Bioenergetics II

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Bioenergetics II
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Learning Objectives
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free-energy, G expresses the amount of energy
available to do work in a chemical reaction at
constant temperature and pressure.
endergonic pertains to a reaction in which DG is
negative, i.e. the reaction proceeds with the
release of energy. exergonic pertains to a
reaction in which DG is positive, i.e. the system
gains free-energy.
heterotroph an organism that requires complex
nutrients such as glucose as a source of energy
and carbon.
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Standard free-energy changes are additive
Two sequential chemical reactions
A B
B C
DG1o
DG2o
A C
DGTo DG1o DG2o
The DGo values of sequential chemical reactions
are additive.
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Glucose Pi glucose-6-phosphate
H2O
DGo 13.8 kJ/mol
ATP H2O ADP Pi
DGo -30.5 kJ/mol
ATP glucose ADP
glucose-6-phosphate
DGo -16.7 kJ/mol
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Phosphoryl group transfers and ATP
Heterotrophic cells obtain free-energy in a
chemical form by the catabolism of nutrient
molecules (i.e. glucose), and they use that
energy to synthesize ATP from ADP and Pi. ATP
then donates some of that energy to endergonic
processes such as synthesis of metabolic
intermediates and macromolecules (DNA, proteins),
the transport of molecules across membranes
against concentration gradients, and mechanical
motion (muscle contraction).
The donation of energy from ATP generally
involves the covalent participation of ATP in the
reaction to be driven. Most case of energy
donation by ATP involve group transfer, not
simple hydrolysis of ATP.
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Chemical basis for the large free-energy change
associated with ATP hydrolysis
(1) Hydrolysis causes charge separation which
relieves electrostatic repulsion among the four
negative charges on ATP.
(2) Inorganic phosphate, Pi, released by
hydrolysis is stabilized by the formation of a
resonance hybrid in which the hydrogen is not
permanently attached to any one of the oxygen
atoms.
(3) ADP2- immediately ionizes, releasing a proton
into a low concentration of H (i.e. pH 7).
(4) The products Pi and ADP are solvated to a
greater extent than the reactant ATP.
Stabilization of products relative to reactants
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Formation of Mg2 complexes with ATP partially
shields the negative charges and influences the
conformation of the phosphate groups. The active
sites of enzymes that utilize Mg2-ATP as
substrate are designed to interact with the
complex, not ATP alone. Thus, most of these
enzymes have an absolute requirement for Mg2 for
catalysis.
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DGo for hydrolysis of ATP -30.5 kJ/mol
Although the hydrolysis of ATP is highly
exergonic, the molecule is kinetically stable at
pH 7 because the activation energy for hydrolysis
is relatively high. Rapid cleavage of the
phosphoanhydride bond occurs only when catalyzed
by an enzyme.
In vivo, the energy released by ATP hydrolysis is
greater than the standard free-energy change.
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For intact human erythrocyte actual DG -51.8
kJ/mol
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DGo -16 kJ/mol
DGo -46 kJ/mol