Metabolism Is the Sum of
Cellular Reactions
• Metabolism - the entire network of chemical
reactions carried out by living cells
• Metabolites - small molecule intermediates in
the degradation and synthesis of polymers
• Catabolic reactions - degrade molecules to
create smaller molecules and energy
• Anabolic reactions - synthesize molecules for
cell maintenance, growth and reproduction
Anabolism and catabolism
Metabolic Pathways Are
Sequences of Reactions
• Metabolism includes all enzyme reactions
• Metabolism can be subdivided into branches
• The metabolism of the four major groups of
biomolecules will be considered:
Carbohydrates
Lipids
Amino Acids
Nucleotides
Forms of metabolic pathways
(a) Linear
(b) Cyclic
Forms of metabolic pathways
(c) Spiral pathway
(fatty acid
biosynthesis)
Metabolic Pathways Are
Regulated
• Metabolism is highly regulated to permit
organisms to respond to changing conditions
• Most pathways are irreversible
• Flux - flow of material through a metabolic
pathway which depends upon:
(1) Supply of substrates
(2) Removal of products
(3) Pathway enzyme activities
Feedback inhibition
• Product of a pathway controls the rate of its own
synthesis by inhibiting an early step (usually the
first “committed” step (unique to the pathway)
Feed-forward activation
• Metabolite early in the pathway activates an
enzyme further down the pathway
Covalent modification for
enzyme regulation
• Interconvertible enzyme activity can be rapidly and
reversibly altered by covalent modification
• Protein kinases phosphorylate enzymes (+ ATP)
• Protein phosphatases remove phosphoryl groups
• The initial signal may be amplified by the “cascade”
nature of this signaling
Regulatory role of a protein kinase,
amplification by a signaling cascade
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
• Overview of
catabolic
pathways
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
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
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
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’
Standard H+ concentration = 10-7 (pH = 7.0) rather
than 1.0M (pH = 1.0)
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
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 = DGo’ + RT ln Q
Where Q = [C]’[D]’ / [A]’[B]’
• Hydrolysis of
ATP
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
Energy of phosphoanhydrides
(1) Electrostatic repulsion among negatively
charged oxygens of phosphoanhydrides of ATP
(2) Solvation of products (ADP and Pi) or (AMP
and PPi) is better than solvation of reactant ATP
(3) Products are more stable than reactants
There are more delocalized electrons on ADP, Pi
or AMP, PPi than on ATP
Glutamine synthesis requires
ATP energy
Phosphoryl-Group Transfer
• Phosphoryl-group-transfer potential - the ability
of a compound to transfer its phosphoryl group
• Energy-rich or high-energy compounds have
group transfer potentials equal to or greater than
that of ATP
• Low-energy compounds have group transfer
potentials less than that of ATP
Transfer of the phosphoryl
group from PEP to ADP
• Phosphoenolpyruvate (PEP) (a glycolytic
intermediate) has a high P-group transfer potential
• PEP can donate a P to ADP to form ATP
Structures of PC and PA
Nucleotidyl-Group Transfer
• Transfer of the nucleotidyl group from ATP is
another common group-transfer reaction
• Synthesis of acetyl CoA requires transfer of an
AMP moiety to acetate
• Hydrolysis of pyrophosphate (PPi) product
drives reaction to completion
Synthesis of acetyl CoA
Synthesis of acetyl CoA
Thioesters Have High Free
Energies of Hydrolysis
• Thioesters are energy-rich compounds
• Acetyl CoA has a DGo’ = -31 kJ mol-1
Succinyl CoA Energy Can
Produce GTP
Reduced Coenzymes Conserve Energy
from Biological Oxidations
• Amino acids, monosaccharides and lipids are
oxidized in the catabolic pathways
• Oxidizing agent - accepts electrons, is reduced
• Reducing agent - loses electrons, is oxidized
• Oxidation of one molecule must be coupled with
the reduction of another molecule
Ared + Box
Aox + Bred
Diagram of an electrochemical cell
• Electrons flow
through external
circuit from Zn
electrode to the
Cu electrode
Standard reduction potentials
and free energy
• Relationship between standard free-energy
change and the standard reduction potential:
DGo’ = -nFDEo’
n = # electrons transferred
F = Faraday constant (96.48 kJ V-1)
DEo’ = Eo’electron acceptor - Eo’electron donor
Reduction Potentials
Cathode (Reduction)
Half-Reaction
Li+(aq) + e- -> Li(s)
K+(aq) + e- -> K(s)
Standard Potential
E° (volts)
-3.04
-2.92
Ca2+(aq) + 2e- -> Ca(s)
Na+(aq) + e- -> Na(s)
Zn2+(aq) + 2e- -> Zn(s)
Cu2+(aq) + 2e- -> Cu(s)
-2.76
-2.71
-0.76
0.34
O3(g) + 2H+(aq) + 2e- -> O2(g) + H2O(l)
F2(g) + 2e- -> 2F-(aq)
2.07
2.87
Actual reduction potentials
(DE)
• Under biological conditions, reactants are not
present at standard concentrations of 1 M
• Actual reduction potential (DE) is dependent
upon the concentrations of reactants and
products
DE = DEo’ - (RT/nF) ln ([Aox][Bred] / [Ared][Box] )
Electron Transfer from NADH
Provides Free Energy
• Most NADH formed in metabolic reactions in
aerobic cells is oxidized by the respiratory
electron-transport chain
• Energy used to produce ATP from ADP, Pi
• Half-reaction for overall oxidation of NADH:
NAD+ + 2H+ + 2e-
NADH + H+ (Eo’ = -0.32V)
Example
Suppose we had the following voltaic cell at 25o C:
Cu(s)/Cu+2 (1.0 M) // Ag+(1.0 M)/ Ag (s)
What would be the cell potential under these conditions?
Example
Suppose we had the following voltaic cell at 25o C:
Cu(s)/Cu+2 (1.0 M) // Ag+(1.0 M)/ Ag (s)
What would be the cell potential under these conditions?
Ag+ + e- ---> Ag0
Cu+2 + 2e- ----> Cu0
E0red = + 0.80 v
E0red = + 0.337 v
Example: Biological Systems
Both NAD+ and FAD are oxidizing agents
The question is which would oxidize which?
OR
Which one of the above
is the spontaneous reaction?
in which DG is negative
To be able to answer the question
We must look into the “electron donation”
capabilities of NADH and FADH2
i.e. reduction potentials of
NADH and FADH2
Remember,
DEo’ = Eo’electron acceptor - Eo’electron donor
For a spontaneous reaction DEo ’ must be positive
Therefore,
rearrange
Add the two reactions
electron
acceptor
electron
donor