Introduction to Electron Transport Chain - Двуязычные субтитры
So now that we have a general idea of what ox data phosphorylation is and what the electron transport chain actually does,
let's begin by discussing briefly the major types of complexes that we find along the electron transport chain.
So we have four major types of complexes, protein complex one, protein complex, two protein complex, three and protein complex, four.
In addition, we also have two very important electron carrier molecules used by the electron transport chain.
One of them is known as Q,
which stands for coenzyme Q, also known as ubiquinone, and the other known as, and the other one is known as, side of protein.
So, in this lecture I'd like to briefly discuss these different types of structures and in
the next many lectures to come we're basically going to look at the details of what actually happens within each one of these complexes.
Let's begin with protein complex one.
Now, by the way, this is a diagram of only one of the electron transport chains of the many electron transport chains that exist in a single mitochondrial
inner membrane.
So this is the inner mitochondrial membrane.
This is the mitochondrial matrix.
And is the intermembrane space that exists between the two membranes.
So protein complex one is also known as NADH dehydrogenase or NADH oxidoreductase and that's because this is the protein complex that actually accepts those high energy electrons from the NADH
molecules which were produced in glycolysis as well as the citric acid cycle.
Now this complex is actually a large complex.
In fact it consists of about 46 individual polypeptide chains.
And we examine the shape of protein complex one, we'll notice that it looks like the letter L.
So, the letter L, which basically looks like this, contains a horizontal component and a vertical component.
Now, the vertical component is basically exposed to the matrix of the mitochondria.
But the horizontal component, this component lies entirely in the membrane, the inner membrane of the mitochondria.
So the entire function of protein complex 1 is to accept those high energy electrons from NADH molecules and to move those electrons along a special pathway within the complex
that we're going to focus on in detail in a lecture to come.
And electrons ultimately transferred onto the electron carrier molecule known as coenzyme QE bicuenone that we're going to look at in just a moment.
Now this protein complex, number one, also acts as a proton pump.
So the fact that we have the movement of electrons within this complex that electrons that generates a flow of
form of energy that allows us to actually move these H plus ions across this
membrane to this side and that ultimately allows us to actually generate a proton electrochemical
gradient that will ultimately be used by the ATP synthase molecule.
to form the ATP molecule.
So is protein complex one.
So once again,
complex one also known as NADHD hydrogenase or NADH oxidore ductase is a very large complex that consists of about 46 individual polypeptide chains.
It has an L shape that contains a horizontal component found within the inner membrane and
the vertical component that lies within the matrix of the mitochondria.
And the transfer of electrons, as we'll see in the future lecture, actually takes place within this vertical component of this complex.
Now, the function of the complex is to actually oxidize the NADH back to NAD+, accept those high
energy electrons and then move those high energy electrons along a specific pathway.
And generates an electric current that basically allows us to pump those H plus ions
across the membrane from the matrix and into the intermembrane space.
Now, let's move on, or actually since we're on the subject, let's look at coenzyme Q.
So coenzyme Q, also known as ubiquinone, is pretty much this small molecule that is dissolved in the membrane of the mitochondria.
This is shown here.
And because it contains a relatively large hydrophobic region, it can dissolve easily in the membrane, and so it can move across the membrane.
And what it does is,
it takes those electrons from protein complex in the received by the NADH molecule and it shuttles, it moves those electrons onto protein complex three.
In this coenzyme Q is also used to actually pick up the electrons received by protein complex one and move those electrons onto protein complex.
3.
So, let's move on to protein complex 2.
Now, the entire function of protein complex 2 is to actually accept and extract those high-end electrons from the FADH2 molecules, which
synthesized in the citric acid cycle.
So, if we recall the citric acid cycle, there's a step in the citric acid cycle where we basically transform, we oxidize, succinate into fumarate.
And in this process, we have an enzyme known as succinate dehydrogenase that essentially reduces the FAD into FADH2.
