Chapter 8: Cell Communication

8.1 General Features of Cell Communication

• All living organisms require cell communication to survive.

• Cell communication involves:

  • Responding to incoming signals.

  • Producing outgoing signals.

• Signals are often chemical molecules but can also be physical parameters like light or temperature.

Why is Cell Communication Necessary?

• To respond to a changing environment, which is critical for survival.

• To communicate with other cells, important for multicellular organisms.

Cell-to-Cell Communication

• Occurs between adjacent cells and cells that are long distances apart.

• Five common ways signals are relayed between cells:

  • Direct intercellular signaling: Cell junctions allow signals to pass directly from one cell to another.

  • Contact-dependent signaling: Molecules bound to the surface of cells serve as signals to other cells encountering them.

  • Autocrine signaling: Cells secrete signaling molecules that bind to their own surface receptors.

  • Paracrine signaling: Cells release signals that affect nearby cells.

  • Endocrine signaling: Signals (hormones) are released into the blood and affect target cells far away.

Three-Stage Process of Cell Signaling

• Receptor activation

• Signal transduction

• Cellular response

• A group of proteins often forms a signal transduction pathway.

8.2 Receptor Activation

Ligand Binding and Conformational Changes

• A ligand (signaling molecule) binds noncovalently to its receptor with high specificity.

• Ligand binding changes the receptor structure, transmitting the signal across the membrane.

• Once the ligand is released, the receptor reverts to the inactive conformation.

Receptor Affinity for Ligands

• The complex formed between the ligand and receptor is stable for a finite time and will dissociate.

• kon [Ligand] + [Receptor] \rightleftharpoons [Ligand-Receptor complex] koff

  • [ ] refers to concentration.

  • kon is the rate at which binding occurs.

  • koff is the rate at which the ligand-receptor complex dissociates.

• Binding and release between a ligand and receptor are generally rapid, reaching equilibrium.

Dissociation Constant (Kd)

• Kd, the dissociation constant, quantifies the affinity receptors have for their ligands.

• At equilibrium, Kd = koff / kon

• Kd reflects the ligand concentration where half of the receptors have bound ligand.

• The Kd value is inversely related to the affinity between the ligand and its receptor.

• When [ligand] > Kd, most receptors are likely to have ligand bound.

• When [ligand] < Kd, most receptors will not be bound to their ligand.

8.3 Cell Surface Receptors

• Most signaling molecules are small and hydrophilic or too large to pass through the plasma membrane.

• Cell surface receptors embedded in the plasma membrane detect these extracellular signals.

• A cell may have hundreds of different cell surface receptors.

• Most receptors are classified as:

  • Enzyme-linked receptors

  • G-protein-coupled receptors

  • Ligand-gated ion channels

Enzyme-Linked Receptors

• Typically have two important domains: an extracellular signal-binding domain and an intracellular catalytic domain.

• Most function as protein kinases, transferring a phosphate group from ATP to a specific amino acid in target proteins.

• Phosphorylation alters the structure and function of a protein.

G-Protein-Coupled Receptors (GPCRs)

• Interact with intracellular proteins called G proteins.

• G proteins can bind GTP and GDP.

• Receptor activation causes the α subunit and β/γ dimer of the G protein to dissociate, then the subunits interact with other proteins.

Ligand-Gated Ion Channels

• When a signaling molecule binds, the channel opens, allowing ions to flow across the membrane.

• Important in communication between neurons and from neurons to muscle cells in animals.

8.4 Intracellular Receptors

• Found in the cytosol or nucleus.

• Bind signal molecules that are small and hydrophobic, which can diffuse across the plasma membrane.

• Activation typically leads to a change in gene expression.

• Steroid hormones like estrogen and testosterone utilize intracellular receptors.

Summary

• Cells detect and respond to signals for survival and communication.

• Cell-to-cell communication occurs in various forms depending on distance and mechanism.

• Cell signaling involves receptor activation, signal transduction, and cellular response.

• Receptors bind ligands with measurable affinity (Kd).

• Cell surface receptors include enzyme-linked receptors, GPCRs, and ligand-gated ion channels.

• Intracellular receptors bind hydrophobic signals, affecting gene expression.