The process of proline hydroxylation, as I noted earlier, requires vitamin C. Without vitamin C,
some really nasty things happen. A deficiency of vitamin C leads to the condition known as scurvy.
Now, scurvy is a pretty serious disease. You can see in the picture on the lower right some drawings
of a person who has scurvy. These are from ancient times. The disease has actually been known
since at least the 13th century. Crusaders that went off on long crusades in the 13th century
went and took with them primarily meat, salted meat because they didn’t have a way of preserving things.
They didn’t have fruits and so forth and they didn’t know of the importance of those fruits.
But as a result of going off and eating this solid salt meat diet, they developed a condition known as scurvy
because they got no vitamin C in their diet. They went away as big hulking brutes and they came back
literally falling apart. So, the value of citrus fruits for preventing scurvy has been known for as long as
1497 when the explorer, Vasco de Gama took off on a voyage. He took with him citrus fruits
because there was this understanding that perhaps it might make a difference. Well, despite that knowledge
and despite the fact that Vasco de Gama’s sailors came back and they had no scurvy unlike many
other sailors at that time, over 2 million sailors died of scurvy between 1500 and 1800 which tells you a lot
about how slowly information actually gets out. Now, proline hydroxylation is important as I noted earlier
for increasing collagen’s thermal stability. Without it, it’s not stable at body temperature.
But that also means that you can break it down by heating things up a fair amount.
One of the reasons we cook meat for example is to make the meat tender and we’re breaking down
the collagen that’s found inside of that meat. Now, collagen is stabilized at body temperature as I said.
The addition of hydroxyl groups to collagen raises the stability of collagen by about 15 degrees,
a pretty critical thing. So, I noted earlier the importance of lysine that appears in the collagen sequence.
Lysine doesn’t occur very frequently but it’s very important for making interstrand cross-links
or chemical reactions between individual lysines as we shall see. These lysine-lysine bonds give collagen
the strength that I’ve described earlier. Now, the reason that this happened is because
lysines are made into an aldehyde form as we shall see. These aldehydes are rarely oxidized.
As they oxidize, they react with each other and form a covalent bond. This, as I noted,
improves the strength in some cases as strong as steel. Lysyl hydroxylase which is equivalent
to the prolyl hydroxylase I described earlier is the enzyme responsible for this reaction.
Another enzyme known as lysyl oxidase can also catalyze it. Now, the reaction that you can see here
on the very top of this slide is the reaction where hydroxyl groups are being placed onto lysine.
Now, it doesn’t have the detailed, the prolyl hydroxylase reaction but the process is actually very similar.
We can see the hydroxyl group being added. We can also see that vitamin C is needed just like it was needed
for the hydroxylation of proline. So again, vitamin C is necessary to provide those electrons to reduce
the iron just as it happened with the prolyl hydroxylase. Hydroxylysine found within the collagen
is then secreted. As I noted earlier, that hydroxylation is necessary for the secretion. In some cases,
the hydroxyl group that’s shown here is a target for glycosylation, meaning that sugar residues
can be added to it that will help to modify and give the protein some of its characteristic features.
A more common thing that happens however to the hydroxyl group is that it can be oxidized to form
either an aldehyde or a ketone depending upon whether it’s as shown in the picture here
or as whether it occurs on the end as I will show you in just a second. The hydroxylation of lysine
within collagen, as I described here, also has to occur inside the endoplasmic reticulum
because if that doesn’t occur, again, the collagen will not be released. The lack of hydroxylation
impairs secretion because again, we’re going to make weak collagen if those residues aren’t present.
So, another reaction that can occur in the side chains of lysine residues within collagen is shown
on the screen here. It also involves oxygen. It’s catalyzed by the enzyme known as lysyl oxidase
as you can see here. This reaction, the amine group that’s at the end of the lysine side chain
is converted into a double bonded oxygen to form an allysine as you can see here.
That allysine is actually the molecule that performs the reactions that go into the cross-links
that I’ve descried to you earlier. Now, instead of the lysyl hydroxylase which was found
in the endoplasmic reticulum, lysyl oxidase is found extracellularly, meaning that this reaction occurs
only after the collagen has been released from the cell. That release from the cell, of course, requires
the hydroxylation reactions that we’ve talked about before. Now, this reaction that has been produced
here allows, as I noted earlier, the aldehyde residues to come into close contact with each other
when those chains intertwine with each other. These chains intertwining allow this aldehyde groups
to come together and form a chemical bond because they’re very, very reactive with each other.
They will also react with the hydroxylysines that I showed you on this previous slide.
Now, aldehydes of the allysine can react as I’ve noted before with the amines of other lysines
and form a compound called pyridinoline. Now, pyridinoline is shown in the screen here.
It’s a fairly complicated structure, so I’m not going to emphasize that in any important way.
But you can see here that there are three lysines that have formed this pyridinoline residue
that you can see here. This is a covalent structure. Just like if you were to take and braid someone’s hair
and for example didn’t put a tie into the hair, what would happen to the braid?
Well, the braid will come apart. The braid will not provide the strength needed over time.
That’s what happens if this cross-link reaction doesn’t occur between the three strands of the collagen.
You put the three strands together, you tie it together with a knot at the end, the braid won’t come apart.
The knot in this case is the pyridinoline. So, the cross-linking that you see on this slide is what gives
collagen the strength that it needs to do the functions that it has. Now, there are a variety
of diseases as I’ve noted that can happen with problems in collagen. One of this is scurvy.
Scurvy happens as a result of deficiency of vitamin C. Other diseases that happen are a result
of genetic diseases or mutations that happen in the coding regions for the various collagens
that are there. So, some of the genetic diseases involved in collagen synthesis or processing
involved osteogenesis imperfecta which produces brittle bones, arises from deficiencies or problems
associated with type I collagen. Chondrodysplasias which is a skeletal disorder arises from the mutations
that happen in collagen type II. Ehlers-Danlos syndrome which happens in connective tissues arises
from problems with collagen type III. Knobloch syndrome which occurs affecting collagen in the brain
and retina happens from mutations within collagen type XVIII. Now, collagen vascular disease
can happen also as a result of autoimmune disorders. So, one autoimmune disorder here
is depicted on the finger of this individual. This gives rise to things like lupus, systemic lupus
and a variety of other things that can cause significant problems for individuals. In this lecture,
I’ve described the biochemistry of collagen, the relationship between its structure and the chemical
modifications of collagen that happen and the requirement of vitamin C to make the overall process occur.