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Introduction to Metabolism
overall process through
which living systems
acquire and use free
energy to carry out
various functions
Synthesis and
utilization of energy
Degradation and
production of energy
Convergence of
different pathways
to form a common
Acetyl CoA is
further oxidized in
the TCA to produce
reduced coenzymes
which will pass
their electrons to
oxygen to form
Metabolic Pathways occur in Specific Cellular
Enzymatic digestion
of cell components
and digested matter
Krebs cycle,
ETC, Fatty acid
Amino Acid
Glycolysis, PPP, fatty
acid biosynthesis,
part of
Lipid and steroid
Synthesis of membrane
bound and secretory
Post transciptional
membrane and
secretory proteins
Thermodynamic Considerations
• The free energy change (ΔG) of a biochemical
process is related to the standard free energy
change (ΔG°’) and the concentrations of the
reactants and products
ΔG = Δ𝐺°′ + 𝑅𝑇 ln [𝐶][𝐷]/[𝐴][𝐵]
• At equilibrium ΔG = 0
Δ𝐺°′ = − 𝑅𝑇 ln 𝐾𝑒𝑞
• Most metabolic reactions are said to be nearequilibrium reactions
Thermodynamic Considerations
• Other pathways are designed to be far from
• As a consequence:
– Metabolic pathways are irreversible
– Every metabolic pathway has a committed step
– Catabolic and anabolic pathways differ
Control of Metabolic Flux
• The flux of intermediates through a metabolic
pathway in a steady state is more or less
• A steady state far from equilibrium is
thermodynamically efficient because only a
nonequilibrium process can perform work
• The flux of intermediates in a pathway is set
by the rate-determining step
Control of Metabolic Flux
• Cells use several mechanisms to control flux
through the rate determining steps of
metabolic pathways
– Allosteric control (i.e. feedback inhibition)
– Covalent modification
– Substrate cycles
– Genetic control
High Energy Compounds
ATP hydrolysis yields an enormous amount of energy per
mole due to:
1. the relief of charge repulsion
2. resonance stabilization of released Pi
3. solvation energy difference
Energy Coupling
Exergonic reactions of “high-energy” compounds can be
coupled to endergonic processes to drive them to completion
Electron Carriers
Electron Carriers