Energy in Chemical and Biochemical...
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Energy in Chemical and Biochemical Reactions
Transcript of Energy in Chemical and Biochemical...
Energy in
Chemical and Biochemical Reactions
ENERGY
Reaction coordinate
Reactants ➝ activated complex ➝ Products
Reactants
Products
Ea
Reaction Progress Diagram for Exothermic Reaction
A + B
C + D ΔHrxn
The energy of a colliding system can be represented using an activation-energy diagram.
When the energy content of the product bonds is less than that of the reactant bonds, the difference in energy appears as heat energy.
Reactions of this type are called exothermic reactions.
ENERGY
Reaction coordinate
Reactants ➝ activated complex ➝ Products
Reactants
Products
Ea
Reaction Progress Diagram for Endothermic Reaction
A + B
C + D
ΔHrxn
When the energy content of the product bonds is less than that of the reactant bonds, the difference in energy appears as heat energy
absorbed. Reactions of this type are called endothermic reactions.
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
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.
Equilibrium Constants and Their Use
A metabolic conversion, such as the conversion of glucose to lactic acid in muscle during glycolysis, is 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
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
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
Free Energy Changes (energy available for work) and Chemical Reactions
(ΔG = Free Energy Change ΔG0’ = Standard Free Energy Change)
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 Changes and Equilibrium Constants
∆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
The previous reaction can be described in terms of free energy changes.
• Reaction 1:
• Glucose + Pi <=> glucose-6-phosphate + H2O
(ΔGo' = + 13.8 kJ/mol, endergonic)
• Reaction 2:
• ATP + H 2O <=> ADP + Pi
(ΔGo' = - 30.5 kJ/mol, exergonic)
• To couple the two reactions , add reactants on left, add products on right, and add ΔGo' values to get ΔGo' for coupled reaction:
• Glucose + ATP <=> glucose-6-phosphate + ADP
(ΔGo' = - 16.7 kJ/mol)
Downward motion of an object releases potential energy(right side, exergonic)
that can be used to do mechanical work,moving another object upward
(left side, endergonic).
A coupling mechanism (rope) is required to enable exergonic process to drive endergonic one.
• The coupled reaction is exergonic; it will go spontaneously (forward, left to right) in the cell, but will it proceed at a rate consistent with cellular needs?
• There's NO information about rates in the value of a ΔG -- we can't answer this question from bioenergetics.
• Most biological reactions would proceed at a very slow rate indeed if they're not catalyzed. The biological catalyst enabling the coupled reaction above to proceed on a biological timescale (as opposed to a geological timescale!) is an enzyme, hexokinase.
• Free energy coupling, with enzymes as catalysts, is the strategy used in metabolic pathways.
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-
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
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
Standard Free Energies of Hydrolysis of Some Phosphorylated Compounds
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 ?
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
