CLASS: 10am-11am
DATE: 9.17.10
PROFESSOR: Whikehart
I.
PEPTIDE HORMONES
Scribe: Megan Guthman
Proof: Spencer Terry
Page 1 of 5
PEPTIDE HORMONES AND HORMONES THAT USE CELL SURFACE RECEPTORS [S1]
a. We will talk about peptide hormones and hormones that use cell surface receptors. We will also go into
internal cell hormones and the connection between them.
II. OUTLINE [S2]
a. We will look at the molecular sizes, origins of these hormones and what their ultimate fate will be.
b. How they occur in blood – how they’re transported in blood either in bound or unbound form.
c. Listing major examples of these hormones.
d. Talk about receptors and receptor binding.
e. Look at a selected peptide hormone (insulin) and a second peptide hormone.
III. SIZES, ORIGINS, AND FATE [S3]
a. Due to the force of custom, “peptide hormones” is a collective name that has been applied to peptides,
polypeptides and proteins that all function as hormones. The name “factor” has also been given to some of
these peptides and originates from a time when their peptide/protein nature was still unknown. In the
endocrine system of hormones originating from the brain, some of the smaller peptides that exist are from the
hypothalamus andanterior and posterior pituitary (hypophysis).
IV. SOME EXAMPLES OF SMALL PEPTIDES [S4]
a. Here’s some examples of small peptides:
i. Thyroid Releasing Hormone (TRH) from the hypothalamus, 360 MW. Its target is anterior pituitary, and it’s
involved with the release of Thyroid Stimulating Hormone (TSH).
1. It’s just a tripeptide, not even a cyclic tripeptide. Glutamyl–Histidyl-Proline with an amino group on
the end, opposite of where you would expect it. The right end should be carboxylic end but its not.
ii. Vasopressin from the posterior pituitary. It’s 3x larger than TRH and it targets kidney and arterioles.
Function is to increase water retention and blood pressure. Water reuptake comes from the kidneys;
increase in blood pressure is in part responsible for contracting the arterioles/blood vessels. It’s a cyclic
peptide with a disulfide bond between the two cysteines.
V. OTHER PEPTIDE HORMONES… [S5]
a. Other peptide hormones fall within the definition of a protein. The arbitrary crossing line between a
polypeptide and protein is 10 kD. Loosely used definition.
b. Growth Hormone (GH) is an example, aka somatrotropin or “body grower”. 22,000 kD and comes from
anterior pituitary. Target is liver and bone (other cells as well but liver and bone are the major ones). It causes
an increase in metabolism and growth in bones.
i. There are examples of excessive somatoropin released in the body. They tend to grow excessively.
Abraham Lincoln supposedly had this problem.
ii. Protein has alpha-helical and random coil structure. See alpha-helical (red), and random coils (yellow).
Blue arrows indicate where hormone binds to cell receptors. Unusual – why at both ends of the molecule?
Hormone can bind to two cells at the same time. These would be cells that are next to each other, and the
hormone can sandwich between them.
VI. PROLACTIN [S6]
a. Another similar structure is a hormone called Prolactin. 22 kD. Note extensive alpha helices with some
random coil structure between them. Binding sites for the receptors located near just one end of the molecule
– meaning binding to the receptor happens at two places at one time. Hormone is made in the anterior
pituitary and it stimulates milk production.
VII. FATE OF PEPTIDE HORMONES [S7]
a. What is the fate of the peptide hormones? What happens after they bind to receptor proteins?
i. Commonly taken up by cells themselves and transported to lysosomes where they’re broken down.
Lysosomes are subcellular organelles with ~30 degredative enzymes and an acidic pH environment – this
favors the breakdown of any molecules taken inside of them.
b. This happens to all peptide hormones after they bind to the receptor site where they deliver their message.
They deliver their message, then they’re taken inside the cell and are broken down.
