00:00
cartilage against impact, again, relevant to
joints and use of cartilage in those joints.
00:02
There are other molecules that carbohydrates
combine to and one of those is proteins.
00:06
Now these come in a variety of different forms and
the names differ mostly relating to the sizes
of the individual molecules that are being
joined together. So for example, glycoproteins
as its name suggests are carbohydrates that
are attached to proteins. Usually these
carbohydrates are oligosaccharides, but
not always. The attachment are in the location
of the glycoproteins usually comes in two
different places. One is they can be found
in cell membranes and one of the functions
that glycoproteins perform in cell membranes
is giving cells identity. A second place where
glycoproteins are commonly found is in the
secreted form. They are for example hormones
like erythropoietin which are secreted that
have numerous oligosaccharides attached to
them. There are many examples in cells of
glycoproteins and some of those glycoproteins
have properties very much like what we have
seen with the glycosaminoglycans. The mucins
for example are proteins that are very very
much glycosylated, meaning they have many
many sugar residues that are on them and these
also are modified residues, giving the sugars
some interesting chemistry that's happening
with the solutions that they're in. Mucins,
when they are in water will give also a very
slimy feel, and mucins are commonly found
in mucous.
01:35
Other places where we find glycoproteins that
play important functions in the body include
the immune system, where we see for example
glycoproteins present in antibodies in some
cases or in the histocompatibility complexes
in others. Now as I mentioned before, one of
the functions of glycoproteins is to give
identity to cells, there is a couple places
we can understand this pretty well. One of
these is the blood types. We all know about
the different A, B and O blood types, and these
A, B and O blood types arise as a result of
the presence or absence of certain sugars
on glycoproteins. Now I've schematically shown
that in this figure right here. Imagine that
we're looking at the surface of a red blood
cell. The red blood cell has embedded in
its membrane a protein, the protein is shown
in blue. This protein has attached to it,
through its asparagine residue, on the side
chain it has an oligosaccharide attached to
it. Now the oligosaccharide has only a few
sugars in it but the presence or absence at
one point in the oligosaccharide of a specific
sugar causes that blood to be type A. If at
that same point a different sugar is present,
then the blood will have type B. If the blood
is missing a sugar at that point completely
the blood will have a type of O. Now the immune
system of a person looks for and recognizes
those individual types. So if you have for
example blood type A, that means your immune
system has antibodies against the blood type
B, so if you inject a person who has a type
A blood with type B blood, the immune system
will attack it and the results can be pretty
drastic. Because type O lacks that sugar,
type O is not attacked by the immune system,
type O is a universal donor as a result
of that.
03:36
Asparagine is an amino acid that's commonly
found linked to oligosaccharides, it's one
of two amino acids that are linked to oligosaccharides
in glycoproteins. We can see the linkage between
asparagine and the oligosaccharide here. Remember
that asparagine has a side chain that has
an amine attached to its R group carboxy making
a carboxamide. We can see the attachment through
this figure in which the oligosaccharide is
highlighted in green and the link to the carboxamide,
amine is shown in yellow. Asparagine is the
form through which we call N-linked glycoproteins
are made. Now there is another type of linkage
that occurs in glycoproteins and it's called
O-linked. And O-linked glycoproteins happen
not through asparagine, but rather through
the side chain of threonine or serine as can be seen here.
These two types of glycoproteins are fundamentally
different, for example the N-linked glycoproteins
have the oligosaccharide attached in the endoplasmic
reticulum and it may be further modified in
the Golgi apparatus. By contrast the O-linked
oligosaccharides have the oligosaccharide
attached to serine only in the Golgi apparatus,
there's no link to the endoplasmic reticulum.