Energetics and energy conversion

1
Lecture 38 Energetics and energy
conversion Loosely based on pages 68 to 108
2
Thermodynamics, Energy, and Life. First law of
thermodynamics The amount of energy of the
universe is constant. Energy gets converted but
the total amount stays constant. This is
relevant because the energy associated with
chemical bonds can be converted to heat, and heat
increases motion. So what? Second law of
thermodynamics The disorder in the universe can
only increase. The highly ordered state of a
living being comes at the cost of increasing the
disorder of the surroundings. Collectively, all
of the chemical reactions that keep a cell alive
release heat to the surroundings. This heat
increases the motion of molecules in the
surroundings and the increased motion results in
greater disorder overall.
system
surroundings
3
The cell stays alive by coupling favorable
reactions that increase disorder with unfavorable
reactions that increase order. What is meant by
favorable and unfavorable reactions? Stated
another way is A B gt C D more or less likely
than C D gt A B. The favored direction of a
reaction is predicted by the change in Gibbs free
energy, ?G, associated with a reaction. ?G lt 0
favorable ?G gt 0 unfavorable ?G 0 the reaction
is at equilibrium and the conversion of reactants
to products is balanced by the conversion of
products to reactants.
4
?G depends on the change in enthalpy (?H) and the
change in entropy (?S) and is described by the
equation ?G ?H - T?S. ?H is the change in
enthalpy of the products and reactants. It
measures changes in bond energies (both covalent
and noncovalent). By definition, bond formation
that releases heat has a negative ?H - reaction
is said to be exothermic. A negative ?H is
favorable because the heat released increases
disorder by increasing the movement of molecules
in the surroundings. Bond formation that takes
up heat has a positive ?H and is said to be
endothermic. A positive ?H is not favored
because the uptake of heat from the surroundings
reduces molecular motion in the surroundings and
hence increases order. ?S is the change in
entropy of the products and reactants. Entropy
measures the randomness of the reactants and
products. By definition, an increase in entropy
(randomness) has a positive value and is
favorable. An endothermic reaction can be
favorable if the uptake of heat from the
surroundings results in an increase in the
disorder of the system. The positive ?S term
exceeds the positive ?H term.
5
?G of a reaction is dependent on the
concentration of products and reactants. ?G
?Go RTlnproducts/reactants For A B gt C
D ?G ?Go RTlnCD/AB ?Go is the
standard free energy change that occurs when the
initial concentrations of reactants and products
is 1M, the pH is 7.0, the temperature is 25 oC,
and pressure is 1 atmosphere. These values for
many reactions have been experimentally
determined and can be found in reference
books. The free energy values given in textbooks
are standard free energy changes of reactions.
It allows one to predict the direction of a
reaction starting with standard conditions.
However, an unfavorable reaction can be made
favorable by altering the concentrations
reactants or products according to the equation
?G ?Go RTlnCD/AB.
6
An unfavorable reaction can be pushed in the
forward direction by increasing reactants or
pulled in the forward direction by depleting
products. As a result of the latter, an
energetically favorable reaction can be used to
pull an energetically unfavorable reaction in the
forward direction.
In this example, the conversion of Y to Z pulls
the reaction of X to Y to the right. A fact
the overall change in free energy equals the sum
of the free energy changes of individual
reactions. For X gt Y, let ?Go XgtY A For Y gt Z,
let ?Go YgtZ B Then for X gt Z, ?Go XgtZ A
B
7
?Go can be determined by measuring the
concentrations of reactants and products after a
reaction has been allowed to reach
equilibrium. At equilibrium, ?G 0 Therefore, 0
?Go RTlnproducts/reactants ?Go -
RTlnproducts/reactants ?Go - RTlnK K is
defined as the equilibrium constant and it is the
ratio of products over reactants when the
reaction has reached equilibrium. Regardless of
the amounts of products and reactants you start
with, the ratio is always the same when the
reaction reaches equilibrium. For the reaction A
B gt C D, K CD/AB Note that just
because nothing appears to be happening in a
reaction, you should not conclude a reaction has
reached equilibrium. A sugar cube has the
potential to react with oxygen to form carbon
dioxide and water. The free energy change of
this reaction is very negative, so virtually all
of the sugar would be converted to CO2 and water
when the reaction reaches equilibrium.
Nevertheless, the sugar cube will sit unchanged
unless something is done to initiate this
reaction. Hence, the rate at which the
unassisted reaction proceeds to equilibrium can
be extremely slow. The reason for this is that
the unassisted reaction has a very high
activation energy.
8
A possible point of confusion You might think
?Go should be zero since the value is determined
by measuring the concentrations of products and
reactants after the reaction has reached
equilibrium. This is not the case because
scientists have defined ?Go as the free energy
change that occurs when one starts with each of
the reactants and each of the products at 1M and
allows the reaction to occur. Hence the value of
?Go gives a measure of which way the reaction
will go if you start with all the components at
1M.
9
A reaction with a positive ?G must be coupled to
a reaction with a negative ?G for the first
reaction to occur.