VIII. PEPTIDE HORMONES IN THE BLOOD [S8]
a. The ability of a hormone to reach its target tissue is crucial.
i. Small peptide hormones have a low concentration. These peptide hormones are soluble enough to be
taken through the blood unbound to any carrier protein.
ii. Large peptide hormones have to be solubilized by a carrier protein. The carrier protein structure and
binding abilities are similar to that of the receptor protein. The carrier protein is probably synthesized in the
same location or in a location close by to where the receptor protein is made.
IX. IMPORTANT PEPTIDE HORMONES [S9]
a. Here’s example lists of important peptide hormones. (There’s many more than this.)
CLASS: 10am-11am
Scribe: Megan Guthman
DATE: 9.17.10
Proof: Spencer Terry
PROFESSOR: Whikehart
PEPTIDE HORMONES
Page 2 of 5
b. For exam purposes: The four bottom peptides  corticotrophin, GH, vasopressin, insulin. Notice insulin is
not part of the hypothalamic pecking order.
i. Corticotropin has a small molecular weight, could be called a small polypeptide. It goes to the adrenal
cortex where its involved with steroid synthesis. Many steroids are affected by the ability of corticotropin to
achieve its function.
ii. GH – comes from anterior pituitary. 22 kD molecular weight and it goes to liver and bone. Involved in
general growth and metabolism, also involved with the growth of muscles.
iii. Posterior pituitary only has 2 hormones from it. One of these is Vasopressin. It has a 1000 molecular
weight and its involved with the kidneys and blood vessels, and it effects water retention to increase or
maintain blood pressure. It’s important with many functions that are related to water volume.
iv. Insulin – 5800 Daltons, effects cells that are dependent upon the insulin molecule in order to get glucose
transported into them. Also involved with other functions related to growth – fatty acid and protein
metabolism. Insulin does many, many other things in addition to affecting the uptake of glucose itself.
X. “TOUR” OF SELECTED PEPTIDE HORMONE: INSULIN [S10]
a. Here’s the insulin molecule in 3D and 2D representation. Structure: 2 peptide chains bound together by
disulfide bonds of two different types.
i. One is intra-, other is an inter-disulfide bond. Inter- meaning it connects two different peptides. Intrameaning the disulfide bond is contained on the same chain rather than different chains.
XI. SINCE ITS DISCOVERY IN 1921… [S11]
a. Insulin has been one of the most heavily studied hormones of all time because of its relationship to diabetes.
Diabetes is a disease in which glucose affects some cells by starving them and poisons other cells by being
present in too high of a concentration. If glucose is present at a concentration in the blood stream, it can’t be
taken up by some cells, and what’s left over is going to go into other cells and cause an osmotic imbalance
and cause those cells to burst.
b. There’s a little bit more to the story than that. Not glucose itself but a metabolite of glucose that causes cells
to be destroyed. Role of insulin is to facilitate uptake of glucose into cells by means of glucose transport
proteins.
c. The general name for glucose transport proteins is GLUT. There’s about 11 GLUT proteins that are known.
Only one of them is insulin-dependent and that’s GLUT-4. This process is countered by two associated
peptide hormones called glucagon and somatostatin (all products of the pancreas).
i. Glucagon causes its effects by releasing glucose from the liver into the bloodstream- it counter-affects the
uptake caused by insulin.
ii. Somatostatin is a hormone that keeps things the way they are. Keeps the release of glucose by the liver
into the bloodstream at the same rate and it also affects the uptake of glucose in the cells at the same rate
that it is, whatever happens to be the state of the cell at that time.
XII. INSULIN IS PRODUCED IN THE ISLET CELLS… [S12]
a. All of the hormones are produced in the pancreas. See a generalized diagram of the pancreas here.
Pancreas associated closely with the duodenum. Look closely at a histological section:
i. Insulin produced in the islet cells (beta or B cells)(red arrow).
ii. Glucagon is made in the A cells (alpha cells), the dark cells that you see in bottom right (green arrow).
iii. Somatostatin in D cells (delta cells), the grey cells (blue arrow).
