In the light reactions of photosynthesis,
the electron flow is also accompanied by the
pumping of protons from stroma to thylakoid
lumen. This proton pumping occurs through
a cyclic process called ‘Q cycle’ in cytochrome
b6f complex. In this video, we will see what
the Q cycle is, its detailed mechanism using
animations and why is it so significant in
the process of photosynthesis.[theme music].Hello
everyone, it’s me Shanty and you are watching
Biology Nowadays. Before we get into the details
of Q cycle, it is important to have a short
look at the initial steps of non-cyclic electron
transfer in photosynthesis. Here is P680,
the special pair chlorophyll a molecule present
in Photosystem II or PSII. When light falls
on P680, it gets excited and gives off an
electron. This electron is accepted by pheophytin,
the primary electron acceptor in PSII. The
lost electron from PSII is replaced by electron
from water splitting. The electron accepted
by pheophytin is transferred to plastoquinone
or PQ. At the same time PQ accepts an H+ ion
from stroma. PQ is a two electron carrier.
So PQ needs one more electron and the electron
transfer processes we saw just now repeat
once again i.e., a second photon hits P680
and it loses one of its electrons. This electron
is accepted by pheophytin. The lost electron
from P680 is replaced by the second electron
from water. The electron with pheophytin is
accepted by PQ. Now PQ received its second
electron. Along with that electron, PQ accepts
another H+ ion from stroma. Now PQ has two
electrons and two H+ ions. In this form it
is called hydroquinone or plastoquinol, PQH2.
Hydroquinone is mobile and it moves to cytochrome
b6f complex through the thylakoid membrane.
PQH2 passes the electrons, one at a time,
to plastocyanin or PC, through cytochrome
b6f complex. When one electron passes from
hydroquinone to PC, one H+ ion is liberated
into the thylakoid lumen. So when it loses
two electrons, two H+ ions are liberated into
the lumen. In addition to these H+ ions, two
more H+ ions are also pumped from stroma to
lumen. This proton pumping takes place through
a cycle called Q cycle in cytochrome b6f complex.
The ‘Q’ in the Q cycle stands for quinone.
The electrons accepted by PC will pass one
by one, through the rest of the electron acceptors
in the chain and finally reaches NADP+ for
the production of NADPH. A detailed description
of this electron transfer process has been
discussed in a previous video and I highly
recommend you to check that link given in
the description box below. In this video,
we will focus on the mechanism of Q cycle.
For this, let’s have a closer look at what
is happening in the cytochrome b6f complex.
Here is the cytochrome b6f complex and PC.
Eventhough, I have represented Cytochrome
b6f complex as a single unit, as the name
indicates it is a ‘complex’ consisting
of mainly 3 units-.cytochrome b6, cytochrome
f and an iron-sulphur Rieske protein. The
name “Rieske” comes from the name of its
discoverer, John S. Rieske. He and his co
workers, first discovered and isolated these
proteins in 1964. Before we move on to Q cycle,
let’s learn a little bit about the chemical
structure of cytochromes. Cytochromes are
based on porphyrin ring system, like... chlorophylls.
You know that chlorophylls have a magnesium
atom as the central atom of the tetrapyrrole
rings. But cytochromes have iron as the central
atom. The porphyrin ring with iron is called
‘heme’. Based on the structure of the
bound heme, cytochromes are divided into 3-
cytochrome a containing heme a, cytochrome
b with heme b and cytochrome c with heme c.
Cytochrome b6, as the name indicates is a
b-type cytochrome. But what about cytochrome
f? Chemically cytochrome f belongs to the
c-type cytochrome containing heme c. Cytochrome
f gets its name from its occurrence in chloroplasts,
‘f’ stands for ‘frons’. This latin
word means ‘leaf’. Cytochrome b6 is embedded
in the thylakoid membrane. It contains two
heme b molecules- heme bp and heme bn. They
are arranged in parallel and stand vertically
to the plane of the thylakoid membrane. The
lumen side of the thylakoid membrane is electrochemically
positive because it is in the lumen side the
H+ ions extrusion occurs. Since heme bp is
situated towards the electrochemically positive
side of the thylakoid membrane, the subscript
‘p’ denotes positive. The stromal side
of the thylakoid memebrane is electrochemically
negative because it is in the stromal side,
the H+ ions uptake occur. Since heme bn is
situated towards the electrochemically negative
side of the thylakoid membrane, the subscript
‘n’ denotes negative. Cytochrome f is
situated towards the lumen of the thylakoid.
As I said before, cytochrome f belongs to
c-type cytochrome and it contains a heme c
molecule. The next component Rieske protein
is only loosely embedded in the membrane.
It is composed of two iron atoms joined to
two sulfur atoms, forming a Fe2S2 centre.
The Q cycle involves 2 distinct catalytic
sites called Qo and Qi. The Qo site is on
the lumenal side of the thylakoid membrane
and Qi site is located on the stromal side.
Now let’s start the Q cycle. Firstly, a
hydroquinone molecule binds to the Qo site.
This site has a higher affinity for hydroquinone.
