- [Presenter] Hey, so we're going to
be talking about amino acids, and specifically
with regards to amino acid structure.
And before we dive on into this topic,
I think it's nice to kind of take a step back
and take a big picture view of amino acids
and kind of figure out, you know,
where do they exactly fit in the grand scheme
of biochemistry, and specifically say, human metabolism.
And I think the best way to do this
is to take a real world example of
a human protein called hemoglobin.
Now what is hemoglobin?
Well hemoglobin is found within the red blood cells
that flow all through our blood stream.
So here's my little red blood cell,
and each red blood cell is chock full
of this hemoglobin protein.
I'm just gonna write it out as Hgb here.
And this hemoglobin protein is what
is actually responsible for picking up oxygen
when this little red blood cell
flows through the vessels in the lungs,
picks up oxygen, and then transports this oxygen
to all the various tissues within our bodies.
And so you can kind of think of hemoglobin
as a car of sorts, and you know my favorite car
is a Porsche 911, so I mean if you want to be
more environmental friendly, you could pick a,
you can think of a Prius or something like that.
So whatever your car is,
oxygen is like the passenger for that care.
And so hemoglobin goes by the lungs, picks up oxygen,
delivers it to the tissues, and then tissues
are just groups of cells that are of a similar type.
And so each of the cells in these tissues,
then takes the oxygen and uses it
to generate adenosine triphosphate, or ATP.
Which is the energy source
for all of the various metabolic processes that go on
within out cells to help keep us alive.
So now, where do amino acids fit into all of this?
Well, amino acids are the building blocks
of this hemoglobin protein.
And so without amino acids, this entire
vitally important process, wouldn't be able to occur.
Now sticking with the car analogy just a little bit longer,
just like we have different types of components
that come together to form different types of cars,
whether it be a Porsche or Prius, what have you,
you can have different types of amino acids
and there are 20 of them to be exact,
that can come together to form countless, countless
different types of proteins.
And so now that you have an idea of where amino acids fit
in this bigger picture of a metabolic process,
let's go ahead and take a closer look
at what the actual structure of an amino acid is.
First we have the amino group,
and then we have the carboxylic acid group,
and already you can start to see
where the name amino acid comes from.
You have amino from the amino group,
and then you have acid from this carboxylic acid group here.
And then linking the two groups is this carbon atom,
which we call the alpha carbon.
And then bound to the alpha carbon is a hydrogen atom,
as well as a unique side chain, or R group.
We just use R to denote any generic side chain.
So each of the amino acids has this same generic structure,
and what makes each of the 20 amino acids
different from each other,
is this R group, or the side chain.
So each of the side chains for the amino acids,
is going to look different.
One thing that's important to note
is that this carbon atom, the alpha carbon
is also known as a chiral carbon.
And what does a chiral carbon mean again?
Well a chiral carbon is a carbon atom
that has four unique groups bound to it.
So if we take a look at this carbon,
we can see that one group that's bound is the amino group,
another group is the carboxylic acid group,
the hydrogen atom make the third group,
and then the fourth group bound to it is
the R group, or the side chain.
And so the alpha carbon in amino acids,
is considered a chiral carbon.
And remember that chirality
really refers to optical activity.
In other words if you were to shoot
plain polarized light at an amino acid,
then because this carbon is chiral
it would rotate to that light.
And so that's what chirality is really referring to.
It's referring to optical activity.
Now it's important to note that there is one exception
among the amino acids, for chirality,
and that is the amino acid glycine.
And that's because the side chain, or R group,
for glycine is just a hydrogen atom.
It is the simplest of all side chains,
just one hydrogen atom.
And so if you were to substitute a hydrogen atom
in place of this R group, you would see that
you have a duplication of atoms
coming off of this alpha carbon in the case for glycine.
And so glycine is the only amino acid,
that does not have a chiral carbon.
So that's just important to make note of.
So let's give ourselves a little bit more room here.
Now another way that you can portray the structure
of an amino acid is with
something called a Fischer projection.
And Fischer projections help to highlight
the relationship of the four groups around a chiral carbon.
So let me draw that for you here.
First you have your amino group,
and up top you have your carboxylic acid,
here is your hydrogen atom,
and at the bottom is the side chain, or R group.
And just to orient you a bit,
here in the center is the chiral carbon,
the alpha carbon here, and then you have the four groups
coming off of the chiral carbon.
And the horizontal bonds here,
you can kind of picture those as coming
out of the plane of the computer towards you.
And then these vertical bonds here
are coming out of the plane of the computer away from you.
And this particular configuration is called an L amino acid.
And conversely you can have the mirror image of this,
which I'll draw for you here.
And this particular configuration is called a D amino acid.
And these two configurations are called enantiomers.
And enantiomers are mirror image molecules
that are not superimposable.
So you can picture, these are mirror images of each other
but if you were to take this D amino acid,
and try to superimpose it on the L,
you wouldn't be able to do that.
It's, you can kind of think of these two configurations
like your left and right hand.
And although your left and right hand
are mirror images of each other,
you can't superimpose them on one another.
And that's the relationship between and L and a D
configuration for Fischer projections.
And these two configurations look awfully similar
and are really easy to mix up,
and so the way that I like to keep them straight is
if I look at where the amino group is,
let's take the L amino acid,
if I look at where the amino group is
I can see that it is to the left of the projection,
so L is for left amino group.
Now if I look at the D amino acid,
I again look for the amino group,
and I see that it's to the right of this configuration.
And so D, which actually means dextro, or right in Latin,
is for right amino group.
And that's kind of how I like to keep them straight.
So why is it important to distinguish
between L and D amino acids?
Well the L form of an amino acid,
is the only form that you will find within the human body.
And so that's really important to remember,
that the L configuration's the kind
that you find within humans.
Alright, now how about we review a little bit
about everything that we learned.
First we sort of got a big picture
of where amino acids fit in a larger metabolic process,
such as in the example of hemoglobin.
And then we learned about the structure of an amino acid,
and the fact that the central alpha carbon,
is a chiral carbon with optical activity.
And the one exception to this rule
is the amino acid glycine, which just has
the simplest side chain of a hydrogen atom.
And therefore it is not a chiral molecule.
And then we also learned about the Fischer projections
for amino acids, and the fact that
the L configuration of an amino acid
is the only one that you find within the human body.
And there you have it.