Molecular Biology Chapter 11 Outline

Front 1

Signal transduction pathway

Back 1

Series of steps linking a mechanical, chemical, or electrical stimulus to a specific cellular response.
1.Evolved in ancient prokaryotes and early single-celled eukaryotes.
2.Later adapted for use by multicellular organisms.
3. Example:Mating yeast cells.

Front 2

Local and Long Distance Signaling

Back 2

1. Locally, cells can communicate through junctions that directly connect adjacent cells.
2. Signaling substances dissolved in the cytosol can pass easily between adjacent cells.
3. Animal cells can communicate by direct contact between membrane-bound cell surface molecules (cell-to-cell recognition).

Front 3

Local regulator

Back 3

Secreted molecule that influences cells near where it is secreted.
a)Growth factors (paracrine signaling).
b)Synaptic signaling.

Front 4

Hormones

Back 4

Type of secreted molecule formed in specialized cells; travels throughout the body in fluids, acting on specific cells, influencing change in target cells (long distance).

Front 5

Hormones

Back 5

Plant hormones travel through vessels or diffuse through cells and air
Animal hormones are restricted largely to vessels
Hormones vary widely in size and type:
Ethylene – promotes fruit ripening, regulates growth = 6 atoms
Insulin – regulates blood-sugar levels = 1,000’s of atoms

Front 6

Cell Signaling

Back 6

In the case of epinephrine’s effect on the body, epinephrine is secreted from the adrenal gland during times of physical or mental stress resulting in the mobilization of fuel reserves. Glycogen to glucose to respiration

Front 7

Three stages of cell signaling

Back 7

Stage 1: Reception
Stage 2: Transduction
Stage 3: Response

Front 8

Stage 1: Reception

Back 8

The target cell detects an external signaling molecule when the molecule binds to a receptor protein on the cell’s surface or inside the cell.

Front 9

Stage 2: Transduction

Back 9

The signal carried by the molecule is converted by the cell to bring about a specific response – sometimes in a single step, but more often in a series of steps.

Front 10

Stage 3: Response

Back 10

Transduced signal triggers a specific response – enzyme activity, rearrangement of the cytoskeleton, or activation of specific genes – at the right time and in coordination with other cellular activities.

Front 11

Receptor Proteins

Back 11

A receptor protein on or in a target cell allows the cell to receive and respond to the signal. The signaling molecule is complementary in shape to a specific site on the receptor and will react only with the target cell.

Front 12

Ligand

Back 12

Signaling molecule that binds specifically to another receptor molecule – usually a larger one.
a) Causes protein receptor to change shape.
b) Receptor is activated, reacting with other cellular molecules or aggregating with other receptors.
c) Results in further molecular events

Front 13

G Protein-coupled Receptors

Back 13

Signal receptor protein in the plasma membrane that responds to the binding of a signaling molecule by activation a G protein.

Front 14

Properties of G Protein-coupled Receptors

Back 14

a) GTP = energy-rich molecule.
b) Receptors are wide-spread.
c) Function: sensory reception.
d) Bacterial infections interfere with proper function.

Front 15

Receptor Tyrosine Kinases

Back 15

Receptor protein spanning the plasma membrane, the cytoplasmic part of which can catalyze the transfer of a phosphate group from ATP to tyrosine
a) Respond to ligands.
b) Activates the signal transduction proteins within the cell.
c) Abnormal function = cancer.

Front 16

Ligand-gated Ion Channel

Back 16

Trans membrane protein containing a pore that opens or closes as it changes shape in response to signaling molecules; regulating ion flow.
a) Example: neurotransmitters.
b) Function: nervous reception, transmission.

Front 17

Intracellular Receptors

Back 17

1. Intracellular receptor proteins are found either in-side the cytoplasm or on the nucleus.
2. Signaling molecule passes through the plasma
membrane – being either hydrophobic or small
enough to pass through.
3. Activation occurs upon binding of signaling molecule to target cell receptor.

