- Now it turns out that relative to the inner mitochondrial
membrane, the outer mitochondrial membrane is a bit more
permeable to many molecules, and specifically
it contains specialized proteins called porins that have
essentially created little tunnels or pores for
molecules to kind of diffuse through, so I'm kind of
just drawing representative pores to remind us of that fact
and it turns out that this carnitine attached to this
fatty acid which is actually commonly referred to now
as an acyl carnitine molecule can readily diffuse from the
cytoplasm into the intermembrane space here via one
of these pores and it's actually a bit unclear to scientists
whether it's the carnitine and the fatty acid,
the acyl-CoA that both diffuse into the intermembrane
space and then get kind of attached via this carnitine
acyl transferase or whether it's more like what we've
drawn up here in that everything occurs in the cytoplasm
and this entire acyl carnitine molecule diffuses
into this intermembrane space.
So I just wanted to mention that, but in any case
the ultimate endpoint is the same.
We get this acyl carnitine into this intermembrane space
via these pores in the outer membrane.
Now at this point you might be wondering to yourself
why did we go through all the trouble to create this
acyl carnitine molecule if these porins are permeable
to a lot of things, including even fatty acids,
so why can't the fatty acids just you know go through
these porins and get into the mitochondrial matrix?
Well, first I want to remind you that
remember we're only halfway home.
We have to still traverse this inner mitochondrial membrane
in order to get to the mitochondria and it turns out
that this inner mitochondrial membrane does not have these
nice pores in its membrane and so it's far less permeable
to molecules than this outer mitochondrial membrane,
but what it does have is it has this protein transporter
within its membrane that's gonna help us out a little bit,
and this protein transporter is called acyl carnitine,
so already it's giving you a clue that it's
going to recognize that molecule acyl carnitine that we
worked so hard to create and it's called acyl carnitine
translocase, so just like its name suggests,
it's gonna essentially translocate this acyl carnitine
from this intermembrane space into the mitochondrial matrix.
And to show that action I'm gonna go ahead and take a quick
shortcut here and actually go ahead and copy and paste
this molecule here from the cytoplasm to the
mitochondrial matrix right here,
and I'll go ahead and extend our path right here
from the intermembrane space, facilitated by this
protein transporter into the mitochondrial matrix.
Now of course we're not finished here because we don't
want to oxidize this entire molecule.
We just want to oxidize this fatty acid chain to obtain
some ATP, so we need some way to remove this fatty acid
from this carnitine molecule, so remember how I said
that we also have a carnitine acyl transferase on
our inner mitochondrial membrane?
Well that's where this enzyme comes in
so I'm gonna go ahead and abbreviate that now as CAT
for carnitine acyl transferase and I'm gonna put the
roman numeral two to indicate that it's a different
enzyme located on the inner mitochondrial matrix
and what this enzyme does is it
catalyzes essentially the reverse reaction
of what happened inside of the cytoplasm.
So what I mean by this is that it take a coenzyme a molecule
from the mitochondrial matrix and in a reverse reaction
now our sulfur atom here is gonna serve as our
nucleophile essentially and essentially displace this
bond that was formed earlier so that we end up forming
a carnitine molecule back to its original form with a
hydroxyl group and we also end up forming back again
simply our acyl-CoA group, so remember that looked
exactly like it does above with our acyl group attached
to an acetyl-CoA molecule via this sulfur atom.
And in fact, just to make that crystal clear,
I'll go ahead and actually write out the full name below it.
So we're calling this is the acyl-CoA molecule.
Now before moving on to the oxidation step I want to point
out one more thing, which is that it turns out that this
acyl carnitine translocase protein transporter is
actually quite efficient because for every acyl carnitine
molecule that it pumps into the mitochondrial matrix
it actually exchanges it for one molecule of carnitine,
so this carnitine molecule that's produced by this
carnitine acyl transferase two enzyme can actually
return via this protein transporter into the intermembrane
space so I'll go ahead and continue its journey in the
intermembrane space and then out through the
outer mitochondrial membrane and back into the cytoplasm
where it can be recycled to help transport another
fatty acid into the mitochondrial matrix.
That's kind of a pretty cool mechanism, I think.
Alright, so now that we've finally gotten our fatty acid
into the mitochondrial matrix in the form of this molecule
acyl-CoA, what happens to this molecule next?
Well, using the enzymes found in the mitochondrial matrix
this molecule undergoes actually a cycle of repeating
steps and each cycle consists of four main steps
and at the end of each of these four steps
we end up producing one molecule of acetyl-CoA
and I'll remind you the structure of this molecule
is a two carbon structure, an acyl group attached to a
coenzyme a and each time we also produce an acyl-CoA
chain that is now two carbons shorter than it was before.
Of course that's because it's losing those two carbons
in producing this acetyl-CoA molecule.
I'm not gonna go in depth into these four steps here
or the enzymes used to complete these steps,
but what I do want to point out big picture
are the major inputs and outputs of these four steps
to get to this molecule of acetyl-CoA
and a fatty acid chain that's two carbons shorter.
