In this lecture, I’m going to describe vitamins D and A,
two of the fat-soluble vitamins
and some of the very different functions that each one has.
Now, vitamins D and A, as I said, are fat-soluble vitamins.
They are important in the case of vitamin A for vision.
In the case of vitamin D, for calcium metabolism.
But that’s not the only thing that these vitamins do.
Vitamin A is also important for gene expression
and differentiation that occurs within organism during development.
Vitamin D is also important for controlling gene expression
and essential for a healthy immune system.
As we will also see, it has some anticancer properties as well.
Now, one of the things to be careful about with
vitamins A and D is that because they’re fat soluble,
excessive amounts of either one of them could be harmful
because they get stored in fat tissue and can be released over a long period of time.
So these are two vitamins you don’t want to take in excess amounts.
Vitamin A, as I said, is fat soluble.
It is toxic at high doses and it’s actually possible for a person to overdose
on eating too much liver from certain organisms like polar bears,
so stay away from polar bears.
Vitamin A is stored in the liver and it occurs in three forms in the body.
The alcohol form is known as retinol and you can see it outlined on the screen.
On the far right of the molecule, there’s an alcohol group
and that’s what gives it the –ol ending of its name.
Retinal is a related compound that differs only
in containing an aldehyde group at the end of its structure.
And finally, retinoic acid has a carboxyl group at its end.
Now these three different molecules have
three very different functions within our body.
Now, retinol is the storage form of vitamin A.
To store vitamin A, retinol is esterified to a fatty acid
as you can see on the structure on the top.
Now, vitamin A comes in a variety of isomeric forms.
And the form that you see on the top for retinol is known as the all-trans form,
that is all the double bonds are in the trans configuration.
Those double bonds can isomerize in the presence of light,
meaning that light can actually physically change their structure.
The summarization of retinal is what gives rise to vision as we shall see.
And one of those isomers you can see in this structure
which is the 11-cis retinal isomer.
And it’s this isomer that is stored within our eyes.
Retinoic acid is important for cell differentiation.
Without retinoic acid, we won’t form into the organisms that we do.
Now, vitamin A is produced in the body using beta carotene as a precursor.
And beta carotene is shown in the figure at the bottom.
Basically, if you cut beta carotene in half,
you will get one of the forms of vitamin A that you see on the screen here.
Now, 11-cis retinal, as I said, is important for vision.
So I need to describe to you that process by which this occurs.
First, 11-cis retinal is bound to a protein called opsin.
And it’s through protein called opsin
that retinal provides us with vision as we shall see.
The absorption of a photon of light by the 11-cis form of retinal
causes the 11-cis form to flip back to the trans configuration.
So we can see this flipping process occurring here
where we see it flipping on the top to the bottom
or from the bottom back up to the top.
and this happens very readily with this form of vitamin A.
It’s the change in structure, the change in form,
that actually provides the very first signal in our eyes
that light has been detected.
So before I talk about the biochemistry of vision,
I’ll just say a little bit about the actual physiology
and the cell structure that gives rise to vision.
So vision happens as a result of light detection
that occurs in specialized cells in our eye known as retina cells.
The opsin that I described earlier is the protein that actually holds
vitamin A containing compound, the retinal,
that allows us to have vision.
Now, notice that retina and retinal are pretty similar in terms of name,
but they’re very different things.
We have two types of cells in our eye that detect light.
The most abundant cells that we have are known as rod cell.
And they provide very basic functions of light detection.
They don’t provide for differences in color,
but rather simply detection of a photon of light.
The photo pigment that they contain, the opsin,
contain links to the retinal, is known as rhodopsin.
The other cells that we have in our eyes are known as cone cells.
And they actually provide the color detection that we see when we see a well lit room.
They also have retinal linked to an opsin.
But the opsin there is a little bit different
and so we call those photopigments, photopsins.
There are three types of cone cells.
One type specialized for the absorption of red wavelengths of light.
One type of cone cells specialized for the detection of green type of light
and one specializes for blue types of light.
Now, to give you an idea of abundance,
there are about a hundred and twenty million rod cells per eye
and about 6 million cone cells per eye.
So eyes have pretty good resolution in terms of
being able to detect small amounts of light.
The rod cells can detect.
They’re so sensitive that they can detect individual photons of light.
Now, that detection comes at a price.
They can’t tell the color of that light,
but they can tell whether or not light impinged upon them.
The opsin as I said earlier contains the 11-cis retinal
and in that form, it’s known as rhodopsin.
The retinal in rhodopsin isomerizes in response to the light
and that isomerization causes the retinal to change form.
So instead of being bent as we saw earlier in the structure, it straightens out.
Well, that straightening out of the retinal that happens in the presence of light
causes the rhodopsin that contains it to also change its structure.
Now, one of the things we’ve seen in other lectures that I’ve given here
relates to small modifications in protein structure.
Small modifications in protein structure can have very big effects
on what the protein actually does.
And that’s very much the case here with rhodopsin.
So these retina cells that I’m describing to you are very sensitive
and that sensitivity allows them to very easily send signals to the brain
that they detected something.
Those signals happen as a result of chemical signals that arise in the nerve cells.
Now, this chemical process is kind of complicated.
So I’m going to take you through it slowly.
The unstimulated optic nerve cells are what we call unpolarized,
meaning that they have an even distribution of ions
on the outside and on the inside of the cell.
This is very different from other nerve cells
because they start out in a polarized fashion.
Light stimulation that happens on the nerve cells of the eye, however,
caused hyperpolarization.
This is exactly the opposite of other nerve cells.
Other nerve cells start out hyperpolarized
and stimulation causes them to become unpolarized.
So we see the reverse of the process that’s happening with nerve cells in our eyes.