ch2210 energy in chemical and biochemical...
TRANSCRIPT
CH2210 Energy
in Chemical and Biochemical Reactions
The energy of a colliding system can be represented using an Reaction Progress Diagram.
The energy difference between reactants and products is called the heat of reaction, ΔHrxn.
For exothermic reactions, ΔHrxn is negative.
For endothermic reactions, ΔHrxn is positive.
Energy in Chemical and Biochemical Reactions
ENERGY
Reaction coordinate
Reactants
Products
Ea
A + B
C + DΔHrxn
Reaction coordinate
Reactants
Products
Ea
A + B
C + D
ΔHrxn
Energy in Chemical and Biochemical Reactions
The terms exothermic and endothermic refer to the energy contained in the
chemical bonds of reactants and products, and not to the total useful energy which can
be obtained from the given reaction.
Equilibrium Constants and Their Use
Any chemical system at equilibrium can be described by a number called the equilibrium constant, Keq, an associated mathematical expression involving the reactants and the products of the reaction.
(The upper-case letters represent the concentrat ions of the chemical substances involved and the lower-case letters r e p r e s e n t t h e b a l a n c i n g coefficients for the reaction.)
a A + b B c C + d D
Keq =[C]c[D]d
[A]a[B]b
Equilibrium Constants and Their Use
The magnitude of the equilibrium constant indicates whether a particular reaction is product-dominated or reactant-dominated at equilibrium.
When Keq >> 1 (i.e. 1 x 103), products dominate the reaction mixture. The equilibrium is called favorable.
When Keq << 1 (i.e. 1 x 10-3), reactants dominate the reaction mixture. The equilibrium is called unfavorable.
Keq =[C]c[D]d
[A]a[B]b
A → B, K1 = 1 x 10-3 (unfavorable) B → C, K2 = 1 x 102 (favorable) C → D, K3 = 1 x 103
(favorable)
A D, K4 = (K1)x(K2)x(K3)= 1 x 102 (favorable)
Equilibrium Constants and Their Use
A metabolic conversion is usually accomplished by a series of connected reactions.
A B C D The equilibrium constant for a set of connected reactions can be obtained by multiplying together the equilibrium constants for the individual reactions:
Equilibrium Constants for a Sequences of Reactions
Keq = 3.8 x 10-3
+ H2OPO43- +
The Hexokinase Reaction:
ATP ADP
Equilibrium Constants and Their Use
Keq = 3.8 x 10-3
+ H2OPO43- +
ATP + H2O ADP + PO43-
Keq = 2.3 x 106
ATP + + ADP
Keq = 8.6 x 103
Equilibrium Constants for “Coupled” Reaction
3)
1)
2)+
Another Way of Writing a “Coupled” Reaction
K3 = K1 x K2 Keq = 8.6 x 103
ATP ADP
O
CH2 O PO32-
OH
OH
OHHO
O
CH2 O H
OH
OH
OHHO
D-Glucose D-Glucose-6-phosphate
Equilibrium Constants and Their Use
Note: A reaction can have a very large equilibrium constant, yet be very slow. Conversely, a reaction can have an equilibrium constant close to one and be very fast. This is because the equilibrium constant is related to the energy difference between the reactant molecules and the product molecules.
The equilibrium constant tells us nothing about the reaction rate which is related to the magnitude of the energy of activation for the reaction.
Nevertheless, equilibrium constants can be useful in calculating the final reactant and product concentrations, given that there is sufficient energy of activation for the reaction to occur.
Free Energy Changes (energy available for work) and Chemical Reactions
ΔG = Free Energy Change
ΔG0’ = Standard Free Energy Change
exergonic endergonic
Free Energy Changes (energy available for work)
Exergonic reactions Release free energy ΔG is negative The forward reaction is favored.
(ΔG = Free Energy Change ΔG0’ = Standard Free Energy Change)
Endergonic reactions Require the input of free energy ΔG is positive The reverse reaction is favored.