10
An example of coupling ATP hydrolysis with an
unfavorable reaction. ATP H2O gt ADP Pi ?Go
-7.3 kcal/mole ( -32 kjoules/mole) The cell
maintains a high concentration of ATP so this
reaction in the cell has a ?G -12 kcal/mole
(note the the absence of the o). Hence, ATP
hydrolysis should be able to drive an
energetically unfavorable reaction that has a ?G
of just under 12 kcal/mole or less if there is a
way to couple ATP hydrolysis with the other
reaction. Consider the conversion of glutamic
acid to glutamine.
Glutamic acid NH3 gt Glutamine H2O ?Go
3.4 kcal/mole ATP H2O gt ADP Pi ?Go -7.3
kcal/mole By coupling the reactions, the overall
change in free energy is the sum of the
individual changes in free energy. Hence ATP
Glutamic acid NH3 gt Glutamine ADP Pi ?Go
-3.9 kcal/mole In the cell, this reaction is
made even more favorable by the high ATP
concentration.
11
ATP hydrolysis can also be used to generate
energetically unfavorable states. Consider the
Na - K ATPase.
Establishment of the asymmetric distribution of
Na and K on each side of the membrane
represents an increase in order. In the language
of thermodynamics, the entropy of the system has
decreased which would correspond to a positive
?G. The reaction is driven by the negative ?G
associated with ATP hydrolysis.
12
The Na K ATPase can be artificially reversed to
generate ATP! Reconstitute a proteoliposome with
high Na inside, high K outside, and ADP and
inorganic phosphate - no ATP.
K
K
ATP
ADP Pi
Na
Na
Proteoliposome provided ADP and Pi plus high Na
inside and high K outside. Efflux of Na and
influx of K will drive ATP synthesis.
Proteoliposome provided ATP. Hydrolysis of ATP
will pump Na in and K out.
13
What keeps an energetically favorable reaction
such as ATP hydrolysis from proceeding before the
reaction can be coupled to something productive
(recall the sugar cube from a few slides
ago)? The activation energy of a reaction slows
reactions even to the point that an energetically
favorable reaction doesnt occur at a significant
rate unless the reaction is catalyzed. Hence,
intrinsically unstable compounds like ATP exist
long enough in the cell to be useful.
Enzymes speed up the rate of reaction by lowering
the activation energy. They do this by
stabilizing the transition state. They can also
guide a reaction in a particular direction by
stabilizing the transition state for one reaction
path and not another. Enzymes do not affect
the overall ?G of the reaction. Hence, they can
not affect the direction of the reaction. The
same ratio of products and reactants is produced
when a reaction reaches equilibrium regardless of
whether it is catalyzed or not. Enzymes only
affect the rate of a reaction.
14
Energy production and metabolism in cells.
15
Our discussion is going to focus on the
production and destruction of glucose, which
turns out to be the major pathway by which the
cell obtains energy and also provides
intermediates for other biosynthetic reactions.
16
The metabolic reactions in the cell can be
separated into those that breakdown organic
molecules to generate energy and those that
utilize energy to synthesize organic molecules.
The first are known as catabolic reactions and
the second are known as anabolic reactions.
Organic biomolecules Low stability
Catabolic reactions Oxidation Dehydrogenation Oft
en generates ATP and NADH
Anabolic reactions Reduction Hydrogenation Often
requires ATP and NADPH
CO2 High stability
17
Glycolysis, citric acid cycle and oxidative
phosphorylation are catabolic reactions whereas
photosynthesis is an anabolic reaction.
18
The oxidation of sugar releases a lot of energy.
The cell controls this oxidation process in such
a way as to capture the released energy in
activated carrier molecules such as ATP and NADH.
19
Glycolysis is the first stage in cells when
glucose is being metabolized to produce energy.
Glycolysis occurs in the cytosol of all organisms
including bacteria and eucaryotes.
Glucose is oxidized to two molecules of pyruvate
and there is a net production of small amounts of
ATP molecules and NADH molecules.
20
The oxidation of carbon is energetically
favorable. The energy released is used to
generate ATP and NADH. To determine if carbon
is being oxidized, count the number of
carbon/oxygen bonds present in the reactants and
the products.
Hydrocarbon.
Alcohol
Aldehyde or ketone
Carboxylic acid
21
Glucose has 7 C-O bonds and two molecules of
pyruvate have 10 C-O bonds. Hence there is an
overall oxidation of carbon releasing energy that
is captured as NADH and ATP.
22
Usually when carbon is oxidized during a
catabolic reaction, a hydride ion is transferred
to NAD to produce NADH. NADH is both an energy
carrier and an electron carrier. As well see
later, NADH is going to reduce other atoms so I
refer to it as reducing power.
H- is equivalent to H and 2 electrons. H- H
2e-
This figure was taken from the 3rd edition of
Alberts
23
During glycolysis, ATP is made by a mechanism
called substrate level phosphorylation. A high
energy chemical intermediate is generated through
formation of a covalent bond with the enzyme
catalyzing the reaction. The intermediate reacts
with inorganic phosphate to form another high
energy intermediate and finally the phosphate is
transferred to ADP to produce ATP.
Note that overall, the terminal aldehyde has been
oxidized to a carboxylic acid. The energy
released is captured in ATP and NADH. Also note
the obvious role that the enzyme plays in guiding
this reaction - it forms a covalent intermediate
with the substrate. Most metabolic steps are
catalyzed by enzymes.
?Go -3 kcal/mole