Signaling Pathways and Hormones

Two Component System

A simple system where a receptor receives a signal and leads to a change in the output domain.
Involves a receptor and a post-regulated protein.
Example: E. coli has 35 different combinations of these systems, each sensing different nutrients or environmental factors.

Signal Transmission and Phosphorylation

Signal transmission in a two-component system often involves phosphorylation.
Ligand binding to the receptor triggers a switch.
The ability to switch off the signal is crucial; otherwise, the signal would be constant.

Speed of Response

Fast responses (seconds to minutes) involve changes in protein function, such as phosphorylation status of an enzyme.
Example: Glycogen metabolism and fat metabolism regulation via protein kinases.
Enzymes and pathways are typically not regulated by transcription and translation due to the energy cost and slow process.
Slower responses (minutes to hours) can involve changes in transcription, such as hormonal signals.

Reception of Signals

Signals are received by receptors, often integral membrane proteins.
Intracellular hormone receptors bind to signals inside the cytoplasm and then translocate to the nucleus.
Signaling pathways involve conversion or translation of the signal into a cellular response.
Effector molecules then carry out specific actions.
Multiple signals may be required for certain processes like survival, growth, and development.

Cell-Cell Communication

Molecules on the cell membrane, such as glycoproteins (proteins with carbohydrate moieties), can be recognized by neighboring cells.
This allows for direct, short-distance signaling between cells.

Paracrine Signaling

Paracrine signaling involves a cell sending a signal that affects only the local area.

Hormones

Hormones are chemical signals that affect pathways in the body.
Hormones are needed in low concentrations due to amplification processes in signaling cascades.

Animal Hormone Synthesis

Not all parts of the body synthesize all hormones.

Plant vs Animal Hormones

Plant hormones are very different from animal hormones.
Every plant cell can synthesize every hormone and can grow a whole another plant
Hormones are transported through the plant, triggering a big effect in the target cell or tissue

Second Messengers

Second messengers are chemical or non-protein molecules that relay signals received at the cell surface to target molecules in the cytoplasm or nucleus.
Examples include calcium ions ( $Ca^{2+}$ ).
Second messengers help distinguish between signals coming from outside the cell (first messenger, e.g., hormones) and triggers inside the cell.