Cell Surface Receptors and Genetic Material Study Guide chapter 8 - intro to 9

Categories of Cell Surface Receptors

  • Cell surface receptors can be classified into three main categories:

    • Enzyme-linked receptors

    • G protein-coupled receptors

    • Ligand-gated ion channels

Enzyme-Linked Receptors

Structure and Function

  • Domains: Receptor proteins generally possess two crucial functional domains:

    • Signal Binding Site: Located extracellularly, responsible for binding the signal or ligand.

    • Catalytic Domain: Located intracellularly, responsible for catalyzing chemical reactions.

  • Transmembrane Proteins: Enzyme-linked receptors are transmembrane, spanning the entire membrane.

Mechanism of Action

  • Upon ligand binding, a conformational change occurs in the receptor, activating the catalytic domain.

  • The activated receptor often functions as a kinase, which catalyzes phosphorylation reactions, transferring phosphate groups to specific target proteins.

    • Kinases: Enzymes that facilitate phosphorylation, requiring ATP as a source of phosphate.

Phosphorylation Process

  • Activation Steps:

    • Initially, the receptor is inactive (no signal bound).

    • Binding of the signal activates the receptor and its kinase domain.

    • Phosphate from ATP is transferred to the target protein.

  • Phosphorylation changes the function of target proteins, often leading to signal transduction:

    • This can activate or deactivate specific proteins, modifying cellular activity accordingly.

G Protein-Coupled Receptors (GPCRs)

Overview

  • GPCRs are another category of cell surface receptors involved in signal transduction for hydrophilic and large molecules that cannot penetrate the membrane.

Structure

  • G Proteins: Composed of three subunits:

    • Alpha: Binds to GDP/GTP.

    • Beta: Often remains associated with gamma.

    • Gamma: Remains associated with beta, forming a trimeric complex.

Activation Mechanism

  • The GPCR interacts with the G protein when the ligand binds, causing:

    • Structural changes in the GPCR and G protein.

    • The alpha subunit dissociates upon activation, aided by the exchange of GDP for GTP.

Functional Dynamics

  • Activation of GPCRs leads to changes in the cellular activities:

    • Signal Processing: The GTP-bound alpha subunit activates downstream signaling pathways.

    • Regulation: Once activated, the G protein undergoes hydrolysis of GTP back to GDP, returning the system to its inactive state.

Ligand-Gated Ion Channels

Structure and Function

  • Defined as channels that can open (permit ion passage) or close in response to binding with a ligand.

Gating Mechanism

  • States: They can be in two states: open or closed.

  • Ligand Gating: The binding of a ligand causes a conformational change that opens the channel, allowing ions to move down their concentration gradient (facilitated diffusion).

Importance

  • Critical for communication in the nervous system and muscle contraction; neurotransmitters often bind to these channels to transmit signals between cells.

Intracellular Receptors

Overview

  • Intracellular receptors can be found in the cytosol or nucleus, responding to small and hydrophobic chemical signals (like steroid hormones).

Example: Estrogen Receptor

  • When estrogen (a small hydrophobic molecule) enters a cell:

    • It diffuses across the phospholipid bilayer.

    • Once inside, it binds to its receptor, triggering a conformational change and activation of the receptor.

  • Gene Expression Changes: The activated receptor interacts with specific DNA sequences, leading to changes in gene expression and protein synthesis, thereby modifying cellular activity.

Summary of Receptor Types

Type of Receptor

Location

Function

Enzyme-linked receptors

Extracellular

Signal binding and catalysis

G protein-coupled receptors

Extracellular

Signal transduction via G proteins

Ligand-gated ion channels

Extracellular

Ion transport in response to ligand binding

Intracellular receptors

Inside the cell

Gene expression regulation

Genetic Material and DNA Structure

Historical Context

  • The understanding of DNA as genetic material only became established in the 1950s.

  • Early ideas of heredity focused on understanding how traits passed from parents to offspring.

Griffith's Experiments

  • Streptococcus pneumoniae: Griffith used two strains of bacteria (smooth and rough), with smooth being virulent due to a capsule.

  • Experiment showed that non-virulent bacteria could transform into virulent forms when exposed to heat-killed virulent bacteria, leading to the hypothesis of a genetic material transfer.

Transformation Example

  • Living rough strain (non-virulent) mixed with heat-killed smooth strain (virulent) -> mouse dies, live smooth strain isolated.

Avery's Experiments

  • Avery and colleagues identified DNA as the transforming principle by purifying macromolecules and showing transformation only occurred with DNA.

    • Enzymes were used to eliminate RNA and proteins to confirm DNA was responsible for the transformation.

Structure of DNA

  • Components: Comprised of nucleotides, each consisting of a phosphate group, deoxyribose sugar, and nitrogenous bases (A, T, C, and G).

  • Bonding: Phosphodiester bonds link nucleotides together to form a polynucleotide strand.

  • Directionality: Strands run anti-parallel; one end labeled 5’ (phosphate) and the other 3’ (hydroxyl).

Base Pairing and Helical Structure

  • A pairs with T (via two hydrogen bonds), and C pairs with G (via three hydrogen bonds).

  • DNA consists of a double helix structure with a sugar-phosphate backbone and nitrogenous bases oriented toward the interior.

Conclusion

  • The understanding of cell surface receptors and genetic material has laid a vital foundation in molecular biology, influencing studies on gene expression, signaling pathways, and heredity.