Mary K. Campbell
Shawn O. Farrell
http://academic.cengage.com/chemistry/campbell
Chapter 15
The Importance of Energy Changes and
Electron Transfer in Biology
Paul D. Adams • University of Arkansas
Standard States for Free-Energy Changes
• Standard states
• for pure solids and liquids, _____________________
• for gases, the gas at _____________________
• for solutions, a concentration __________________
• 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˚)
Standard States for Free-Energy Changes (Cont’d)
• When the reaction is at equilibrium, G = _____
• If we can determine the concentration of reactants
and products at equilibrium, we can determine Keq
and, from it, the change in free energy for conversion
of one mole of reactant to product(s)
Modified Standard State for Biochemical Rxns
• Standard free energy change, G°, assumes a molar
concentration of 1 M
• if [H+] = 1 M, then pH = _____
• 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+] = _____________________
• this modified standard state is given the symbol _____
• Summary
The usual thermodynamic standard state implies that
the system involved is at pH=0, which is seldom, if
ever, found in living things. The modified standard
state explicitly states that the system is at pH=7
Biochemical Connections: Thermodynamics
The Nature of Metabolism
• _____________: the chemical rxns of biomolecules
• Metabolism is the biochemical basis of
• _____________: the breakdown of larger molecules
into smaller ones; an oxidative process that releases
energy
• _____________: 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 biochemical
reactions 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
• ________: the loss of electrons; the substance that
loses the electrons is called a ________ agent
• ________: the gain of electrons; the substance that
gains the electrons is called an ________ agent
• Carbon in most reduced form- ___________
• Carbon in most oxidized form- ___________
(final product of catabolism
Summary
• In ________, large molecules are broken down to
smaller products, releasing energy and transferring
electrons to acceptor molecules of various sorts. The
overall process is one of oxidation.
• In ________, small molecules react to give rise to
larger ones; this process requires energy and
involves acceptance of electrons from a variety of
donors. The overall process is one of reduction
Coenzymes used in Biologically important Redox Rxns
• Conversion of ethanol to acetaldehyde is a twoelectron oxidation
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 moiety. It is involved in the reaction. It
is a derivative of nicotinic acid
• NAD+ is a two-electron oxidizing agent, and is
reduced to NADH
Structures & Redox States of Nicotinamide Coenzymes
FAD/FADH2
• Flavin adenine dinucleotide (FAD) is also a biological
______________ agent
• Protons and electrons are accepted by FAD
The Structures of Riboflavin, Flavin Mononucleotide
(FMN), and Flavin Dinucleotide (FAD)
Coupling of Production and Use of Energy
• The coupling of ________-____________ and
________-____________ 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 require or release convenient
amounts of energy, depending on the direction of the
reaction
• ______ is an essential high energy bond-containing
compound
• Phosphorylation of ADP to ATP _________ energy
• Hydrolysis of ATP to ADP _________ energy
The Phosphoric Anhydride Bonds in ATP
“High Energy” Bonds are in red
ATP
• Four – charges on ATP and 3 on ADP, therefore
ATP is ______________ ______________ .
• Why is ATP less stable, charge-wise, than ADP?
• Energy must be expended to put an additional
___________________________________________
• Also, entropy loss when ADP is phosphorylated
because there is a potential loss of ______________
______________ of inorganic phosphate (Pi) on
phosphorylation of ADP to ATP
Loss of a Resonance-Stabilized Phosphate Ion
…in Production of ATP
ATP Hydrolysis Decreases Electrostatic Repulsion
• Marked decrease in
________________
_______________ of
-phosphate of GDP
upon hydrolysis of
ATP to ADP
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
CoA in Activation of Metabolic Pathways
• A step frequently encountered in metabolism is
activation
• activation: the formation of a ___________________
substance
• A metabolite is bonded to some other molecule and
the free-energy change for breaking the new bond is
______________ .
• Causes next reaction to be ______________
Two Ways of Looking at Coenzyme A
Coenzyme A (CoA-SH) contains pantothenic acid, 3’,5’-ADP
and 2-mercaptoethylamine
The Hydrolysis of Acetyl-CoA
The metabolically active form of a carboxylic acid is the acylCoA thioester with a high-energy thioester bond
The Role of Electron Transfer & ATP Production
in Metabolism
Summary
• Metabolic pathways proceed in many stages,
allowing for efficient use of energy
• Many coenzymes, particularly coenzyme A(HS-CoA)
play a crucial role in metabolism