So remember that the production
of acetyl-CoA involves oxidation.
So oxidation, we're losing electrons which means we need
the help of electron carrier molecules to accept those
electrons so they can shuttle them to the electron transport
chain to produce some useful energy in the form of ATP,
so remember our two electron carrier molecules are
FAD and NAD and of course these are in their oxidized
forms so when they accept electrons they become reduced
into FADH and NADH and of course this also produces
a proton as well and the kind of convenient point that I
want to point out to you is that it turns out
remember that the electron transport chain is not far away.
The electron transport chain is located on the
inner mitochondrial membrane, right, so that's quite a
quick journey and so these are readily reoxidized to their
oxidized form once they donate to the
electron transport chain and so this cycle can continue
quite conveniently in this mitochondrial matrix.
Now as a couple of minor points,
remember that we need some source of oxygen to be forming
all of these carbons bonded to oxygen in each
of these subsequent acetyl-CoA molecules that we produce,
so there's actually an insertion of water somewhere
along these four steps as well as an insertion of an
additional coenzyme A molecule for each subsequent
acetyl-CoA group that's formed.
Now before we wrap things up and briefly talk about the
regulation of this oxidation process I just
want to mention something that I didn't get quite the
chance to talk to you in depth to you about.
It's something you might hear when referring to this process
which is the statement beta oxidation,
and it sounds super intimidating but it's actually quite
simple and so I just wanted to take a minute to just explain
this to you briefly, like what the big idea behind this is.
So this beta oxidation simply refers to the carbon,
the position of the carbon in this fatty
acid chain that's being oxidized.
So remember the carbon that's being oxidized is this
C double bond O bond right here in this acetyl-CoA
molecule and so to explain this to you I wanna go ahead
and actually, I know I've been abbreviating this
fatty acid chain using the letter R but I just want
to draw in a couple of these carbon carbon bonds
here so I'll draw in a couple of CH2 groups
attached to the carboxylate group here.
So it turns out that the way that chemists label these
carbons is based on their position to this carboxylate
group here, so the carbon closest to this carboxylate
group gets the designation of being the alpha carbon
and the one, the next furthest one away is called the
beta carbon and of course if you're further away
you also get other names as well,
but just because we're talking about beta oxidation
I'll stop there and so you can see that it's this
carbon, this beta carbon, so in other words,
every other carbon relative to this carboxyl carbon here
that is going to be turned into this C double bond O
to form this acetyl-CoA molecule.
So, that in a way is probably pretty intuitive
but this beta oxidation is just kind of
a fancy way to refer to that.
Alright, so we're hitting the home stretch here
and the only other thing that I want to mention is how
this oxidation process is regulated
and it turns out that among all these reactions
the rate limiting reaction is catalyzed by this
enzyme carnitine acyl transferase.
I'll actually go ahead and write that up.
This is a rate limiting step and remember that means
that kinetically it's the slowest among
all of these reactions and also remember that's
important to know when we're thinking about regulation
because since the rate limiting step determines the
overall rate of this entire reaction it's a good
step to regulate, essentially to turn on or off.
And it turns out that in order to figure out how this is
regulated we need to remind ourselves a little bit about
fatty acid synthesis, so I'm going to remind you of a
molecule called malonyl CoA and if you remember this
molecule was kind of a charged up version of acetyl-CoA
that was used to synthesize fatty acids inside of
the cytoplasm and it turns out that this molecule is
actually an allosteric inhibitor of this enzyme,
so I'm gonna kind of just squeeze this arrow through
to indicate with a red line and a minus sign here
that this inhibits this enzyme
carnitine acyl transferase one.
So how can we think about this?
Well the way I kind of reason it in my head is,
is if we have a lot of malonyl CoA lying around in our cell
it means that we're making a lot of fatty acids,
so if we're making fatty acids, if we're synthesizing them,
then we don't want to be breaking them down,
and so we can essentially inhibit the rate limiting step of
this oxidation process to make sure that we have
a net production of fatty acids.
Now another cool thing is that I think is worth noting about
this type of regulation is that by using a substrate in
fatty acid synthesis to regulate an enzyme
in fatty acid oxidation, the body has essentially made these
processes mutually exclusive, so when one is on,
so when there's a lot of malonyl CoA, the other,
you know being oxidation will have
to be kind of essentially turned off, right?
And so that's exactly what the body wants.
It doesn't want to be in a gray zone.
It either wants to be essentially producing a net
amount of fatty acids for storage or it wants to
be breaking them down to extract all of that ATP from.
And finally the last point that I would make is that,
even though it kind of seems like a pain to traverse
through all of these compartments to get to where we
need to go, having these compartments makes this kind of
regulation also pretty cool, because by kind of regulating
this oxidation at this step right here which is the key step
for transporting the fatty acids from the cytoplasm to the
mitochondria, we can essentially keep these reactions
separate from one another, literally, so we're actually
keeping them in separate compartments which is a great way
to regulate whether something is on or off.