- [Instructor] Alright, let's go through
the classification of amino acids.
And, I've highlighted the word CLASS within classification
for you because I'm going to paint for you
a picture of a classroom
that is full of 20 different amino acids.
Just picture this as the most diverse classroom
you've ever seen because each amino acid
has their own unique side chain,
and this makes them distinctly different
from the amino acid next to them.
And, just like a real classroom full of kids,
even though each amino acid is unique and special
in their own way, you can start to see
that some of these amino acids are more alike
than they are different.
We can start to see these similarities
in the chemical properties of the side chains,
and this allows us to group them together
into various categories.
Those chemical properties include:
the charge of the side chain,
the ability of the side chain to undergo hydrogen bonding,
and also whether or not we can classify
that side chain as being either acidic or basic.
The 20 amino acids can be split broadly
into two main groups.
The first group includes the non-polar amino acids,
and then the second group includes the polar ones.
The non-polar amino acids can also be thought of
as the hydrophobic, or water-fearing, amino acids.
Conversely, you have the polar ones.
Those can be considered hydrophilic, meaning water-loving.
Yet another way that I like to think about
these two main groups are, the hydrophobic amino acids,
they're kind of like the water-haters.
They don't really want to interact with water at all.
They'd rather just interact with themselves.
Whereas the hydrophilic amino acids are very open
and welcoming to interacting with water,
so they're water-lovers.
Then, within the two groups of non-polar, hydrophobic
and polar, hydrophilic amino acids,
you then have a further breakdown into subgroups.
Those subgroups include those amino acids that have
alkyl side chains, aromatic side chains,
neutral ones, acidic ones, or basic ones.
So, let's take a closer look at those amino acids
that have alkyl groups as side chains.
As you can see here, we have seven different amino acids,
and I've just drawn out the side chain for you.
I've left the rest of the molecule out
just to fit everything in here.
And, we have glycine, alanine, valine, methionine,
leucine, isoleucine, and proline.
Proline is the exception.
I have drawn out the entire amino acid there,
because as you can see, its side chain
forms this interesting ring structure
with the amino group in the backbone of the molecule.
So, I just included it there for our completeness.
All of these side chains are made up of alkyl groups,
with the one exception being glycine,
because its side chain has only a hydrogen atom in it.
But, because it behaves similarly
to an alkyl chain side group, it gets slumped
into this category of amino acids.
Whenever you see an amino acid
with an alkyl group as its side group,
you should be thinking that this amino acid is non-polar.
So, they're also going to be hydrophobic.
Now let's take a closer look at those amino acids
that have aromatic groups as part of their side chain.
Remember, we're still under the umbrella
of non-polar, hydrophobic amino acids, here.
So, I've drawn out for you, here, two amino acids:
phenylalanine and tryptophan.
What should you be thinking
when you're looking at these amino acids?
So, besides thinking, "Oh, those amino acids
"Must smell really good, because
"They're called 'aromatic' amino acids."
Well, that might be true, but you should also be thinking
the same thing that you think when you see amino acids
with alkyl groups as their side chains.
These amino acids that you see here
are also non-polar and hydrophobic.
You weren't wrong if you thought
that aromatic compounds might smell really good,
because many of our most aromatic herbs and spices
that we're all familiar with,
like basil, or cinnamon, and vanilla
are composed of the same sorts
of ring structures that we see here.
So, now that we've tackled the non-polar,
hydrophobic amino acids, let's dive on into
the polar and hydrophilic amino acids.
The first group that we will look at is the neutral group.
Here we have serine, threonine, asparagine,
glutamine, cysteine, and tyrosine.
The way that I remember that these are the polar amino acids
is that these amino acids have a side chain that contain
an oxygen or a sulfur atom, which tends to hog
all the electrons around them to create
a localized negative charge over that atom,
and then a positive charge over the rest of the side chain.
So, you can kind of see why these amino acids
like to hang out with water, now,
since water is also polar in the same way.
These amino acids are considered neutral
because, although they are polar enough to interact
with water, they're not strongly polar enough
to qualify as an acid or base.
Which of the polar, hydrophilic amino acids
do qualify as acidic?
That would be these two amino acids, here:
aspartic acid and glutamic acid.
As you can see, these amino acids
have a carboxylic acid as part of their side chain,
which is a very willing, strong hydrogen donor,
which qualifies these amino acids as acidic.
When these side chains do donate their hydrogen,
and they're left in anion form,
in that case we refer to them
as aspartate and glutamate, respectively.
But, you might see them referred to in that way.
Last, but not least, we have the basic amino acids,
and they are histidine, lysine, and arginine.
The way I remember that these amino acids are basic
is that if you take a closer look at their side chains,
you see a few nitrogen atoms.
Remember that nitrogen is a very willing proton accepter.
This is why they qualify as basic.