Vitamin A: Introduction – Biochemistry | Lecturio

February 25, 2020 0 By Ewald Bahringer

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.