Free Energy Change is Related to the Enthalpy Change, Temperature, and Entropy
ΔG = Free Energy Change
ΔG0’ = Standard Free Energy Change
Enthalpy Change, Temperature, and Entropy
ΔG under Nonstandard Conditions
⋅ ΔG = ΔG° only when the reactants and products are in their standard states
⋅ there normal state at that temperature
⋅ partial pressure of gas = 1 atm
⋅ concentration = 1 M
⋅ under nonstandard conditions, ΔG = ΔG° + RTlnQ
⋅ Q is the reaction quotient
⋅ at equilibrium ΔG = 0
⋅ ΔG° = ─RTlnK
Free Energy Change is Related to the Equilibrium Constant
For an exergonic (spontaneous) reaction:
For an endergonic (nonspontaneous) reaction:
ΔG is -
ΔG is +(ln K is positive)
(ln K is negative)
∆Gº (kJ/mol) Keq% product at equilibrium
-17 1000 99.9%
-11 100 99%
-6 10 90%
0 1 50%
+6 0.1 10%
+11 0.01 1%
+17 0.001 0.1%
Relationship Between ∆G and Keq
Free Energy Changes
Reaction#1:
Glucose + Pi Glucose-6-phosphate + H2O
ΔG0’ = +13.8 kJ/mol , endergonic Reaction#2:
ATP + H2O ADP + Pi
ΔG0’ = -30.5 kJ/mol , exergonic
The previous reactions can be described in terms of free energy changes:
To “couple” the reactions we add the reactants and products together and add the ΔG0’ values
Glucose + ATP Glucose-6-phosphate + ADP
ΔG0’ = -16.7 kJ/mol , exergonic
Mechanical Model
Downward motion of an object releases potential energy(an exergonic process)
that can be used to do mechanical work, moving another object upward.(an endergonic process)
A coupling mechanism is required to enable the exergonic process to drive the endergonic process.
ΔG>0 ΔG<0
Chemical Model
+
Reaction #1
endergonicReaction #2
exergonic
“Coupled” Reaction
exergonic
The coupled reaction is exergonic; it will go spontaneously (forward, left to right) in the cell, but will it proceed at a
are consistent with cellular needs ?
There is no information about rates in the value of a ΔG.
Most biological reactions would proceed at a very slow rate if they are not catalyzed. the catalyst is usually an enzyme.
Chemical Model
The Energy of Activation must still be considered !!!
Energy released during oxidation of nutrients is trapped in the form of a few energy-rich or "high energy" compounds.
A "high-energy" compound is a compound with a functional group (in many cases, phosphoryl group) whose free energy of transfer to
another compound proceeds with a large negative ΔG.
free energy of transfer to water = ΔG for hydrolysis
High Energy Compounds
X-OPO32- + H2O X-OH + HOPO32- + Energy
We can compare how "high" the energy is in compounds by comparing their free energy of transfer to
a common compound.
Hydrolysis of High Energy Compounds
N
N
N
NH2
N
O
OH
HH
H
CH2
HOH
PO
O
O
O
PO
O
O
P
O
O
O
N
N
N
NH2
N
O
OH
HH
H
CH2
HOH
PO
O
O
O
PHOO
O
P
O
O
O
OH
H2O
Hydrolysis of High Energy Compounds
+ Energy !!!
Standard Free Energies of Hydrolysis of Some Phosphorylated Compounds
X-OPO32- + H2O → X-OH + HOPO32- ∆G’º(kJ/mol) ∆G’º(kcal/mol)
Phosphoenolpyruvate + H2O → Pyruvate + Pi -61.9 -14.8
1,3-Bisphosphoglycerate + H2O → 3-Phosphoglycerate + Pi -49.3 -11.8
Phosphocreatine + H2O → Creatine + Pi -43 -10.3