This hydroquinone actually comes from a group
of 7-10 PQ molecules attached to Photosystem
II. This group of PQ molecules is called PQ
pool. Some of the PQ molecules in this pool
will be in their reduced form i.e, hydroquinone.
Remember that the hydroquinone formed in the
photosystem II during the light reaction enters
this PQ pool. And, one hydroquinone from that
pool is now at the Qo site. Hydroquinone passes
one of its electrons to PC through Rieske
protein and heme c molecule. This path is
called the ‘high potential chain’. Now
let’s draw an arrow here. In the left hand
side of the arrow, let’s write the chemical
species entering the process of Q cycle. For
example now we have PQH2. In the right hand
side, we will write those who are coming out
of the Q cycle. So now, here we have the electron
which is with PC. When PQH2 loses an electron
it loses an H+ ion, which will come into the
thylakoid lumen. So let us write it in the
product’s side. PC will take that electron
to photosystem I and finally reaches NADP+.
Now the hydroquninone has one electron and
one H+ ion. In this form it is called ‘semiquinone’.
The next electron is accepted by heme bp .This
time the Rieske protein doesn’t have a chance
to get that electron. When heme bp accepts
the electron, an H+ ion is released into the
lumen. So in the equation, here it becomes
2. Actually the subscript ‘o’ in Qo stands
for ‘output’...proton output...because
it is at this site the protons are released
into the lumen. At the Qo site, now there
is a PQ molecule. So let us write in, PQ,
in the product’s side. As soon as it is
formed, it detaches from the Qo site and will
join the PQ pool. The electron which is with
heme bp is then transferred to heme bn. This
path of electron transfer is called the ‘low
potential chain’ because the redox midpoint
potentials of heme bp and heme bn is lower
when compared to that of Rieske protein and
cytochrome f in the high potential chain.
As you noted here, hydroquinone transferred
its electrons in two different ways. One electron
to the high potential chain, and the other
electron to the low potential chain. This
kind of differential transfer of the two electrons
by hydroquinone is called electron bifurcation.
At this time, a PQ molecule comes in from
the PQ pool and binds to the Qi site. A PQ
molecule entered the Q cycle, so...let us
write it in the reactant’s side. The Qi
site has a higher affinity to PQ. From heme
bn, the electron is transferred to the PQ
molecule at Qi site. As PQ accepts the electron
from bn, it also accepts an H+ ion from stroma.
So in the reactant’s side, we have to write
one H+ ion from stroma. The subscript ‘i’
in Qi stands for ‘input’...proton input...because
it is here where protons are added to the
PQ molecule. Now there is a semiquinone at
the Qi site. Until here it is called a half
cycle. A complete Q cycle consists of 2 half
cycles. So let’s see what will happen in
the next half cycle. At the start of the second
half cycle, another hydroquinone from the
PQ pool binds to Qo site. Let’s add it in
the reactant’s side. OK. Now the whole processes
which we saw in the last half cycle repeat
once again i.e, one of its electrons is passed
to PC through the high potential chain. So
here it will be 2. At this time, an H+ ion
is released into the lumen. So in the equation,
here it will become 3H+ ions. PC will take
the electron to photosystem I and finally
that electron reaches NADP+. Now at the Qo
site we have a semiquinone. The second electron
is passed first to the heme bp molecule of
the low potential chain. At this time, an
H+ ion is released into the lumen. So here
it will become 4H+ ions. PQ formed at the
Qo site detaches. Since a PQ molecule is formed
at Qo site, let’s add that also, in the
product’s side. The electron from heme bp
is then transferred to the heme bn molecule
of the low potential chain. From heme bn,
the electron is transferred to the semiquinone
molecule at the Qi site. As it accepts the
electron from bn, it also accepts an H+ ion
from stroma. So in the reactant’s side,
here it will be 2H+ ions from stroma. Now
there is a hydroquinone at the Qi site. So
let us add PQH2 in the product’s side. As
soon as it becomes hydroquinone, it detaches
from the Qi site and joins the PQ pool. Now
the second half cycle is also completed. These
two half cycles make one complete Q cycle.
Now let’s check the equation. On the reactant’s
side there are 2PQH2, one PQ molecule and
2H+ ions taken from stroma. On the product’s
side 2PQ molecules, one PQH2, 4H+ ions were
released into the lumen and 2 electrons were
transferred to PC. Let me cancel PQH2 and
PQ on both sides, the net result is PQH2+
2H+ from stroma give PQ+ 4H+ ions in lumen+
2 electrons i.e, one complete oxidation of
one of the PQH2 molecules along with the transfer
of 2H+ ions from stroma to lumen occurred.
Now, you may be remembering, PQH2 was formed
by combining PQ with 2 electrons and 2H+ ions
from stroma. So in effect 4H+ ions which were
originally in stroma have now landed safely
in the thylakoid lumen. You may be wondering
why I didn’t cancel the 2H+ ions from both
sides of the equation. Yes you can, because
they are chemically the same. But why I kept
them there because I wanted to show that the
H+ ions have changed their location from stroma
to lumen during our Q cycle. Now you look
at the molecular species that we have cancelled
from the equation. We had a PQH2 and PQ on
both sides of the equation. It means that,
in this reaction, PQH2 and PQ are consumed
and the same reaction produced PQH2 and PQ
itself or we reach at the same point where
we started. That’s what we call as a ‘cycle’.
