Hormones are constantly floating through our

bloodstream.

0:00:15.000,0:00:24.033

At any given point in time you may have growth

hormone, thyroid hormone, or

luteinizing hormones coursing through your

0:00:24.033,0:00:32.066

circulation. Some of these hormones, such as

the steroid hormones, can pass directly into

cells and bind to intracellular receptors.

0:00:32.066,0:00:40.066

Others, such as protein and peptide hormones

are hydrophilic, and must bind to receptors in

the plasma membrane of target cells.

0:00:40.066,0:00:48.000

This brings up an important point: How does the

extracellular signal of a hormone get transmitted

into the cell?

0:00:48.000,0:00:55.066

This is commonly accomplished using second

messengers: small molecules such as cAMP or

calcium.

0:00:55.066,0:01:01.066

Second messengers relay information from the

first “messenger,” the hormone, into the cell.

0:01:01.066,0:01:09.066

These second messengers are often produced

using common proteins associated with the

plasma membrane called G-proteins.

0:01:09.066,0:01:18.000

G-proteins are “coupled” to receptors in the

plasma membrane of the cell. G-protein coupled

receptors can mediate the responses to signals

0:01:18.000,0:01:28.066

such as hormones and neurotransmitters.

Many different types of ligands can activate G-

proteins such as fatty acids, proteins, peptides,

0:01:28.066,0:01:37.066

or amino acids.

Interestingly, about half of all known drugs work

through G-protein coupled receptors.

0:01:37.066,0:01:45.066

Let’s take a closer look at how G-protein

signaling mechanisms work. Hormones floating

through the bloodstream may circulate freely or

0:01:45.066,0:01:54.066

may be complexed with binding proteins.

In the bloodstream, the hormone dissociates

from any associated binding proteins and moves

0:01:54.066,0:02:11.000

out of the capillary and into the interstitial fluid.

The hormone then binds to a hormone receptor

in the plasma membrane of a target cell.

0:02:11.000,0:02:19.033

The hormone receptor is associated with a G-

protein, as shown here, which is attached to the

cytoplasmic side of the plasma membrane.

0:02:19.033,0:02:26.000

The G-protein is responsible for relaying the

hormonal information to downstream signaling

pathways within the cell.

0:02:26.000,0:02:31.066

They can be coupled to enzymes or ions

channels in the plasma membrane.

0:02:31.066,0:02:35.033

Each type of G-protein is specific for one of

these signaling pathways.

0:02:35.033,0:02:43.000

G-proteins have 3 subunits: an alpha-subunit, a

beta-subunit, and a gamma-subunit.

0:02:43.000,0:02:50.033

When the G-protein is in an inactive state, the

alpha-subunit has a bound guanosine-

diphosphate, or GDP.

0:02:50.033,0:02:57.000

The binding of the hormone to the G-protein

coupled receptor initiates a conformational

change in the G-protein.

0:02:57.000,0:03:07.033

This stimulates the alpha subunit of the G-

protein to exchange its bound GDP for a GTP.

With this GTP bound,

0:03:07.033,0:03:21.066

the G-protein is in an active state. The activated

G-protein dissociates into the alpha subunit, and

a beta-gamma complex.

0:03:21.066,0:03:26.066

The actual target of the activated subunit

depends on the G-protein that is activated.

0:03:26.066,0:03:32.000

In this video, we will first examine the pathway is

which cAMP serves as a second messenger.

0:03:32.000,0:03:37.066

The G-protein in this case is a stimulatory

protein called GS.

0:03:37.066,0:03:45.033

The activated alpha-subunit of GS binds to the

enzyme adenylyl cyclase.

0:03:45.033,0:03:54.033

This enzyme converts adenosine tri-phosphate

(ATP) into cAMP.

0:03:54.033,0:04:02.033

cAMP can serve directly as a signaling

molecule, or it can act indirectly through

activation of proteins within the cell.

0:04:02.033,0:04:12.033

For example, 4 cAMP molecules can bind to the

regulatory subunits of Protein Kinase A (PKA).

0:04:12.033,0:04:23.033

This allows the catalytic subunits of PKA to

dissociate, and PKA can then phosphorylate

intracellular targets.

0:04:23.033,0:04:29.066

The response of a cell to cAMP and PKA activity

depends on the cell itself.

0:04:29.066,0:04:41.033

A wide variety of hormones utilize cAMP and G-

protein signaling such as ACTH, Glucagon, LH,

PTH and TSH.

