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BOGDAN FEDELES: Hi, everyone.
Welcome to 5.07 Bio
Chemistry Online.
I'm Dr. Bogdan Fedeles.
I'm going to help you
work through some more
biochemistry problems today.
I have here question
2 of Problem Set 8.
Now, this is the
question I put together
to get you thinking about
the electron transport chain.
As you know, the
electron transport chain
is a fundamental redox
process through which
we convert the chemical
energy of the covalent bonds
into an electrochemical
gradient.
This electrochemical
gradient is like a battery,
and it can be used inside
the cell to generate,
for example, ATP, which is the
energy currency of the cell,
or it can be dissipated
to generate heat.
We're going to see both
of these modes in action
in this problem.
Now in most organisms, the
electron transport chain
helps to transfer electrons all
the way to molecular oxygen.
However, in this
problem, we're dealing
with an organism that
lives deep inside the ocean
where the atmospheric
oxygen is not available.
And it turns out this organism
transfers its electrons
to sulfate.
Sulfate is the final
electron acceptor.
Part A of this problem
asks us to write
the order of the
electron carriers
as they would function
in an electron transport
chain for this organism.
Now, for a number
of redox processes,
the problem provides a table
with the electrochemical
reducing potentials,
as you see here.
Now, I've selected the ones that
are mentioned in the problem,
and I put them into
a smaller table here.
As you can see, we're dealing
with cytochrome A, B, C, C1.
This is the flavin
mononucleotide.
This is the sulfate, the
fine electron acceptor,
and ubiquinol.
Now, on this column here we
have the redox potential,
which are the electrochemical
reduction potentials denoted
by epsilon, or e0 prime.
Now, e0, as you know from
physical chemistry or physics,
denotes the
electrochemical potential
in standard conditions.
However, in biochemistry,
we use the e0 prime notation
to denote that the pH
is taken into account,
and it's not what
you would expect,
like of hydrogen ion's
concentration equals 1 molar,
but rather it's a pH of 7.
The hydrogen ion's concentration
equals 10 to the minus 7.
So therefore, these numbers are
adjusted to correspond to pH 7.
The electrochemical
potentials we
see in this table are
reduction potentials,
and they tell us how easy it is
to reduce a particular species.
Therefore, the higher
the number, the easier it
is to reduce that particular
species and the more energy
the reduction of that
species will generate.
Therefore, the electron
transport chain
will go from the
species that hardest
to be reduce towards the species
that are easiest to be reduced.
Therefore, the order of
the electron carriers
will be from the ones that
have the lowest reductive
potential to the ones
that have the highest
reductive potential.
So now if we're going to sort
all these electron carriers
in order of their
potential, we're
going to get the following
order as you see here.
So the electrons are going
to flow from the flavin
into the coenzyme Q,
and then the electrons
are going to flow coenzyme
Q to cytochrome B, and then
Cytochrome C1, C,
A, and sulfate.
And as you can see, flavin has
a negative reduction potential.
It's like the hardest
to be reduced.
And the next one is ubiquinol.
It's barely positive.
And then the highest number
is sulfate 0.48 volts.
Now, let's take a closer
look how the electrons
are going to be transferred
through this proposed electron
transport chain.
In the first reaction,
here we have the flavin,
I've written the flavin
adenine dinucleotide,
FADH2, the reduced
version, is going
to be converted to the
oxidized FAD version of it.
And in this redox
reaction, we're
going to use the coenzyme
Q, the oxidized version
and reduce it in the process.
So the electrons get transferred
from FADH2 to coenzyme Q.
Now, in the next reaction, the
reduced version of coenzyme Q
is going to get oxidized
back to coenzyme Q
and in the process
cytochrome B is
going to go from its oxidized
form to its reduced form.
Now, this process continues
with every single step,
every single electron carrier
up until we get to the sulfate
where the reduced form
of the cytochrome A
will donate its
electrons to the sulfate,
and sulfate would get
reduced to its reduced form.
It's called sulfite.
So if we were to draw
how the electrons move
through this chain,
the electrons
are going to start at
FADH, and then they're
going to be transferred to
coenzyme Q in the reduced form.
And then coenzyme Q is going
to pass it to the cytochrome B.
That's going to be
in its reduced form.
And then cytochrome
B is going to pass
it to cytochrome C1, and then
cytochrome C, cytochrome A,
and finally, they're going
to end up in sulfite.
Another thing to
notice here is that
except for the initial
flavin and the final electron
acceptor, sulfate, all the other
intermediates get regenerated.
So we go from the oxidized
version to the reduced version
and back to the
oxidized version.
So all these electron carriers
are going to be sufficient only
in catalytic amounts.
So the only thing
that gets consumed
is the FADH2 and the sulfate.
These are two reactants.
And we get in this
reaction FAD and sulfite.
What we just said will
help us segue into the Part
B of the problem, which asks us
to calculate how much energy do
we get by converting
one molecule of FADH2
and one molecule of sulfate into
FAD and sulfite, respectively.
Now as we pointed out here,
only the FADH2 and sulfate
are consumed in this reaction.
All the other electron carriers
are recycled and regenerated
in the course of the
electron transport chain.
In order to
calculate the energy,
it's useful first to write
the half reaction of the redox
processes.
Here are the two half reactions
of this redox process.
FADH2 gets oxidized through FAD
and donates its two electrons.
And the epsilon, or e0
prime is minus 0.22 volts.
Now, this is the
potential from the table,
and that's a
reduction potential.
The equation as written
is an oxidation,
and therefore, the
potential that we
need to take into account
is the minus of this one.
Sulfate is then going to
accept the two electrons
and going to get reduced
to the sulfite and water.
And the electrochemical
potential for this
is 0.48 volts.