XIII. INSULIN IS INITIALLY FORMED… [S13]
a. Insulin is formed as a single polypeptide chain – preproinsulin. Proteolytic enzymes remove the first part of
that molecule, called the signal sequence/peptide. Peptide tells the enzyme what to do, tells it to begin the
process of making insulin molecule by removing itself first.
b. One that takes place, you have the formation of pro-insulin by forming disulfide bonds. Notice where the
disulfide bonds are formed. They’re formed between the remaining three portions of the peptides called A, B,
and C. None are formed on C, only on A and B. Inter and intra disulfide bonds formed, all insulin-catalyzed.
c. Now you have proinsulin. Proinsulin is kept inside the beta calls. Some of it is excreted. Proinsulin has very
little activity as a hormone (1-4%).
d. Before most proinsulin is excreted it’s converted to mature insulin state. Done by removal of the C peptide.
This is now mature insulin that will cause the uptake of glucose to take place.
e. ** Slide says hormone is not active in the proinsulin state. Not true, there’s a small amount of activity – about
1-4%.
XIV.
BINDING OF INSULIN TO ITS RECEPTOR PROTEIN [S14]
a. Insulin will bind to a receptor protein as all peptide hormones do. Insulin has a specific receptor protein:
simply called the insulin receptor protein. It’s a tetrapeptide – four polypeptide chains – 2 alpha and 2 beta
chains.
CLASS: 10am-11am
Scribe: Megan Guthman
DATE: 9.17.10
Proof: Spencer Terry
PROFESSOR: Whikehart
PEPTIDE HORMONES
Page 3 of 5
b. Insulin causes a conformational shift that activates a kinase activity in the cytoplasmic portion in the beta
chains. Kinase activity present in this region is a self-modulating/self-activating enzyme. Inactive before
insulin binding, active after insulin binds to the two alpha-chains. It autophosphorylates – adds phosphate
groups to itself. When it does that, it activates other kinase activity within that region. First
autophosphorylation, then activity of kinase itself on other molecules takes place.
i. Substrate for this is IRS1 (insulin receptor substrate 1). It produces a series of phosphorylations - a
cascade mechanism that involves delivery of GLUT-4 to the cell surface.
c. **Don’t get tripped up on an examination: Autophosphorylation, then activation of the main kinase activity,
which causes phosphorylation of IRS1.
XV. POST RECEPTOR ACTIVITY: GLUT-4 SURFACE DELIVERY [S15]
a. Here’s a diagram of the whole mechanism as far as it is known. The insulin receptor is responsible for about
3-4 different cascade mechanisms, we’re only looking at one of them – the one that involves GLUT-4.
b. Different diagram but same thing. Only difference is that this diagram indicates the disulfide bonds holding the
alpha and beta subunits together.
c. Cytoplasmic region is where kinase activity takes place (kinase activity means adding a phosphate group to
some substrate.)
d. Powering this whole reaction is ATP. Phosphorylation occurs with the addition of a phosphate group on IRS1.
This, in turn, activates another protein: phosphatidylinositol-3-kinase. When you activate that, it, in turn, is an
active kinase that phosphorylates a number of other enzymes. It finally causes the initiation of translocation of
GLUT-4 - translocation meaning it is released from a subcellular vesicle and transported to the plasma
membrane on the surface of a cell.
XVI.
SURFACE HORMONES THAT USE cAMP AS A MEDIATOR [S16]
a. This is a list of all these hormones that have this role. The ones that are peptides or polypeptides are
indicated by little blue stars. Other ones that have that role that are not peptides are indicated without little
blue stars.
b. I would like you to be familiar with the ones that are underlined. Know what they do, and be careful
because some are peptides and some are not. These will come up again, especially when you get to talking
about organ systems.
XVII. GENERAL cAMP MECHANISM [S17]
a. Now comes the fun! This is a general cAMP mechanism. Acutally two mechanisms: one is stimulatory and
one is inhibitory (we will emphasize the stimulatory one).
b. In the stimulatory pathway, it’s called the beta effector. The beta effector (a hormone) will go to its receptor
protein (blue arrow). Associated with the receptor protein is a G-protein. G-protein has 3 polypeptide
subunits, also bound to GDP. When the effector binds to the receptor protein it causes a conformational
change in the protein that, in turn, affects the dissociation of the gamma and beta subunits away from the Gprotein. And it causes GTP to replace GDP. Now we have GTP bound to the alpha subunit of the G-protein.