Front 18

Signal Transduction Pathways

Back 18

The binding of a specific signaling molecule to a receptor in the plasma membrane triggers the first step in a chain of molecular interactions that leads to a particular response within the cell. Transduction stage usually consists of multiple steps

Front 19

Signal Transduction Pathways- Multiple steps

Back 19

a) Results in signal amplification.
b) Larger number of activated molecules.
c) More opportunities for coordination and regulation.
(e.g.Protein phosphorylation and dephosphorylation, Cyclic AMP, Calcium ions and Inositol triphosphate (IP3)).

Front 20

Second Messenger

Back 20

Small, non-protein, water-soluble molecule or ion (calcium ion, cyclic AMP), that relays a signal to a cell’s in-terior in response to a signaling molecule bound by a signal receptor protein.

Front 21

Nuclear and Cytoplasmic Responses

Back 21

1. Genes may be turned on or off as in the case of
protein synthesis.
2.Protein activity may be regulated, directly affecting
proteins outside the nucleus as in the case of
enzyme function.
3. Signaling pathways may regulate the activity of
proteins rather than their synthesis.
4. Liver cells respond to epinephrine by regulating
cellular energy metabolism by affecting the activity
of an enzyme which catalyzes the breakdown of
glycogen into glucose
5. Response may affect cell shape.
6. Nonmotile yeast mate by the growth of localized projections in one cell toward the opposite mating type.

Front 22

Signal Amplification

Back 22

a) Elaborate enzyme cascades amplify the cell’s response to a signal.
b) At each catalytic step in the cascade, the number of activated products is much greater than in the preceding step.
c) For instance, a small number of epinephrine molecules can lead to the release of hundreds of millions of glucose molecules from glycogen.

Front 23

Specificity and Coordination of the Response

Back 23

a) Different kinds of cells turn on different genes resulting in different kinds of cells possessing different collections of proteins.
b) Two different cells can respond differently to the same signal (heart & liver cells to epinephrine).
c) Economical to use some of the same proteins in more than one pathway.

Front 24

Scaffolding proteins

Back 24

Type of large relay protein to which several other relay proteins are simultaneously attached, increasing the efficiency of signal transduction.
1. Enhances speed and accuracy of signal transfer between cells due to structure alignment.
2. May more directly activate other relay proteins

Front 25

Termination of the Signal

Back 25

a) The ability of the cell to receive new signals depends on reversibility of the changes produced by prior signals.
b) Each molecular change must last only a short time, resulting in the cell’s readiness to respond to a fresh signal.

Front 26

Apoptosis

Back 26

Type of programmed cell death which is brought about by activation of enzymes that break down many chemical components in the cell.
1. DNA and other cellular components are fragmented.
2. Cell shrinks and becomes lobe-shaped.
3. Cell’s parts are packaged in vesicles and engulfed and digested by scavenger cells.

Front 27

Apoptotic Pathways and Signals

Back 27

1. Mitochondrial proteins are triggered to form molecular pores in the mitochondrial membrane causing it to leak and release other proteins that promote apoptosis.
2. When a death-signaling molecule, which was presumably released by a neighboring cell, occupies a cell surface receptor, it activates caspases and other enzymes that carry out apoptosis (no mitochondria).

Front 28

Caspases

Back 28

Family of proteins that are one of the main executors of the apoptotic process; they form active enzymes following the induction of apoptosis.
-Signals may originate from the nucleus, generated when the DNA has suffered irreparable damage.
-Signals may also originate from the endoplasmic reticulum when excessive protein misfolding occurs.
-In vertebrates, apoptosis is essential in:.
a) Normal development of the nervous system.
b) Normal operation of the immune system.
c) Morphogenesis of hands and feet.
-In humans, apoptosis may be responsible for:
a) Parkinson’s and Alzheimer’s disease.
b) Cancer due to the failure of apoptosis to occur.