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Major Pathways in Cells
• Metabolic fuels
Three major nutrients consumed by mammals:
(1) Carbohydrates - provide energy
(2) Proteins - provide amino acids for protein
synthesis and some energy
(3) Fats - triacylglycerols provide energy and
also lipids for membrane synthesis
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• Overview of
catabolic
pathways
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Catabolism produces compounds
for energy utilization
• Three types of compounds are produced that
mediate the release of energy
(1) Acetyl CoA
(2) Nucleoside triphosphates (e.g. ATP)
(3) Reduced coenzymes (NADH, FADH2, QH2)
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Reducing Power
• Electrons of reduced coenzymes flow toward O2
• This produces a proton flow and a transmembrane
potential
• Oxidative phosphorylation is the process by
which the potential is coupled to the reaction:
ADP + Pi
ATP
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Compartmentation of
metabolic processes
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Thermodynamics and Metabolism
A. Free-Energy Change
• Free-energy change (DG) is a measure of the
chemical energy available from a reaction
DG = Gproducts - Greactants
• DH = change in enthalpy
• DS = change in entropy
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Relationship between energy and entropy
• Both entropy and enthalpy contribute to DG
DG = DH - TDS
(T = degrees Kelvin)
-DG = a spontaneous reaction in the
direction written
+DG = the reaction is not spontaneous
DG = 0 the reaction is at equilibrium
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The Standard State (DGo) Conditions
• Reaction free-energy depends upon conditions
• Standard state (DGo) - defined reference conditions
Standard Temperature = 298K (25oC)
Standard Pressure = 1 atmosphere
Standard Solute Concentration = 1.0M
• Biological standard state = DGo’ or DG’o
Standard [H+] = 10-7 M (pH = 7.0) rather than 1.0M
(pH = 1.0); [H2O]=55.5M; [Mg2+]=1mM
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B. Equilibrium Constants and
Standard Free-Energy Change
• For the reaction: A + B
C+D
DGreaction = DGo’reaction + RT ln([C][D]/[A][B])
• At equilibrium: Keq = [C][D]/[A][B] and
DGreaction = 0, so that:
DGo’reaction = -RT ln Keq
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C. Actual Free-Energy Change Determines
Spontaneity of Cellular Reactions
• When a reaction is not at equilibrium, the
actual free energy change (DG) depends
upon the ratio of products to substrates
• Q = the mass action ratio
DG = DG’o + RT ln Q
Where Q = [C]’[D]’ / [A]’[B]’
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The Free Energy of ATP
• Energy from oxidation of metabolic fuels is
largely recovered in the form of ATP
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• Hydrolysis of
ATP
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Complexes between ATP and Mg2+
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ATP is an “energy-rich” compound
• A large amount of energy is released in the
hydrolysis of the phosphoanhydride bonds of
ATP (and UTP, GTP, CTP)
• All nucleoside phosphates have nearly equal
standard free energies of hydrolysis
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