Transcript Thermodynamics & ATP

Lecture 24
Thermodynamics in Biology
A Simple Thought Experiment
48 hours
1 E. coli cell (10 -11 mL)
1 mL H 2O
5 mg glucose
1 mg (NH 4)SO4
Mg++, PO4=, Fe3+, etc...
Glucose + (NH 4)SO4
109 cells
1 mL H 2O
0 mg glucose
<1 mg (NH 4)SO4
CO2
Cells + CO 2
Driving Forces for Natural Processes
• Enthalpy
– Tendency toward lowest energy state
• Form stablest bonds
• Entropy
– Tendency to maximize randomness
Enthalpy and Bond Strength
• Enthalpy = ∆H = heat change at constant pressure
• Units
– cal/mole or joule/mole
• 1 cal = 4.18 joule
• Sign
– ∆H is negative for a reaction that liberates heat
Entropy and Randomness
Decreased
randomness
Myoglobin
153 free
amino acids
Entropy and Randomness
• Entropy = S = measure of randomness
– cal/deg·mole
• T∆S = change of randomness
• For increased randomness, sign is “+”
“System” Definition
System
Surroundings
Closed system:
No exchange of
mass or energy
“System” Definition
Isolated system:
Energy is exchanged
E
E
“System” Definition
M
Open System:
Mass and energy
are exchanged
E
M
E
Cells and Organisms: Open Systems
• Material exchange with surroundings
– Fuels and nutrients in (glucose)
– By-products out (CO2)
• Energy exchange
– Heat release (fermentation)
– Light release (fireflies)
– Light absorption (plants)
1st Law of Thermodynamics
• Energy is conserved, but transduction is allowed
• Transduction
One form
of E
Light
Another form
of E
Plants
Mayer: 1842
Chemical
bonds
2nd Law of Thermodynamics
• In all spontaneous processes, total entropy
of the universe increases
2nd Law of Thermodynamics
• ∆Ssystem + ∆Ssurroundings = ∆Suniverse > 0
• A cell (system) can decrease in entropy only
if a greater increase in entropy occurs in
surroundings
• C6H12O6 + 6O2  6CO2 + 6H2O
complex
simple
Entropy: A More Rigorous Definition
• From statistical mechanics:
– S = k lnW
• k = Boltzmann constant = 1.3810–23 J/K
• W = number of ways to arrange the
system
• S = 0 at absolute zero (-273ºC)
Gibbs Free Energy
• Unifies 1st and 2nd laws
• ∆G
– Gibbs free energy
– Useful work available in a process
• ∆G = ∆H – T∆S
– ∆H from 1st law
• Kind and number of bonds
– T∆S from 2nd law
• Order of the system
∆G
• Driving force on a reaction
• Work available  distance from equilibrium
• ∆G = ∆H – T∆S
– State functions
•
•
•
•
Particular reaction
T
P
Concentration (activity) of reactants and products
Equilibrium
• ∆G = ∆H – T∆S = 0
• So ∆H = T∆S
– ∆H is measurement of enthalpy
– T∆S is measurement of entropy
• Enthalpy and entropy are exactly balanced at
equilibrium
Effects of ∆H and ∆S on ∆G
Voet, Voet, and Pratt. Fundamentals of Biochemistry. 1999.
Standard State and ∆Gº
• Arbitrary definition, like sea level
• [Reactants] and [Products]
– 1 M or 1 atmos (activity)
• T = 25ºC = 298K
• P = 1 atmosphere
• Standard free energy change = ∆Gº
Biochemical Conventions: ∆Gº
• Most reactions at pH 7 in H2O
• Simplify ∆Gº and Keq by defining [H+] = 10–7 M
• [H2O] = unity
• Biochemists use ∆Gº and Keq
Relationship of ∆G to ∆Gº
• ∆G is real and ∆Gº is standard
• For A in solution
– GA = GA + RT ln[A]
}
º
• For reaction aA + bB  cC + dD
[C]c [D]d
– ∆G = ∆Gº + RT ln
[A]a [B]b
– Constant Variable
(from table)
Relationship Between ∆Gº and Keq
[C]c [D]d
• ∆G = ∆Gº + RT ln a
[A] [B]b
• At equilibrium, ∆G = 0, so
[C]c [D]d
– ∆Gº = –RT ln
[A]a [B]b
– ∆Gº = –RT ln Keq
Relationship Between Keq and ∆Gº
Keq ²G º (kJ/mol)
10-6
34.3
10-5
28.5
10-4
21.4
-3
10
17.2
-2
10
11.3
10-1
5.9
1
0.0
1
10
-5.9
102
-11.3
103
-17.2
Will Reaction Occur Spontaneously?
A+B
C+D
• When:
– ∆G is negative, forward reaction tends to occur
– ∆G is positive, back reaction tends to occur
– ∆G is zero, system is at equilibrium
c [D]d
[C]
∆G = ∆Gº + RT ln
[A]a [B]b
A Caution About ∆Gº
• Even when a reaction has a large, negative
∆Gº, it may not occur at a measurable rate
• Thermodynamics
– Where is the equilibrium point?
• Kinetics
– How fast is equilibrium approached?
• Enzymes change rate of reactions, but do
not change Keq
∆Gº is Additive (State Function)
Reaction
AB
BC
Sum: A  C
Free energy change
∆G1º
∆G2º
∆G1º + ∆G2º
Also: B  A
– ∆G1º
Coupling Reactions
∆Gº
kcal/mol kJ/mol
Glucose + HPO42–  Glucose-6-P
+3.3
+13.8
ATP
 ADP + HPO42–
–7.3
–
ATP + Glucose
 ADP + Glucose-6-P 30.5
–4.0
–
16.7
Resonance Forms of Pi
O
HO
P
O
O
HO
O
P
O
–
O
O
– O
O
HO
P
O
O
–
O
O
HO
P
P
O
O
So: resonance stabilization
etc...
O –
Phosphate Esters and Anhydrides
Esters:
H2O
O
ROH + HO
P
O
O
R
O
O
P
O
O
H2O
Anhydrides:
O
R
C
H2O
O
+ HO
OH
P
O
O
R
C
O
O
O
P
O
H2O
= Hydrolysis
O
Hydrolysis of Glucose-6-Phosphate
O
HO
P
O
O
HO
CH2
O + H2O
O
CH2
O
+ HO
P
O
∆Gº = –3.3 kcal/mol
= –13.8 kJ/mol
Ionization,
resonance
Product
stabilization
OH