So remember FAD is flavon adenide dinucleotide, which capable of actually receiving extracting those high energy electrons.
So these two H plus ions and one electron from each one of these two bonds is extracted
and then placed onto the FAD to form FADH2.
And actually, this succinate dehydrogenase that is used by the citric acid cycle is found within complex 2.
So complex 2 found on the inside portion of the inner membrane of the contains the succinate dehydrogenase that is used by the citric acid cycle.
And so actually,
when we synthesize the FADH2 in the citric acid cycle,
it remains attached onto the succinate dehydrogenase of this protein complex too,
and within that complex,
we that FADH2 is actually oxidized back into FAD and that allows those two high energy electrons to be extracted by this complex.
And as we'll see in the future, those electrons are then taken by this coenzyme Q carrier, our ubiquinone.
so ubiquinone is able to shuttle to move the electrons not only from protein complex one to three,
but also from protein complex two to three.
So protein complex two,
also known as succinate reductase,
contains the succinate dehydrogenase enzyme that is used in by the citric acid cycle to transform succinate into fumarate in the process, forming FADH2.
Now, one important distinction between protein complex 2 and the other protein complexes is
protein complex 2 is not a proton pump, it does not act actually move the protons from the matrix side to the intermembrane side.
Only protein complex two and as we'll see in just a moment protein complex three and four are actually
proton pumps and are used to generate electrochemical gradients for proton.
So let's move on to complex 3.
So complex 3 is shown here.
And this complex is also known as cytochrome C oxidoreductase or Q cytochrome C oxidoreductase and sometimes known as cytochrome reductase.
And what this proton comp or what this complex does is it accepts those electrons from the cue carrier molecule,
the and then it takes those electrons and transfers them onto another electron carrier known as cytochrome C.
Now just like protein complex,
one is also proton pump,
and as a result of the movement of those electrons within this complex that allows the structure to actually pump those
protons into the intermembrane space from the matrix of the mitochondria.
And finally if we examine protein complex 4, this is essentially where we take those electrons and we use electrons.
use them to reduce oxygen molecules into water molecules.
So is where we find the final electron acceptor of the electron transport chain.
And this also uses the movement of those electrons to pump those protons out of the matrix and into the space between the two membranes.
And so together protein complex 1, 3, and 4 are basically proton pumps which help establish a proton gradient.
And as we'll see in a future lecture, the ATP synthase uses that proton gradient to generate those ATP molecules via oxidative phosphorylation.
And it's called oxidative because protein complex for use as oxygen as that final electron acceptor So,
we see that coenzyme Q,
also known as ubiquinone,
shown here,
is basically a small hydrophobic molecule that is dissolved in the inner mitochondrial membrane and it acts
as an electron carrier that shuttles electron from protein complex 1 or protein complex 2, ultimately 2 protein complex 3.
Now in its oxidized form, we call it ubiquinone Q, and this is what the structure actually looks like.
And no- a long hydrophobic tail in humans, the N value is usually 10, and why we call it coenzyme Q10.
And happens is,
the two electrons and two H plus ions are accepted,
so one electron and one H plus ion is accepted by each one of these groups shown here and here.
form the fully reduced form we call ubiquinol or QH2.
So structure accepts electrons from either complex 1 or 2 to form the reduced form QH2 and then that travels to complex
3 where it gives off those electrons.
to complex 3, which ultimately move along the complex and then to cytochrome C.
And C, unlike coenzyme Q, is a small water-soluble protein.
So remember, this structure is not a protein.
Coenzyme Q is not a protein, but cytochrome C is a protein.
and into what a soluble protein it is bound onto the intermembrane region of
complex 3 and once it accepts those electrons it moves and attaches
onto protein complex 4 on this side of the complex and then it transfers those
electrons which ultimately use to actually reduce that oxygen and form the water molecules, as we'll see much more detail in lecturers.
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