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Energy
Change and Electron Transfer in Metabolism
Molecules
taken in as nutrients must be broken down for energy or use as building
blocks.
Homeostasis relies on a steady stream of energy.
Energy extraction process takes place in a series of reaction in
which electron donors transfer energy to electron acceptors.
Oxidation-reduction reactions are essential for the extraction of
energy from molecules such as glucose.
Final step in the oxidation of glucose is the transfer of two
electrons from NADH and protons to oxygen form H2O.
Energy generated in this reaction is conserved by transforming ADP
(lower-energy)to ATP (higher-energy).
Energy and Change.
Spontaneous:
something that occurs without outside intervention (not necessarily fast)
energetically favorable.
The Criterion for Spontaneity.
Free energy, G, (J. William Gibbs) for a substance is the most useful predictor of
spontaneity. Defined in terms of its enthalpy (H), entropy (S), and temperature (T) at constant pressure. We are interested in changes in free energy for biochemical transformations. Change in free energy is a measure of spontaneity.
DG < 0 exergonic,
spontaneous (energy released), to the right as written.
DG = 0 at
equilibrium, no chemical change.
DG > 0 endergonic,
nonspontaneous (energy required), to the left. Ex: Spontaneous process C6H12O6 + 6O2 ΰ 6CO2 + 6H2O DG < 0. Ex: Non Spontaneous ATP + Pi ΰ ATP DG > 0.
This reaction can take place because other metabolic reactions
supply energy.
Standard
States and Standard Free Energy Change.
We can
arbitrarily define standard conditions and use those as a basis for
comparing reactions.
For pure solids and liquids, the pure substances.
For gases, at pressure of 1 atm.
For solutions, at a concentration of 1 mol liter-1. »
Valid for all but the most exact work. For any general reaction: The o in DGo indicates standard conditions, brackets indicate molar concentrations, R is the gas constant (8.31 J mol-1K-1) and T is the absolute temperature.
There is only one DGo for a
given reaction at a given temp. When the reaction is at equilibrium DG = 0 we can rewrite the equation as DGo = -RT ln Keq.
Where Keq is the equilibrium constant for the reaction. Therefore, if we can determine the concentration of reactants and products at equilibrium, we can determine Keq, and, from it, the change in free energy (DGo) for conversion of one mol of each reactant to product(s).
Nature of Metabolism:
Metabolism:
the chemical reactions of biomolecules.
Nature of Redox (Oxidation-Reduction Reactions):
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.
Oxidation
and reduction are most easily recognized by writing balanced-half
reactions. Conversion of ethanol to acetaldehyde is a two-electron oxidation. Conversion of pyruvate to lactate is a two-electron reduction. Nicotinamide adenine dinucleotide (NAD+) is a biological oxidizing agent.
NAD+ is a two-electron oxidizing agent, and is
reduced to NADH Ex: Ethanol to Acetaldehyde. Ex: Pyruvate to Lactate. Flavin adenine dinucleotide (FAD) is also a biological oxidizing agent.
FAD participates in several types of enzyme-catalyzed
oxidation/reduction reactions. »
One is in the oxidation of a C-C bond in a fatty acid hydrocarbon
chain to a C=C bond as shown in these balanced half-reactions.
Coupling
of Production and Use of Energy.
How
is energy released by one reaction used by another?
The formation of ATP (from ADP and Pi) is directly linked with the release of energy from the oxidation of nutrients.
Phosphorylation of ADP requires energy that is released from
nutrient oxidation.
Phosphorylation of ADP to ATP requires energy.
Hydrolysis of ATP to ADP releases energy.
Making ATP requires 30.5 kJ/mol (positive DGo) while hydrolysis of ATP releases 30.5 kJ/mol (negative DGo). »
Hydrolysis of a high-energy bond, one that releases a useful
amount of energy.
Many organophosphates release energy when hydrolyzed some produce
enough energy to drive ATP formation. »
Table 11.1 page 411. Ex: The conversion of glucose to lactate is exergonic
Phosphorylation of ADP is endergonic
The two are coupled for a conservation of 61.0 kJ mol-1 (33%)
for use in anaerobic metabolism.
Under aerobic conditions, glucose is oxidized to carbon
dioxide and water (efficiency 34%).
Metabolism
Proceeds in Stages:
Steps allow for efficient production and usage of energy. The electrons produced by the oxidation of glucose are passed onto oxygen the final electron acceptor, by intermediate electron acceptors.
Many of these intermediate stages are coupled to ATP synthesis.
A step frequently encountered in metabolism is activation.
Acetyl-CoA (coenzyme A) is an important metabolite (compound
involved in a metabolic process). »
Coenzyme A is an important factor that is involved in activation to
form Acetyl-CoA which is important for energy metabolism.
Other important coenzymes include NAD+ and FAD+.
Anabolism proceeds in stages also and involves reducing agents like
NADH, NADPH and FADH2.
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