0:04:41.033,0:04:48.000

For example, the hormone glucagon can travel

through the bloodstream to the liver and bind to

G-protein coupled receptors.

0:04:48.000,0:04:54.000

This initiates an increase in cAMP, which leads

to the breakdown of glycogen in the liver.

0:04:54.000,0:04:59.000

Since many hormones and neurotransmitters

rely on the cAMP signaling pathway, the

response of a

0:04:59.000,0:05:03.000

cell will depend on the cell type itself.

0:05:03.000,0:05:14.033

An increase of cAMP in a liver cell will cause a

very different response than an increase in

cAMP in a renal cell or in an adipocyte.

0:05:14.033,0:05:22.033

For proper cell function, the cell must also be

able to stop the G-protein signaling pathway

after it has accomplished its task.

0:05:22.033,0:05:35.066

To terminate this signal, the cAMP must be

broken down using the enzyme cAMP

phosphodiesterase.

0:05:35.066,0:05:41.033

The catalytic subunits of PKA then reassociate

with the regulatory subunits.

0:05:41.033,0:05:54.033

In order for the G-protein to become inactivated,

the alpha-subunit must hydrolyze its bound GTP

back into GDP using its GTPase activity.

0:05:54.033,0:06:03.000

The alpha subunit then reassociates with the

beta-gamma complex, and the G protein is once

again back in an inactive state.

0:06:03.000,0:06:20.000

The cell is then ready to be stimulated by

another hormone.

0:06:20.000,0:06:29.066

G-proteins can also initiate another common

signaling pathway that utilizes intracellular

calcium as a second messenger.

0:06:29.066,0:06:36.033

Once again, the hormone dissociates from any

complexed binding proteins and moves out of

the capillary and into the interstitial fluid.

0:06:36.033,0:06:47.000

The hormone then binds to a G-protein coupled

hormone receptor in the plasma membrane of

the target cell.

0:06:47.000,0:06:52.033

The G-protein in this signaling pathway is called

Gq.

0:06:52.033,0:07:02.033

The alpha subunit of the G-protein exchanges its

bound GDP for GTP, and the activated alpha

subunit dissociates from the rest of the G-

0:07:02.033,0:07:13.000

protein.

In this particular pathway, the alpha subunit

activates phospholipase C, or PLC.

0:07:13.000,0:07:22.033

This enzyme acts on the molecule

phosphatidylinositol 4,5-bisphosphate, also

called PIP2.

0:07:22.033,0:07:33.000

PLC cleaves PIP2 into two molecules: inositol

1,4,5 triphosphate (IP3) and diacylglycerol

(DAG).

0:07:33.000,0:07:41.066

IP3 is a small, water soluble molecule that is

released into the cytosol, and travels to the

endoplasmic reticulum.

0:07:41.066,0:07:49.066

As you have seen in previous lectures, the

endoplasmic reticulum stores a large of amount

of calcium in the lumen.

0:07:49.066,0:08:01.066

IP3 binds to a ligand-gated calcium release

channel in the membrane of the endoplasmic

reticulum, and calcium flows into the cytosol.

0:08:01.066,0:08:13.066

At the same time that IP3 is initiating calcium

release, DAG is migrating through the plasma

membrane to activate Protein Kinase C (PKC).

0:08:13.066,0:08:20.000

The “C” in PKC is named because calcium is

necessary for full activity of this kinase.

0:08:20.000,0:08:27.033

The calcium released from the ER by IP3

assists in full activation of PKC.

0:08:27.033,0:08:35.033

Once activated, PKC phosphorylates a number

of intracellular targets, thus transmitting the

initial message of the hormone binding to the

0:08:35.033,0:08:50.000

hormone receptor.

0:08:50.000,0:09:09.033

In order to terminate this signal, calcium is

resequestered in the endoplasmic reticulum and

PIP2 is reformed.

0:09:09.033,0:09:20.033

The alpha subunit of the G-protein hydrolyzes its

bound GTP into GDP, and the G-protein

reassociates.

0:09:20.033,0:09:28.000

This restores the resting state of the cell so that

another hormone can initiate cellular effects.

0:09:29.033,0:09:35.066

Today we have looked at two of the major

mechanisms by which G-proteins operate within

a cell.

0:09:35.066,0:09:41.066

This is one of the major ways in which

hydrophilic hormones are able to exert

intracellular effects.

0:09:41.066,0:09:47.033

Please review this video as many times as

needed to familiarize yourself with the G-protein

signaling pathways.