The importance of energy
changes and electron transfer in
metabolism
Chapter 15
The Nature of Metabolism
• Metabolism: the sum total of the chemical
reactions of biomolecules in an organism. It is
the biochemical basis of life processes.
– Catabolism: the breakdown of larger molecules
into smaller ones; an oxidative process that
releases energy
– Anabolism: the synthesis of larger molecules from
smaller ones; a reductive process that requires
energy
A Comparison of Catabolism and
Anabolism
• Metabolism is the sum total of the chemical
reactions of biomolecules in an organism
The Role of Oxidation and Reduction in
Metabolism
• Oxidation-Reduction reactions are those in
which electrons are transferred from a donor
to an acceptor
– oxidation: the loss of electrons - the substance
that loses the electrons is called a reducing agent
– reduction: the gain of electrons - the substance
that gains the electrons is called an oxidizing
agent
Summary
• In Catabolism - large molecules are broken
down to smaller products, releasing energy
and transferring electrons to acceptor
molecules of various sorts - Oxidation
• In Anabolism - small molecules react to give
rise to larger ones; this process requires
energy and involves acceptance of electrons
from a variety of donors - Reduction
NAD+/NADH: An Important Coenzyme
• Nicotinamide adenine dinucleotide (NAD+) is
an important coenzyme
• Acts as a biological oxidizing agent
• The structure of NADH is comprised of a
nicotinamide portion. It is involved in the
reaction. It is a derivative of nicotinic acid
• NAD+ is a two-electron oxidizing agent - is
reduced to NADH
The Structures and Redox States of the
Nicotinamide Coenzymes
FAD/FADH2
• Flavin adenine dinucleotide (FAD) is also a
biological oxidizing agent
• Protons, as well as, electrons are accepted by
FAD
Coupling of Production and Use of
Energy
• The coupling of energy-producing and energyrequiring reactions is a central theme in the
metabolism of all organisms
• Energy cannot be used directly, must by shuttled
into easily accessible forms of chemical energy
• “High Energy” bonds- bonds that require or
release convenient amounts of energy, depending
on the direction of the reaction
• ATP is essential high energy bond-containing
compound
• Phosphorylation of ADP to ATP requires energy
• Hydrolysis of ATP to ADP releases energy
The Phosphoric Anhydride Bonds in
ATP are “High Energy” Bonds
ATP
• 4 (-) charges on ATP and 3 on ADP, therefore
ATP is less stable.
• Why is ATP less stable, charge-wise, than
ADP?
– Energy must be expended to put on additional
negative charge on ADP
– Also, entropy loss when ADP is phosphorylated
because there is a potential loss of resonance
hybridization of inorganic phosphate (Pi) upon
phosphorylation of ADP to ATP
Loss of a Resonance-Stabilized
Phosphate Ion in Production of ATP
Role of ATP as Energy Currency
Summary
• Hydrolysis of ATP to ADP releases energy
• In the coupling of biochemical reactions, the
energy released by one reaction, such as ATP
hydrolysis, provides energy for another
Coenzyme A in Activation of Metabolic
Pathways
• A step frequently encountered in metabolism
is activation
– activation: the formation of a more reactive
substance
– A metabolite is bonded to some other molecule
and the free-energy change for breaking the new
bond is negative.
– Causes next reaction to be exergonic
Two Ways of Looking at Coenzyme A
• Coenzyme A (CoA-SH) contains units of 2mercaptoethylamine, pantothenic acid, and
3’,5’-ADP
The Hydrolysis of Acetyl-CoA
• The metabolically active form of a carboxylic
acid is the corresponding acyl-CoA thioester,
in which the thioester linkage is a high-energy
bond
Summary
• Metabolic pathways proceed in many stages,
allowing for efficient use of energy
• Many coenzymes, particularly coenzyme
A(CoA) play a crucial role in metabolism
What are standard states?
• Standard states
– for pure solids and liquids-the pure substance
– for gases, the gas at a pressure of 1 atm
– for solutions, a concentration of 1 mol/L
Standard States for Free-Energy
Changes
• For the reaction
We can rewrite the equation that relates the
G for the reaction under any conditions to
the free-energy change under standard
conditions (G˚)
A Modified Standard State for
Biochemical Applications
• Standard free energy change, G°, assumes a
concentration of 1 M
– if [H+] = 1 M, then pH = 0
– but the pH in most cells is near the neutral range
• For biochemical reactions, we define a different
standard state for the concentration of H+
– standard state for [H+] = 10-7 M, pH = 7.0
– this modified standard state is given the symbol G°’
Loss of a Resonance-Stabilized
Phosphate Ion in Production of ATP
ATP Hydrolysis Decreases in
Electrostatic Repulsion
• Marked decrease in
electrostatic repulsion
of -phosphate of
GDP upon hydrolysis
of ATP to ADP
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