Complex I and II of Electron Transport Chain - 双语字幕
So now that we have a general idea of what the electron transport chain actually does,
let's discuss the details of what happens in the complexes of the electron transport chain.
And so we're going to begin our discussion by examining the details of what happens in complex one and complex two.
complexes are found in the inner membrane of the mitochondria.
So this is complex one,
this is the inner membrane of the mitochondria, this is the matrix of the mitochondria, and this is the intermembrane space.
Now, complex one is a very large complex.
It's an L-shaped multisub, you know, that kind of contains about 46 individual polypeptide chains, so one is a very massive complex.
Now, complex one is also known as NADHD hydrogenase or NADH oxyl reductase.
And the reason we call it this is because this is the complex.
of the electron transport chain that ultimately accepts the high energy electrons from NADH
molecules that we generate in processes such as the citric acid cycle and glycolysis.
So, this L-shaped structure contains the horizontal component that lies in the membrane of the mitochondria,
the inner membrane, and we have a vertical component that extends into the matrix of the mitochondria.
And the NADH actually binds onto this extension that lies in the matrix of the mitochondria.
And along with the NADH, an H plus ion, actually use and we'll see why that H plus I is needed in just a moment.
And so in the process,
we ultimately oxidize the NADH back into NAD plus, and those two electrons are extracted by a group known as F.M.N.
and the F.M.N.
stands for Flavin Mine.
a nucleotide.
So within this vertical component of complex one, we have flavin mononucleotide, which accepts those two high energy electrons from the NADH molecule.
Now, if we take a look at the structure of FMN, this is what it's going to look like.
in its fully oxidized form.
So basically means before it accepted those electrons.
So we have this R component that contains a phosphate group, not shown here.
And we have this three-ring structure.
And this three-ring structure, so one, two, three, is known as the isoloxazene electron.
ring.
And the isoloxazine ring in this flavin mononucleotide is the same exact isoloxazine ring that is found in FAD molecules.
Remember FAD stands for flavin adenine dinucleotide and we find FAD in the citric acid cycle and the FAD is
able to extract those two electrons in the same exact way that FMN is able to use this
same iso-loxazine ring to actually extract those two electrons, but the two electrons cannot actually bind onto the FMN by themselves.
They need 2H plus...
ion.
So we have two H plus ions and two electrons,
one H atom basically binds onto this nitrogen and the other H atom binds onto this nitrogen.
And we formed the reduced form of flavon mononucleotide known as fmn H2.
And that's why we need an additional H ion.
So one H ion comes from here,
the other H ion comes from here,
and the two electrons are found on the NADH, and we ultimately oxidize the NADH into the NAD plus, and we form the FMN H2.
And this takes place on the matrix matrix.
side of this complex 1.
So once again the NADH molecule donates the two electrons onto and accept the group found on the vertical component of complex 1 known as flavon
mononucleotide FMN.
The FMN is reduced into FMN H2 and this prosthetic is this group contains the same isoloxazine ring that we find on the FAD molecule.
So FAD is to FMN in the sense that they contain this same three-member ring
that is used to actually form that or actually use Ticstract and collect those electrons.
Now, once we reform the NAD+, that NAD+, can be re-used by the process of the citric
acid cycle or glycolysis, where the NAD+, molecules are needed to actually the glucose derivative and abstract those electrons.
But what happens to the electrons once they are abstracted by FMN?
Well, once the electrons are abstracted by FMN, those electrons begin to move
along a series of other groups known as iron-selfle clusters, ion-selfle groups.
And as these electrons actually move along these different groups,
we know from basic physics that whenever electrons flow along a certain area, that flow of electrons is what we call an electrocurrent.
And that electrocurrent can be used to power some type of process.
And in this particular case, the process that we power is the pumping of H plus ions.
So what we want to do is we want to establish a proton electrochemical gradient that will ultimately be
used by ATP synthase to form ATP molecules.
And protein complex one is actually a proton pump.
And as these electrons move and ultimately end up on ubiquinone as we'll see in just a moment,
four H plus ions are actually pumped by protein complex one from the matrix side to the intermembrane space of the mitochondrion.