ATP + H2O → AMP + PPi -45.6 -10.9
ADP + H2O → AMP + Pi -32.8 -7.8
ATP + H2O → ADP + Pi -32.5 -7.3
PPi + H2O → Pi + Pi -19 -4
AMP + H2O → Adenosine + Pi -14.2 -3.4
Glucose-1-phosphate + H2O → Glucose + Pi -20.9 -5
Fructose-6-phosphate + H2O → Fructose + Pi -15.9 -3.8
Glucose-6-phosphate + H2O → Glucose + Pi -13.8 -3.3
Glycerol-1-phosphate + H2O → Glycerol + Pi -9.2 -2.2
ATP’s “energy of hydrolysis” :
ATP ---------------> ADP + Pi ∆G’º = -30.5 kJ/mol
Energy needed to synthesize ATP from ADP:
ADP + Pi ---------------> ATP ∆G’º = +30.5 kJ/mol
Glu-6-P “energy of hydrolysis” :
Glu-6-P --------------> Glu + Pi ∆G’º = -13.8 kJ/mol
Energy needed to synthesize Glu-6-P from Glu:
Glu + Pi -------------> Glu-6-P ∆G’º = +13.8 kJ/mol
ATP + Glu -------------> Glu-6-P +ADP ∆G’º = -16.7 kJ/mol
1
2
3
4
+
∆G’º= -16.7 kJ/mol (i.e. -30.5 + 13.8)
ATP ADP
O
CH2 O PO32-
OH
OH
OHHO
O
CH2 O H
OH
OH
OHHO
D-Glucose D-Glucose-6-phosphate
Hexokinase
ATP’s “energy of hydrolysis” :
ATP ---------------> ADP + Pi ∆G’º = -30.5 kJ/mol
Energy needed to synthesize ATP from ADP:
ADP + Pi ---------------> ATP ∆G’º = +30.5 kJ/mol
Phosphoenol (PEP) Pyruvate “energy of hydrolysis” :
PEP --------------> Pyruvate + Pi ∆G’º = -61.9 kJ/mol
Energy needed to synthesize PEP from Pyruvate:
Pyruvate + Pi -------------> PEP ∆G’º = +61.9 kJ/mol
ADP + PEP -----------> ATP + Pyruvate ∆G’º = -31.4 kJ/mol
1
2
3
4
+
∆G’º= -31.5 kJ/mol (i.e. -61.5 + 30.5)
CC
C
P
O
O
O
OO
H
H
CC
CO
O
H
H
O
H
O
Phosphoenolpyruvate Pyruvate
ADP ATP
Pyruvate kinase
ATP ADP
O
CH2 O PO32-
OH
OH
OHHO
O
CH2 O H
OH
OH
OHHO
D-Glucose D-Glucose-6-phosphate
Hexokinase
Pyruvate kinaseC
CC
P
O
O
O
OO
H
H
CC
CO
O
H
H
O
H
O
Phosphoenolpyruvate Pyruvate
ADP ATP
What is the difference between these two reactions ?
X-OPO32- + H2O → X-OH + HOPO32- ∆G’º(kJ/mol) ∆G’º(kcal/mol)
Phosphoenolpyruvate → Pyruvate + Pi -61.9 -14.8
1,3-Bisphosphoglycerate → 3-Phosphoglycerate + Pi -49.3 -11.8
Phosphocreatine → Creatine + Pi -43 -10.3
ATP → AMP + PPi -45.6 -10.9
ADP → AMP + Pi -32.8 -7.8
ATP → ADP + Pi -32.5 -7.3
PPi → Pi + Pi -19 -4
AMP → Adenosine + Pi -14.2 -3.4
Glucose-1-phosphate → Glucose + Pi -20.9 -5
Fructose-6-phosphate → Fructose + Pi -15.9 -3.8
Glucose-6-phosphate → Glucose + Pi -13.8 -3.3
Glycerol-1-phosphate → Glycerol + Pi -9.2 -2.2
Standard Free Energies of Hydrolysis of Some Phosphorylated Compounds
All of the reactions have reverse reactions with + ΔG values which
are therefore NONSPONTANEOUS.
Standard Free Energies of Hydrolysis of Some Phosphorylated Compounds
X-OPO32- + H2O → X-OH + HOPO32- ∆G’º(kJ/mol) ∆G’º(kcal/mol)
Phosphoenolpyruvate → Pyruvate + Pi -61.9 -14.8
1,3-Bisphosphoglycerate → 3-Phosphoglycerate + Pi -49.3 -11.8
Phosphocreatine → Creatine + Pi -43 -10.3
ATP → AMP + PPi -45.6 -10.9
ADP → AMP + Pi -32.8 -7.8
ATP → ADP + Pi -32.5 -7.3
PPi → Pi + Pi -19 -4
AMP → Adenosine + Pi -14.2 -3.4
Glucose-1-phosphate → Glucose + Pi -20.9 -5
Fructose-6-phosphate → Fructose + Pi -15.9 -3.8
Glucose-6-phosphate → Glucose + Pi -13.8 -3.3
Glycerol-1-phosphate → Glycerol + Pi -9.2 -2.2