Previously we talked about the structure and the different types of
collagen molecules. Now we're going to switch gears and
talk about the ways that our cells synthesize collagen and
the way that our body breaks down collagen.
So let's begin with synthesis.
So in our body we have specialized cells called fibroblasts.
And the entire point of these fibroblasts
is to actually synthesize collagen.
We have different types of fibroblasts depending on
what organ or tissue we are talking about. For example,
in our bone the fibroblasts are known as
osteoblasts and these generate
type I collagen while in our cartilage for
example, we have fibroblasts known as
chondroblasts and these generate
type II collagen. Now, regardless of the fibroblast that
we're talking about, all of these actually
carry out the same function in the same exact way -
they generate the collagen. And so
collagen synthesis begins inside the cell
and ends outside the cell. And
its carried out in the following fashion.
Lets begin in the nucleus.
This is the nucleus - we have our nuclear
membrane, the nuclear pores.
We have cytoplasm. We have the rough
endoplasmic reticulum, the Golgi apparatus, the
cell membrane and then outside the cell.
So this is our fibroblasts.
Within the nucleus, we have the nuclear
machinery that undergoes the process of transcription.
We take the genes, transcribe the genes,
and we form our mRNA molecule.
The mRNA molecule leaves via these nuclear
pores, it enters the cytosol of the
cell - the fibroblasts - and then it associates
with free ribosomes present in the
cytosol.
And this begins the process of translation.
Now we begin the process of building
the alpha-molecule, that alpha-chain. So remember
our single collagen monomer consists of
three individual alpha-chains
and so we begin building these alpha-chains
in the following way in the cytosol.
Now the initial molecule that we first begin producing
is known as prepro-alpha-chain.
The pre- component of the prepro-alpha-chain
means it contains this blue region
on the end terminal end of the translating
polypeptide
and this is known as the signal
sequence peptide. And this
signal sequence peptide is used to actually
direct this polypeptide to the
rough endoplasmic reticulum. Now
when the entire free ribosome with this
translating polypeptide and the mRNA
ends up being transported to the rough endoplasmic
reticulum because of this signal peptide,
this signal sequence peptide is actually cleaved by an enzyme
and that forms the
pro-alpha-chain. So we remove the blue component
- the signal peptide - and we form the pro-alpha-chain.
And we have many of these pro-alpha-chains
being formed at the same time
within the lumen of the rough endoplasmic reticulum.
Now, as we're forming
that pro-alpha chain, we are also at the same time
adding hydroxyl groups onto the proline and
lysine to form the hydroxyproline
and the hydroxylysine, and this is carried out by
an enzyme called hydroxylase. And
hydroxylase uses a molecular oxygen
for the oxidation process
and vitamin C for the reduction
process, to actually attach those
hydroxy groups. Now, in patients
with Scurvy - which is basically
vitamin C deficiency - this process is affected
and people with Scurvy cannot actually
hydroxylate the collagen and ultimately that
affects the ability to actually form the covalent
cross-linkages at the end of this process. In fact in
patients with Scurvy, they actually have
collagen that is not very stabilized and so
for example those patients with Scurvy actually have
blood vessels that will bleed readily.
And that is one of the signs and symptoms of patients
with Scurvy. So once we form the
hydroxyl groups, some of the hydroxyl
of the hydroxylysine are actually are modified
even further via the process of
glycosylation.
And this process involves adding things like glucose
or galactose. So we add these sugar moieties
and ultimately they help and assist with
the process of forming that final collagen
structure. Now
Inside the lumen of the rough endoplasmic
reticulum, many of these individual alpha-chains
are actually being formed at the same exact time.
And once we hydroxylate the residues,
three of these alpha-chains
known as pro-alpha-chains can
actually associate to form
a molecule known as pro-collagen. Now
a pro-collagen consists of three
of these pro-alpha-chains and on the end
of each of these chains - so we have the
N-terminal end of the protein and the C-terminal end
of the protein.
These regions are known as propeptide regions.