Now as these electrons move they ultimately end up on a carrier molecule known as coenzyme Q or ubiquinone.
Now, ubiquinone accepts two electrons and it also actually accepts two H plus ions.
So it takes up two H plus ions from the matrix and it forms the fully reduced form of ubiquinone,
QH2, which is known as ubiquinone.
So, once again, the electrons
once they abstracted by flavin mononucleotide, then along a series of iron sulfur groups shown here, and are ultimately transferred to coenzyme Qubiquinone.
The ubiquinone also updates to protons,
and that helps us establish that electrochemical gradient and those two H ions bind onto Q to basically form the reduced
form of a ubiquinol known as ubiquinol.
And as these electrons move along this proton complex, it pumps four H plus ions from the to the intermembrane side.
Now let's move on to complex two.
Now complex two is actually not a proton pump.
So that's the main difference between complex one, three and four, and complex two.
Complex two is not a proton pump, it will not pump any protons across the membrane.
And actually because of that less ATP molecules will be formed from FADH2 than from NADH.
Now, the reason we mention FADH2 is because complex one is actually complex two is actually responsible for extracting those electrons from FADH2 molecules.
Now, let's think back to the citric acid cycle.
In the citric acid cycle, the step that allowed us to form the FADH2 molecule is this step here.
In this step, succinate.
into fumorade and the FAD molecule is reduced into FADH2.
So these two H atoms along with one electron each are extracted, they bind onto FAD to form the FADH2.
And these two electrons left over here, one here and one here, create a double bond.
to form this fumarate molecule.
And enzyme that catalyzes this step of the citric acid cycle is known as succinate dehydrogenase.
And actually succinate dehydrogenase,
the enzyme that catalyzes this step of the citric acid cycle is found within So complex 2 is actually involved in the citric acid cycle,
informing the FADH2 molecule and extracting those electrons from succinate to form fumarate.
Now, complex 2 for that reason is also known as succinate reductase because we essentially
abstract those electrons from succinate, we oxidize it into fumarate and we reduce the FAD into FADH2.
Now once we form the FADH2,
the FADH2 remains bound to complex 2 and in complex 2,
electrons are abstracted from FADH2 and they move on to a series of iron sulfur clusters,
iron sulfur groups,
and ultimately those two electrons end up being bound to ubiquinone, coenzyme Q, the same coenzyme Q that we discussed in this particular case.
And again, the co-anzyme Q once it bonds those two electrons, it abstracts to H plus ions to form ubiquinol.
The ubiquinol then departs, it detaches from the complex and moves on to complex 3.
So once we form ubiquinol here,
and once we form ubiquinol here, they detach and move on onto complex 3 as we'll see in the next lecture.
So, to summarize, complex 2, also known as succinate reductase, is a protein complex that contains succinate dehydrogenase, which functions in the citric acid cycle.
to actually convert, succinate, into fumarate and generates that FADH2 molecule and the FADH2 molecule doesn't actually detach.
It remains attached onto complex 2, so this is complex 2.
The FADH2 remains bound onto the compound.
complex, and then it basically is oxidized back into FAD, it basically kicks off those
two H ions as well as those two electrons, and those two electrons then travel through a series of these iron sulfur clusters.
and ultimately end up being bound onto that co-anzyme Q, the ubiquinone.
When the ubiquinone uptakes those two H ions,
it then forms ubiquinol, which detaches and moves on to complex three, as we'll see in the next lecture.
And a very important The important complex one and complex two is the fact that complex one
actually pumps those protons and helps generate that electrochemical gradient for hydrogen ions, but this one doesn't actually pump any protons.
And that's precisely why, as we'll see in a future.
NADH is able to actually form a greater number of ATP molecules compared to the FADH2.
So the complex oxidizes succinate into fumarate in the process forming the FADH2 which is then oxidized back into fumarate.
FAD and that release the two electrons which ultimately move through these FES clusters and onto
ubiquinone to then form ubiquinole and the ubiquinone is the electron carrier that shuttles these electrons from either complex one or two onto complex one.
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