Exam 3, Class 1: Cell Signaling

Peptide Messengers

• chains of amino acids

• water soluble/hydrophilic

• travel freely dissolved in blood plasma

• cannot cross cell membrane and bind to receptors on cell surface

Steroid Messengers

• derived from cholesterol (four ring structure)

• lipid soluble/hydrophobic

• must be bound to transport proteins (carrier proteins) to be carried through bloodstream

• easily diffuse across the cell membrane to bind to intracellular receptors in the cytoplasm or nucleus

Amine Messengers

• Modified from single amino acids

• Variable solubility: some are water-soluble (catecholamines like epinephrine), while others are lipid-soluble (thyroid hormones)

• Travel in blood variable; water-soluble ones travel freely, while lipid-soluble ones require transport proteins

• Entry into cells variable based on solubility: water-soluble amines bind to cell-surface receptors, while lipid-soluble amines enter the cell to bind to intracellular receptors

Receptor specificity

A chemical messenger can only influence a cell if that cell possesses the specific receptor to which the messenger can bind. This is a "lock and key" relationship

Response depends on the target cell

The same chemical messenger can cause different effects in different cell types. For example, epinephrine binding to receptors in one tissue might cause smooth muscle contraction, while binding to a different type of receptor in another tissue might cause it to relax

Receptor location

Depends on the messenger's solubility

Water-soluble messengers

Bind to receptors on the outer surface of the cell membrane, activating intracellular signaling pathways

Lipid-soluble messengers

pass through the membrane to bind to receptors inside the cell (in the cytoplasm or nucleus), often affecting gene expression

Small signal, big response

A very low concentration of a chemical messenger can still produce a large response through a process called signal amplification.

The cascade effect

This happens because the binding of one messenger molecule to a single receptor can trigger a cascade of intracellular events.

Second messengers

A common example involves G protein-coupled receptors (GPCRs), where one receptor activation can activate multiple G proteins, which in turn can activate many enzymes. These enzymes produce many "second messenger" molecules (like cAMP), and each of these can activate even more downstream proteins.

Result

The signal is magnified at each step, so a tiny initial signal becomes a massive intracellular response

Up-regulation

An increase in the number of receptors makes the cell more sensitive to the messenger.

Down-regulation

A decrease in receptor numbers makes the cell less sensitive.

Change in receptor binding affinity

Mutations can alter the receptor's structure, affecting how tightly it binds to the messenger. A higher affinity means the cell can respond to lower messenger concentrations.

Loss of receptor function

A mutation could prevent the receptor from activating the intracellular pathway, making the cell unable to respond.

Gain of receptor function

A mutation could cause a receptor to be permanently "on" even without a messenger, leading to a constant, inappropriate cellular response.

Blocking the receptor

Other molecules (like drugs or autoantibodies) can bind to and block a receptor, preventing the natural messenger from activating it.