The GTP+Galpha subunit will dissociate from this area where it encounters adenylyl cyclase. This enzyme
takes ATP and converts it to cyclic AMP. This is the point at which a cascade begins.
c. Over on right side of the figure (inhibitory pathway), alpha2 effector causes an inhibition to take place using a
Gi protein. [Writes on board] Gs causes stimulation. Gi causes inhibition (of adenylyl cyclase). This is
essentially the same thing except its an inhibitory complex that forms. When it gets to the adenylyl cyclase it
will shut it down.
d. Usually adenylyl cyclase is operating to some set rate. It will be increased in its rate by stimulation of GTP,
Gs – to convert ATP to cAMP. Gi will shut it down, and it will also have some effect on the GTP+G-protein as
well (that’s why the arrow is going to the left of the figure).
e. Cyclic AMP is going to be broken down eventually. [Writes on board] cAMP  stimulates cascade.
Something has to break down cAMP to AMP, prevents too much cAMP in a cell. This is a cyclic AMP
phosphodiesterase. cAMP phosphodiesterase is inhibited by caffeine and other kinds of related compounds.
[Skip to S21] Methyl xanthines (general name – caffeine and theobromine are examples of methyl xanthines
which are found in coffee, tea, chocolate cake(?)) What do they do? If you inhibit cAMP phosphodiesterse,
you’re going to get more cAMP and less AMP. These things increase alertness and GI motility. Be aware that
phosphodiesterase inhibitors cause this effect by affecting the amount of cAMP.
f. [Back to S17] Be familiar with left side of the diagram and know about the cAMP phosphodiesterase
inhibitors.
g. Buck asks for clarification. Answer: phosphodiesterase is the enzyme that breaks down cyclic AMP. If you
inhibit the enzyme, you get more cyclic AMP.
XVIII. AN EXAMPLE OF A HORMONE AND ITS RECEPTOR [S18]
CLASS: 10am-11am
Scribe: Megan Guthman
DATE: 9.17.10
Proof: Spencer Terry
PROFESSOR: Whikehart
PEPTIDE HORMONES
Page 4 of 5
a. What we’re looking at here is Thyroid Stimulating Hormone (TSH) and TSH receptor protein. Looking at the
hormone (blue and green molecule) and receptor protein (brown). Blue arrow is where G-protein is possibly
located.
XIX.
WHEN A G PROTEIN IS AFFECTED… [S19]
a. Here’s more information about the G-protein in a diagrammatic fashion.
b. G-protein (stimulatory variety) has alpha, beta, and gamma subunits.
i. When a hormone receptor associated with the G-protein becomes bound and causes conformational shift,
GTP replaces GDP, and the gamma and beta subunits dissociate away from it. The activated alpha subunit
affects adenylyl cyclase. After G-alpha binds, it loses a phosphate group, now in the GDP form again.
Gamma and beta subunits join back with the alpha subunit, and the process starts over again.
XX. G PROTEINS COME IN SEVERAL VARIETIES… [S20]
a. Table of all the different types of G-proteins that exist.
i. Optometry students – learn about Gt in ocular biochemistry.
b. Be familiar with 3 Gs and 1 Gi – top 4 G-proteins in the chart. Locations, stimulating hormones, effectors,
physiological effects. Actually, don’t worry about Gi, just know some examples of Gs.
XXI.
AT THIS STAGE, GTP HAS BEEN FORMED… [S21]
a. Repeat of what I told you before, including info about cAMP phosphodiesterase and methyl xanthines.