Because these regions actually do not
contain the helical structure and instead
they contain disulfide bonds.
So we have a bunch of disulfide bonds here and
a bunch of disulfide bonds here. And these
disulfide bonds in these
pro-peptide sections.. so this is known as "pro-peptide"
"pro-peptide".. They are important to actually hold the structure of the
pro-collage together, at least initially.
So once we form the pro-collagen that
consists of three individual pro-alpha chains
being help together at the ends by these disulfide bonds
they then go into the Golgi apparatus
where we form the vesicles and we
essentially release the
vesicles outside via the process of exocytosis.
So once outside, we have these pro-collagen
molecules that are acted on by an
enzymes known as peptidases. And what
peptidases do is, they snip the
pro-peptides off.
Because the entire point of these pro-peptides was
to contain these disulfide bonds to hold the
structure together
but now that they are outside we want to remove them and so
the peptidases essentially clip them. So they clip this one off
and they clip this one off, so that
now we form that final
collagen monomer. So now we have the
collagen monomer without the pro-peptide
ends and they spontaneously arrange
themselves to essentially form
these stacking units.
And this is known as a fibril.. so we have
one monomer here
and another monomer here overlapping it, and
a third one, a fourth one... and this fibril is not very
stabilized and so what we want to do finally is
we want to form covalent bonds
- strong bonds - between the
individual
collagen monomers. And this is carried out by the enzyme
known as lysyl oxidase.
So lysyl oxidase essentially
uses copper as a cofactor to form covalent
bonds between the individual collagen
monomers, so ultimately form a
very strong, reinforced and stabilized
structure.
Now in a disease known as Menke's disease, in which
there is a problem in actually being able to
take the copper and attach the copper onto the
this enzyme - and other enzymes - the lysyl
oxidase does not function as well and so
we cannot form these cross-linkages and so
with patients with Menkes disease
are going to have problems associated with collagen
instability.
This is the way we form collagen in our body.
So to summarize, we have these specialized cells
known as fibroblasts, which are actually
responsible for initiating synthesis
in the cell and finishing it outside the cell.
So in the cell,
we transcribe the genes to form the
mRNA that ends up in the cytosol. We have
free ribosomes that begin the process of translation
initially forming the prepro-alpha-chain and
then once we cleave that signal sequence,
we form the pro-alpha-chain, which ends up being transferred to the
rough endoplasmic reticulum
where the translation continues. As we are
continually translating the pro-alpha-chain, we are also
hydroxylating it and glycosylating
the different residues and eventually when we glycosylate and we hydroxylate,
the three individual pro-chains can combine
to form the pro-collagen. The pro-collagen consists (at both ends)
of these propeptide regions in which
we have disulfide bonds to actually
hold the procollagen together. Then that transfers
to the Golgi apparatus
and that forms vesicles and the vesicles undergo
exocytosis to actually end up outside the cell.
Once outside the cell, we cleave the
propeptide ends with peptidases, thereby forming
those individual collage monomers
that essentially spontaneously form these relatively
stabilized structures known as fibrils
in which we have the monomers stacking
like so, but to actually reinforce and stabilize
the final structure, we have the enzyme called
lysyl oxidase that uses copper as a cofactor
to actually cross-link and form those covalent bonds
thereby greatly stabilizing and
reinforcing the structure. Now
the half-life of collagen is several months.
and actually collagen is a very...
collagen metabolism is a very dynamic process, so
we are always breaking down and forming new collagen.
And so the set of enzymes known as
metalloproteases that are responsible for breaking down and degrading collagen are known as
collagenases. So we have all this process
that takes place inside and outside of the
cell to actually form collagen, is
then actually reversed by these collagenases
that breakdown the collagen. And that makes way
for us to actually be able to form new collagen.
So this process of collagen remodeling
is important and we will talk more about these
collagenases when we'll talk about the process of
forming granulation tissue. So in the
next several lectures, we are going to talk about the different types of
medical conditions that arise because of
problems with collagen synthesis.