Classes of hormones by Ryan Patton of Khan Academy: OK. So when I introduced the endocrine system, I mentioned that hormones can be classified by where they function.
Classes of hormones: there are endocrine, paracrine, and autocrine hormones based on function on their target cells, but they are best categorized by structure. So we talk about autocrine function, paracrine function and endocrine function. Perhaps even more significantly, hormones can also be categorized by structure. I say more importantly because the structure of a hormone truly defines how it works. That is what I want to talk about today, the three major Classes of hormones. The first major types of hormones are proteins and polypeptides. Just as a refresher, proteins and polypeptides are composed of amino acids. These amino acids are joined together by petide bonds. Thus, many peptide bonds join amino acids together to form a polypeptide or a protein.
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Classes of hormones: Protein or polypeptide hormones
Proteins and polypeptides form most of our body’s hormones. The sizes of these hormones range from small to large. To give you an example, imagine we have three or so amino acids linked together forming a hormone. That would be a small polypeptide. In three amino acids, we are talking about a handful of atoms, maybe 20 or so atoms. Just to illustrate, a cell in your body has about a trillion atoms just inside that one cell. And there are 100 trillion cells in your body. So we’re talking about very, very, very small things.
Proteins can range from small collections of amino acids, all the way to hundreds of amino acids. So they can be quite large. The break point becomes right around 100. And that’s where we shift from calling them polypeptides to proteins. And just like all proteins in your body that are going to be excreted, protein and polypeptide hormones are made in the rough endoplasmic reticulum (RER) of the cell. And they go from the RER to the Golgi apparatus. And from the Golgi apparatus, they’re kind of repackaged into vesicles that can eventually be excreted from the cell. And because proteins and polypeptides are made of amino acids, they’re typically charged, which makes them water soluble. However, it also gives them a really hard time crossing cell membranes. And so typically the hormone receptors are located in or on a cell surface.
Because these protein and polypeptide hormones are unable to actually travel into the cell, they initiate a cascade effect of secondary messengers inside the cell. The main idea is that when these protein and polypeptide hormones bind to the cell surface, they initiate a response inside the cell. And we refer to that as a secondary messenger system. Some examples are given in the figure. The peptide bonds are really what make this class of hormones unique. These peptide bonds are carbon-nitrogen bonds.
So they can be small and they can be large. But these building blocks of amino acids that are used as chemical messengers to convey the message of a hormone binding to a receptor are called proteins and polypeptides. And one example is insulin. Insulin is a relatively large hormone, and it is a protein hormone. The second major type of hormones are steroids. When I think of steroids, the first thoughts that comes to my mind are a bunch of athletes getting in trouble with their regulating committees. But steroids are actually one of the major Classes of hormones used in our body to communicate.
Classes of hormones: Steroid hormones
And so there are a lot of steroids in your body. Steroids come from lipids, a major difference from the protein and peptide hormones. And cholesterol is the major lipid from which these steroids are derived through specific enzymatic pathways. Because they come from cholesterol, steroids have a really distinctive structure that all of them share. So steroids have a characteristic backbone. There are four ring structures here. And these rings are made of carbon atoms. And so there are three cyclohexane rings or six-membered carbon rings and one cyclopentane ring.
And I’m going to label these rings A, B, C, and D. This characteristic structure has a characteristic way of signaling a cell. Unlike proteins and polypeptides, whose receptors are located on the cell surface, steroids, because they’re made of lipids, have an actually easy time passing through the cell membrane. As a result, their receptors are located inside the cell. So steroids usually go all the way inside of the cell to signal the receptor as primary messengers.
They’re actually doing the signaling without using second messengers. And often times, their receptors are located either in the cytoplasm or all the way in the nucleus. But steroids typically go in, and their effect goes all the way down to the transcription and translation level of proteins. And so as primary messengers, they’re going inside the cell, and they’re effecting a change in that cell resulting in the transcription and the translation of new proteins and new products inside the cell.
But for now, I want you to be thinking of steroids as one of the major hormones that are in our body, not just a means for athletes getting an edge on the competition. Some examples of big steroids in the body are those that come from the adrenal cortex, such as cortisol and aldosterone, and those hormones that come from the gonads such the sex hormones, testosterone,estrogen and progesterone. So far we described Steroids and proteins or polypeptides as two major classes of hormones. The third major type of hormones by structure are tyrosine derivatives. And tyrosine derivatives come from the amino acid tyrosine. As I told you earlier that protein and polypeptide hormones are made of amino acids, you might ask yourself why do these get their own major class if these are also made of an amino acid? What makes them really special? Is that because they are made of one amino acid.
Classes of hormones: amine or tyrosine-derived hormones
So one amino acid, tyrosine, is manipulated to make these hormones. Do these hormones that are derived from tyrosine end up being able to sometimes act like proteins and polypeptides and sometimes act like steroids? So they really get their own class. And an example of tyrosine derivatives in the body are those that come from the thyroid gland, like T3 and T4, or triiodothyronine and thyroxine, that stimulate metabolism. And these tyrosine derivatives act really similarly to steroids. And then another example of tyrosine derivatives are catecholamines. Catecholamines are those hormones that are produced in the adrenal medulla that are involved in our fight or flight responses, like epinephrine and norepinephrine. And these thyrosine derivatives act really similarly to peptides by binding on the outside of the cell and releasing those secondary messengers inside the cell.
And so the thyroid hormones that are tyrosine derivatives act like steroids. And the catecholamine tyrosine derivatives act like proteins and polypeptides. However, it is important to remember that they form their own unique class because they’re all derived from the amino acid tyrosine.
And so I know it’s hard to make learning these Classes of hormones fun. But maybe at least we can let our minds blow up a little bit over the fact that the structure of these hormones dictates almost everything we think or do, from fear, to hunger, to urinating, and pushing babies out. All of our responses to the world around us are signaled by hormones.
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