XXII. CASCADE MECHANISMS [S22]
a. Example of a cascade mechanism.
b. You have seen this before. Well known cascade involves breakdown of glycogen into glucose when hormone
is epinephrine. Many of the other mechanisms have not been worked out with certainty.
c. Epinephrine binds to its receptor protein and there’s a formation of the activated G-subunit. This causes the
activation of adenylyl cyclase to make cAMP. At this stage, we’re producing 20 molecules because each
enzyme causes an increase in the overall effect. Each enzyme amplifies the reaction.
d. In this case, cAMP activates Protein Kinase A (a 10-fold amplification), that in turn effects Phosphorylase B
Kinase and activates it. It, in turn, takes inactive Glycogen Phosphorylase B and makes it active (another
amplification). This will take glycogen and break it down into gluclose-1-phosphate, giving you still another
amplification. From the original molecule, you get a signal increase that involves 10,000 molecules.
e. Next slide focuses on Protein Kinase A…
XXIII. PROTEIN KINASES EXIST AT THE START… [S23]
a. Protein Kinase A (PKA) - enzyme in inactive form has four polypeptides: Two of which are R, regulatory, and
two of which are C, catalytic. This is not an enzyme you would consider to be one of the classes we looked at
before. Simply involves activation of the enzyme.
b. Four sites on regulatory polypeptides. These sites take up cAMP. This causes dissociation away from the
catalytic unit. Once that dissociation occurs, enzyme is in active state. Back to previous slide – see catalytic
subunit as active PKA – substrate for the enzyme is inactive Phosphorylase B Kinase. (That’s what the red
molecule is on this S23.) One enzyme sits on another enzyme and becomes activated by a phosphorylation
process. Each time this occurs, there’s an amplification of the signal.
c. Other protein kinases (like Protein Kinase B which uses a derivative of inositol, or Protein Kinase C which
uses calcium) all work the same way. Have regulatory subunits bound to them, and when cAMP binds to
them (or phosphorylation happens) they become active.
XXIV. THE LAST MECHANISM THAT WE WILL LOOK AT… [S24]
a. Calmodulin is a protein that will store or bind to calcium itself. More complex – don’t need intimate details for
exam but will walk us through it.
b. Double mechanism – requires two membranes, two enzymes, as well as the usual hormone receptor protein
and a G-protein (in this case a Gq protein). Calcium and calmodulin participate in the mechanism. Calcium
bound to calmodulin will function, and calcium itself will function independent of one another.
c. Hormones associated with the mechanism include peptide hormones like Vasopressin (made in posterior
pituitary) and TRH. Mechanism makes use of Protein Kinase C.
XXV. CALCIUM/CALMODULIN MECHANISM [S25]
a. Top membrane is the plasma membrane of a cell. Polypeptide hormone at the top left. Bottom is membrane
of ER.
b. The polypeptide hormone binds to receptor protein. The G-protein (Gq protein) in turn binds to another
enzyme (NOT adenylyl cyclase) but Phospholipase C Beta. This causes breakdown of membrane lipid called
PIP2 (phosphoinositol diphosphate). It will release inositol-1,4,5-triphosphate. This will migrate to another
protein on the ER that opens up a pore and allows the release of calcium ions. Calcium ions will either
operate independently or bind to calmodulin. This causes a cascade too.
c. The activation of protein kinase phosphorylates an inactive target protein making it active and producing a
cellular response. At the same time, and independent of this, calcium also binds to a Protein Kinase C, along
CLASS: 10am-11am
Scribe: Megan Guthman
DATE: 9.17.10
Proof: Spencer Terry
PROFESSOR: Whikehart
PEPTIDE HORMONES
Page 5 of 5
with the remainder of what was here – a diacylglycerol – both of them will activate this protein kinase (upper
right of figure) that will phosphorylate an inactive target protein giving you an active target protein and
producing a cellular response. Two mechanisms involved.
d. On test, will ask very general question about this, nothing very specific. All peptide hormone does is cause
initial activation on receptor protein, and everything else is involved internally.
XXVI. SUMMARY [S26]
a. Read slide.
XXVII. SUMMARY (continued) [S27]
a. Read slide.
b. Guanylyl cyclase operates by the same mechanism as adenylyl cyclase but it produces opposing effects.
[End 44:13 mins]