Hey it's professor Dave, I'm going to tell you about steroids.
When people hear the word steroids, they typically think of
anabolic steroids, which are synthetic derivatives of testosterone, but in
actuality, steroids refer to a class of molecules that includes not just
hormones, but also certain vitamins and cholesterol. Let's learn a bit about the
structure and function of steroids. Steroids are all derivatives of this
ring system, which is comprised of four rings. The A, B, and C rings are six
membered rings while the D ring is a five-membered ring. There are also always
methyl groups on these two carbons, and for ease of communication we will always
number this ring system the following way:
This is carbon 1, and numbers 1-10 will wrap around rings A and B this
way. Then carbon 11 jumps over to ring C and we wrap around rings C and D, lastly
numbering these two methyl groups like this. All the steroids we examine will be
based on this structure. Now we know that six-membered rings like to be in chair
conformations, so let's see what that would actually look like.
First we should note that there are two common conformations for the A ring
either with a trans ring junction between A and B like this, or a cis junction
putting ring A like this.
Also notice that the methyls are in the axial position pointing upwards, and we
will commonly refer to the substituents on the same side as those methyls as
beta substituents, whereas ones on the other side, trans to the methyls, will be
alpha substituents. The orientation of a ring will depend on the hydrogen on
carbon 5. If that hydrogen is alpha, or on the dash bond you get the trans AB
junction. If the hydrogen is beta, or on the wedge bond, you get the cis AB junction.
Now, where else do steroids vary in structure? A common location for a
substituent is carbon 17. Different alkyl groups at this location will generate
different steroids, one of which is cholesterol. Cholesterol has eight
stereocenters which means 256 different stereoisomers of this structure are
possible, but only this specific one is cholesterol.
This is an intermediate in the biosynthesis of all the steroids we need
in our body, and although we often consume cholesterol, we don't need to
because we also make it ourselves, so we will typically have more cholesterol
than we need, and too much cholesterol in the diet leads to clogged arteries and
heart problems.
Let's look at another type of steroid, the sex hormones. These can be divided
into three classes: the female hormones or estrogens, the male hormones or
androgens, and pregnancy hormones or progestin. Let's look at the female
hormones first. These are estrone and estradiol,l which differ only in the level
of oxidation at carbon 17. Estradiol is secreted by the ovaries and kicks off
the changes that occur during puberty, and also causes development of the
mammary glands during pregnancy,
whereas estrone is just a metabolized form of estradiol that is excreted.
The male sex hormones, and androsterone and testosterone, are incredibly similar to
the female hormones, with the only differences occurring in the A ring.
Similarly to the female hormones, testosterone does all the work, promoting
the growth of body hair and facial hair, lowering of the voice and other traits
associated with puberty, whereas androsterone is just the metabolized
version that is excreted into the urine. And progesterone is the most important
pregnancy hormone, which prepares the uterine lining for implantation of a
fertilized egg and suppresses further ovulation during pregnancy.
Other hormones like cortisol and cortisol regulate many other biological
activities, like metabolism and controlling inflammation. And that wraps our survey of lipids.
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