 reaction 2, C D: Go
2 > 0 •Coupled reaction:
A + C B + D: GoC = Go
1 + Go2
•If GoC < 0,
then reaction 1 is driving reaction 2!
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How else can we win?How else can we win?
• Concentration of product may play a role
• As we’ll discuss in a moment, the actual free energy depends on Go
and on concentration of products and reactants
• So if the first reaction withdraws product of reaction B away,that drives the equilibrium of reaction 2 to the right
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Adenosine TriphosphateAdenosine Triphosphate
• ATP readily available in cells• Derived from catabolic reactions• Contains two high-energy phosphate bonds
that can be hydrolyzed to release energy: O O-
|| |(AMP)-O~P-O~P-O-
| || O- O
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Hydrolysis of ATPHydrolysis of ATP
• Hydrolysis at the rightmost high-energy bond:ATP + H2O ADP + Pi Go = -33kJ/mol
•Hydrolysis of middle bond:ATP + H2O AMP + PPi
Go = -33kJ/mol•BUT PPi + H2O 2 Pi,
Go = -33 kJ/mol•So, appropriately coupled, we get roughly
twice as much!
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ATP as energy currencyATP as energy currency
• Any time we wish to drive a reaction that has
Go < +30 kJ/mol, we can couple it to ATP hydrolysis and come out ahead
• If the reaction we want hasGo < +60 kJ/mol, we can couple it toATP AMP and come out ahead
• So ATP is a convenient source of energy — an energy currency for the cell
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Coin analogyCoin analogy
•Think of store of ATPas a roll of quarters
•Vendors don’t give change•Use one quarter for some reactions,
two for others• Inefficient for buying $0.35 items
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Other high-energy compoundsOther high-energy compounds
•Creatine phosphate: ~ $0.40•Phosphoenolpyruvate: ~ $0.35•So for some reactions, they’re more
efficient than ATP
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Why not use those always? Why not use those always?
•There’s no such thing as a free lunch!
• In order to store a compound, you have to create it in the first place
•So an intermediate-energy currency is the most appropriate
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Dependence on ConcentrationDependence on Concentration
•Actual G of a reaction is related to the concentrations / activities of products and reactants: G = Go + RT ln [products]/[reactants]
• If all products and reactants are at 1M, then the second term drops away; that’s why we describe Go as the standard free energy
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Is [A] = [B] = 1M… realistic?Is [A] = [B] = 1M… realistic?
• No, but it doesn’t matter;as long as we can define the concentrations, we can correct for them
• Often we can rig it so[products]/[reactants] = 1even if all the concentrations are small
• Typically [ATP]/[ADP] > 1 so ATP coupling helps even more than 33 kJ/mol!
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How does this matter?How does this matter?
•Often coupled reactions involve withdrawal of a product from availability
• If that happens,[product] / [reactant]shrinks, the second term becomes negative,and G < 0 even if Go > 0
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How to solve energy problems How to solve energy problems involving coupled equationsinvolving coupled equations•General principles:
– If two equations are added, their energetics add
– An item that appears on the left and right side of the combined equation can be cancelled
– Reversing a reaction reverses the sign of G.
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A bit more detailA bit more detail
•Suppose we couple two equations:A + B C + D, Go’ = xC + F B + G, Go’ = y
•The result is:A + B + C + F B + C + D + GorA + F D + G, Go’ = x + y
•… since B and C appear on both sides
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Slightly more complex…Slightly more complex…
•Suppose we couple two equations:A + B C + D, Go’ = xH + A J + C, Go’ = z
•Reverse the second equation:J + C A + H, Go’ = -z
•Add this to 1st eqn. & simplify:B + J D + H, Go’ = x - z
•… since A and C appear on both sides
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What do we mean by What do we mean by hydrolysishydrolysis??• It simply means a reaction with water•Typically involves cleaving a bond:•U + H2O V + W
is described as hydrolysis of Uto yield V and W