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--