In this lecture, I’m going to describe the biochemistry of collagen, one of the most important proteins
in our body for helping to hold all the components together. The word collagen originally came from
the Greek, the word kolla, meaning glue and the word gen, producing. Collagen was originally made
by boiling the hooves of ungulate animals to make glues to stick other things together.
Gelatin, of course, is a source of collagen that has been irreversibly hydrolyzed to produce the things,
the characteristics of gelatin that we know. There are many medical uses for collagen.
One of the more common uses of collagen today are used in ointments or beauty treatments
to help keep the skin looking young and youthful. Collagen is the most abundant protein in our body.
About 20%-30% of all the body protein that we have is made up of collagen. The synthesis of collagen
changes as we age. It decreases in terms of quantity and it also changes in terms of its flexibility.
One of the reasons that we get stiff and cranky as we get older is because our collagen is less flexible.
There are 29 different types of collagen found in the human body. Over 90% of the collagen
in the human body, however, is just type I. So, there’s a very big difference in the amounts
of the different collagens that are actually made. The most common types that we have in our body
are types I, II, III, IV, and V. That’s pretty much in terms of decreasing order of each of those.
The type I collagen fibrils are the stronger by weight than steel which really says a lot
about the importance of the strength of this important protein. The basement membrane
of the extracellular matrix is composed of collagen. It is this basement membrane that actually helps
to hold all the different components of our body together. One of the very important characteristics
of collagen is that it surrounds the blood vessels. When blood vessels are damaged, the collagen gets exposed.
It’s the exposure of the collagen that helps to signal to the clotting system that there’s been damage
and it’s time to start the clotting process. Now, there are five different types that are primarily found
in the body, as I said among the different 29. Collagen type I is found in skin, tendon, vascular ligature,
in organs, and in bone. Collagen type II is comprised primarily of cartilage. Collagen type III is found
in the reticular fibers. These are the fibers that help to give the organs the characteristic shapes
and structure that they have. Collagen type IV is found in the basal lamina in the epithelium-secreted layer
of the basement membrane as I mentioned previously. Finally, collagen type V is found on cell surfaces,
primarily around the hair and in the placenta. This shows the basement membrane that I was describing earlier.
If we look at the top row, we see a line of epithelial cells. We could think of this as the outer portion
of our body. Beneath that, we have the basement membrane which I’ll describe in a second.
Beneath that, we have individual blood vessels which themselves are lined by a little red layer
of epithelial cells. Those epithelial cells are a signal that everything is okay and intact.
If damage happens to a blood vessel, that epithelial layer is broken and the basement membrane
is exposed letting the blood clotting system be exposed to the collagen. That is a signal that damage
has occurred and the clotting process must begin. Now, collagen comes in a variety of states.
The state of the collagen actually depends on the amount of mineralization that a given type of collagen
has experienced. So for example, bones and tendons and cartilage tend to be harder and have had
more extensive mineralization, whereas fibrillar and non-fibrillar types may be more flexible
and not quite as hard. The most abundant fibrous tissue that we have is in the ligaments, the tendons,
and in the skin. Now, collagen is a fibrous protein. Fibrous proteins have long repeating helices
that go on for quite a long distance. The collagen repeating unit is a triple helix. That’s a fairly unusual
protein in our body. It’s even more unusual in the way the individual units are made
as I will describe in a second. Now, each polypeptide chain that’s found in the triple helix of collagen
contains about 1400 amino acids. Well, it’s a fairly long protein. The type I collagen contains
two identical α-1 units and one α-2 chain. These are arranged in a right-handed helix.
Now, that’s kind of unusual because each chain of the α-1 and α-2 is itself aligned in a left-handed nature.
Now, when I talk about the left-handed and right-handed orientations of helices,
that’s a little hard to imagine. So, let me just take a second to describe what that’s like.
A left-handed helix can be described as left-handed if you point it away from you and in pointing it away,
the helix coils away from you in a counter clockwise direction. A right-handed helix will coil away from you
in a clockwise direction as the helix moves away from you. So, the individual chains are left-handed
but the left-handed chains are wound together in a right-handed form, a very unusual structure.
Now, the strands are combined together to make fibers. These fibers as I’ve noted have considerable strength.
The strength arises from chemical bonds that occur between the individual helices as we shall see.