During one complete cycle involving the consumption
and release of a PQH2 and PQ, another PQH2
enters through one side to the cycle and it
leaves the cycle as PQ+ 2H++ 2 electrons through
the other side. Likewise, 2H+ ions enter from
stroma to the cycle and leaves to lumen. Now
let us draw a schematic diagram of the Q cycle.
The PQH2 molecule comes in, at the Qo site
and transfers an electron to PC through the
high potential chain consisting of Rieske
protein and cytochrome f. One H+ ion is liberated
at this point and PQH2 becomes semiquinone.
SQ transfers the second electron to the heme
bp molecule of the low potential chain. During
this time, one more H+ ion is released into
the lumen. So there will be 2H+ ions in the
lumen. SQ now becomes PQ. From heme bp, the
electron is transferred to PQ bound at the
Qi site. This PQ will take up an H+ ion from
stroma and becomes SQ. So the first half cycle
is completed. In the start of the second half
cycle, there is a SQ here, but remember that
this SQ is the same SQ that was formed during
the first half cycle. Another PQH2 comes in
and transfers one of its electrons to PC through
the high potential chain. The second electron
is transferred through the low potential chain
to the SQ. Two H+ ions are liberated into
the lumen during the second half cycle. A
PQ is also formed at the Qo site. The SQ at
the Qi site accepts an H+ ion from stroma
and becomes PQH2. Thus the second half cycle
is also completed. Here is the net reaction
we saw earlier. So I hope it is clear how
cytochrome works as a proton pump during the
non-cyclic electron transfer. Now let me ask
you a question. What will happen under over
reducing conditions? i.e, when the availability
of PQ is very less at the Qi site or in other
words when the PQ pool consists of mostly
hydroquinones. How will the plant deal with
this problem? In such situations, instead
of the Q cycle, a similar cycle called ‘semiquinone
cycle’ or ‘SQ cycle’ will operate. Semiquinone
cycle was proposed by Wikstroem and Krab in
1986. There is only a small difference when
we compare Q cycle to semiquinone cycle. Since
the name of the cycle is ‘semiquinone cycle’,
the difference should be in the step related
to semiquinone formation. Let’s see in detail
what the difference is. At the start of the
semiquinone cycle, as in the q cycle, a hydroquinone
molecule from the PQ pool binds to Qo site
and transfers one of its electrons to PC.
At this time, the hydroquinone releases an
H+ ion into the lumen. Let us write it in
the equation. PC will take the accepted electron
to PSI. At the Qo site there is a semiquinone.
Until here it is the same as in the Q cycle.
Now remember that the plant is dealing with
a situation where there is less PQ in the
pool to bind to the Qi site. In such conditions,
the semiquinone formed at the Qo site is transferred
to the Qi site. Simple! Now a second hydroquinone
binds to Qo site. So let’s write it in the
reactant’s side. It transfers one of its
electrons to PC. So here it will be 2. An
H+ ion will be released into the lumen. So
here it will be 2. PC will take the electron
to PSI. The second electron of the hydroquinone
will be passed to heme bp. At this point the
other H+ ion will also be released into the
lumen. So now in the lumen there are three
H+ ions. There is a PQ molecule at Qo site
and it detaches from there. So let us write
it in the equation, in the product’s side.
The electron accepted by heme bp is passed
to heme bn and ultimately reaches the semiquinone
bound at Qi site. The semiquinone accepts
an H+ ion from stroma. In the equation, let
us write it in the reactant’s side. Now
there is a hydroquinone molecule at the Qi
site. So let us write that, in the product’s
side. The hydroquinone detaches from the site.
Let us check the equation. PQH2 on both sides
can be cancelled. So the final equation becomes
PQH2+ one H+ from stroma give PQ+ 3H+ in the
lumen+ 2 electrons to PC. In effect one complete
oxidation of one of the PQH2 molecules along
with the transfer of one H+ ion from stroma
to lumen occurred. One advantage of semiquinone
cycle is that it regenerates PQ in the pool
while creating a proton gradient across the
thylakoid membrane. In both Q cycle and semiquinone
cycle, there is an H+ ion build up in the
thylakoid lumen. This generates a chemiosmotic
potential across the thylakoid membrane. Due
to this, the H+ ions pass back to the stroma
driving the ATP synthase and result in the
production of ATP molecules. ATP is one of
the products of light reactions in photosynthesis.
So the proton pumping that takes place here
through Q cycle has a great significance as
it is responsible for the production of ATP.
Actually, the current mechanistic model for
the b6f complex derives from the Q cycle model
of cytochrome bc1 complex. Cytochrome bc1
complex, is seen in the inner membrane of
mitochondria and participates in the mitochondrial
respiratory chain. The Q cycle was originally
proposed by Mitchell of Glynn research laboratories,
UK in 1975 and later it was modified by Crofts
et al., University of Illinois, US in 1983.
And that’s all for this video and stay tuned.