So now when we add
these two together,
we get the overall process where
FADH2 gets oxidized by sulfate
to generate FAD and sulfite.
And the electromotive force
is just the mathematical sum
of these two keeping
in mind that this has
to be taken as a negative sign.
Because, again, as
written, this is
an oxidation and this the
potential for the reduction
reaction.
So electromotive force
is actually 0.7 volts.
Now, we can easily convert
from the electromotive force
to a delta g0 prime value,
and the relationship
is written here, delta g0 prime.
It's minus nF delta e0 prime
and is the number of electrons
in the process as we see here.
Two, F is the Faraday's
constant and delta e0
prime is going to be
the electromotive force.
And if we go through
the number crunching,
we get a delta g0 prime minus
135 kilojoules per mole.
Notice because it's a
negative number that means
there's a spontaneous
process as written.
And as you know,
the negative delta g
will correspond to a
positive electromotive force.
Now, we're just one step
away from calculating
how much ATP we can
produce with this energy.
As you know, we generate ATP
out of ADP and phosphate,
and this is the reaction that's
catalyzed by ATP synthase.
And it takes about 30.5
kilojoules per mole
to form ATP out of
ADP and phosphate.
Therefore, the 135
kilojoules per mole
that we generated
from 1 mole of FADH2,
it's going to be enough for
about 4 molecules of ATPs.
This is in contrast, which
was the normal processes that
use oxygen as their
final electron acceptor
where out of one FADH2
molecule, will generate
at most 2 molecules of ATP.
So in some ways, sulfate is
actually a better electron
acceptor and can
give us more energy.
Part C of these problem
deals with a culture
of this microorganism
in the lab.
And we're adding to this
culture dinitrophenol,
a compound we're told
has a pKa of about 5.2.
So let's explore what happens
to the electron transport
chain of the organism
when we add dinitrophenol.
Here I put together a
cartoon representation
of the electron transport
chain of our organism.
So as you can see here, this is
the extracellular environment.
This is the outer membrane.
This is the inner membrane where
we have all these complexes
I denoted here with these
rectangles of the electron
transport chain.
And FADH2, for example, is
going to donate its electrons.
They're going to be passed
along all the way to sulfate.
And in the process,
protons are going
to get pumped into this
intermembrane space.
Now, these protons can be
used in the ATP synthase
as they travel back into
the intercellular space.
Their energy can be used to
convert ADP and organophosphate
to ATP as we just
discussed in Part 2.
Now, to this organism,
we said we're
going to add dinitrophenol.
Here is the structure
of dinitrophenol.
And we're told the
pKa of this proton,
right here, the
pKa is about 5.2.
When this compound diffuses
through the membrane,
it's going to go through this
intermembrane space, which
has a very low pH and
also in the intercellular
space in the cytosol,
which has a much higher pH.
So because pKa 5.2, it's a
relatively low, much lower
than 7, pKa, in the
intermembrane space where
it's more acidic, it's
going to be protonated.
So we can write, for example,
dinitrophenol OH in equilibrium
with dinitrophenol O
minus plus a proton.
Now, because here we
have a lot of protons,
this equilibrium will
be shifted to the left.
That is the protonated
form of dinitrophenol.
However, here in the
cytosol, the NPOH,
it's going to be in the
same equilibrium O minus
plus H plus.
But because the pH is
fairly high, that is
there are not a lot of
protons, this equilibrium
is going to be
shifted to the right.
This equilibrium is going
to be shifted to the left.
So now look what happens.
So because this equilibrium
has shifted to the left,
it's going to keep soaking
up a lot of these protons.
Then the neutral
dinitrophenol molecule
is going to diffuse through
the membrane as such
and enter the intercellular
space to cytosol where
it's going to be deprotonated.
The equilibrium is
shifted to the right.
So in effect,
dinitrophenol is going
to carry the protons from
the intermembrane space
inside the cell.
Now it's going to
do that in parallel
with the protons that are going
to be flowing through the ATP
synthase to generate ATP.
So in effect, we're
discharging this battery
where the concentration
of protons
is basically our
electrochemical gradient.
It's going to be discharging the
battery without producing ATP.
So as you know, if you
short circuit a battery,
the battery is going to heat
up because you're discharging
an electrochemical gradient.
Similarly, dinitrophenol,
by taking these protons
from the intermembrane space
and bringing them inside
into the intercellular
space, it's
going to be generating heat.
Therefore, we can
answer Part C by saying
that the medium in which
these cells are growing
is going to heat up when
we add dinitrophenol to it.
The processes described in this
problem are fairly universal.
Now, in eukaryotes, like
more evolved organisms,
they would happen
in the mitochondria.
Now, if you look
back at this diagram,
if this was the double
membrane of the mitochondria,
this would be the
inside of the cell that
contains the
mitochondria, this would
be the intermembrane
space, and this
will be the inside
of the mitochondria
or the mitochondrial matrix.
Similarly, by adding a
compound like dinitrophenol,
who can dissipate the
electrochemical gradient
in the mitochondria and
cause the cell to heat up.
In fact, this process
is actually used
by a number of organisms
to generate heat instead
of chemical energy, or ATP.
For example, the brown fat
cells in newborns in mammals
have a special
protein that allows
to dissipate this
electrochemical gradient
in the mitochondria
to generate heat.
Another good example is
the seeds of many plants.
When they germinate,
they actually
generate a lot of
heat that can be
used to melt the ice or
the snow around them.
That's why some
of the plants can
start growing even
before the snow has
melt in the early spring.
I hope that working
through this problem
will help you understand
better the inner workings
of an electron
transport chain and how
it can convert the chemical
energy of chemical bonds
into an electrochemical
gradient, which
can then be used to generate
high energy compounds like ATP.
Or it can be dissipated
to generate heat.
