In previous videos, I introduced steroids to you
as one example of the chemical messages
that our body parts use to communicate with each other.
And we call those chemical messages hormones,
but I haven't really gotten to tell you all about steroids,
and where they come from and how they're made,
and that's what I want to do today.
To accomplish that, I really need
to start by introducing you to terpenes.
Terpene might not be the most familiar of words,
but it refers to a class of lipid molecules
that are made of a repeating distinct set of carbon
atoms called isoprene.
So I'll draw in that carbon unit called isoprene.
Isoprene has this really distinct structure
of four carbons chained together-- one, two, three,
four-- with a fifth carbon branching off
of one of the middle carbons.
And this five carbon unit repeats
to build larger and larger molecules.
And so as two five carbon isoprenes come together,
you get a molecule that has 10 carbons.
And we call that a monoterpene.
And one of my favorite monoterpenes
right now is menthol, which looks like this.
And this little guy is super helpful to me
right now, because I've had a pretty rough cough
for the past week, and my cough drops are filled with menthol.
And then if you add one more isoprene,
you'd have 15 carbons.
And that would give you three units of isoprene.
Supreme And we'd call it a sequiterpene.
And one example of a sequiterpene is ginger.
And ginger looks like this.
But you can see that ginger is made up
of three isoprene units, and it fills up gingersnap cookies,
and those are my favorite types of cookies.
Really, you can just keep adding these isoprene units
and making larger and larger molecules,
so I'll just go ahead and make a chart here.
Remember, monoterpenes had two isoprene units,
and sequiterpenes had three isoprene units.
And if you added another isoprene unit to that,
you'd have a diterpene, and it would
have four isoprene units, making 20 carbons.
And it's called a diterpene now because it's
really just two monoterpenes put together.
And then you could add another one,
and that would give us five.
And five isoprenes for 25 carbons,
and that would be a sesterterpene.
And you could just keep going.
And you could have six isoprene units for 30 total carbons,
and that would be a triterpene.
Because essentially now we've got three monoterpenes.
And then, if you added another monoterpene to that,
you'd have a tetraterpene, and that
would have 40 carbons and eight isoprene units.
But really, this could keep going,
and we could keep adding isoprene units.
And we'd find lots of nice plant oils
that we'd make and vitamins.
But that, unfortunately, is not our goal today.
We're still talking about steroids.
I'm going to shift to how our body uses
these isoprene building blocks to create the chemicals that it
needs.
And that's a process that's called biosynthesis.
So let me clear some room so that we
can talk about biosynthesis.
In biosynthesis, our body starts off
with isoprene bound to something called pyrophosphate,
which looks something like this.
And I'm going to use the letter R
to represent that isoprene unit for now.
Just so I can really show you what pyrophosphate looks like.
So that's pyrophosphate.
This green area is pyrophosphate.
And you probably won't be expected
to know too much about pyrophosphate.
But what you should know is that it's a really weak base, which
makes it a good leaving group in organic chemistry,
and that's the important part.
Because for the reaction that our body uses
to build with these blocks of isoprene,
it needs a good leaving group, and that's pyrophosphate.
And our body really begins with two different isoprene
pyrophosphates, and they're found in nearly all living
organisms.
And those are dimethyl allyl pyrophosphate
and isopentyl pyrophosphate.
I've just shortened the pyrophosphate to OPP
because that's what most textbooks abbreviate it as,
and it saves quite a bit of time.
But I will go ahead and write out the names of these,
just so I don't confuse you too badly.
And those names make pretty good sense
if you've practiced naming carbon molecules in, say,
an organic chemistry class or something.
But they probably sound like a foreign language if you
haven't.
So let's not get too hung up on them here,
because they don't help us much with the concept anyway.
But what happens is the electrons of one pi bond
act like a nucleophile, and they attack this carbon,
allowing the pyrophosphate to leave.
And that results in a 10 carbon molecule
called geranyl pyrophosphate.
And so that's geranyl pyrophosphate.
And then that process might happen again,
leading to a 15 carbon farnesyl pyrophosphate.
And forgive me if I'm butchering the pronunciation,
but I'll draw it in.
That's farnesyl pyrophosphate.
And then as these chemical reactions continue,
our body eventually produces a triterpene, or if you remember,
that's a 30 carbon molecule made of six isoprene units.
And that triterpene is called squalene.
So let me draw that in.
OK, I just wanted to make sure that I actually got 30 carbon
molecules in there, and I think I did.
And that is squalene.
It was important for me to get to squalene because this
is the molecule the forms the basis of all
of the steroid hormones that our body--