HuBi 2001: Receptor Signaling and GPCRs

Receptor Signaling and GPCRs

• The course is HuBi 2001 – Introduction to Biochemistry, taught by Renan Danielski.
• Learning objectives include understanding receptor proteins, signal transduction pathways, general signal transducers, extracellular chemical signaling, and G-proteins and GPCRs.

Receptor Proteins

• Animal cells exchange information regarding ion and glucose concentration in extracellular fluids.
• They also use this information for various metabolic processes in different tissues and proper cell development and positioning during various stages.
• Information in cells comes in the form of signals.
• Signals are converted into cellular response via receptors, a chemical process.
• Signal transduction converts information into chemical change in cells.

Receptor Proteins (continued)

• Receptors are not always in the cell membrane; they can be in other structures or even in the cytosol.
• Receptors have specific ligand binding.
• Binding usually leads to a conformational change.
• Receptors often have effector specificity.
• Receptors can mediate various actions including cell-cell signaling, adhesion, endocytosis and so on.
• Signal transduction often amplifies signals, which occurs quickly.

Signal Transduction Basics

• A signal (ligand) interacts with the receptor.
• The activated receptor interacts with cellular machinery.
• A second signal/protein activity change is produced.
• Metabolic activity changes in the target cell.
• Transduction event ends.

8 Features of Signal Transduction Pathways

• Specificity: Signaling ligand binds to the correct receptor; others do not fit.
• Sensitivity: Receptors have high affinity for specific ligands (low Kd).
• Amplification: Enzyme activation leads to a geometric increase in the number of affected molecules.
• Modularity: Diverse signaling complexes are formed via interchangeable proteins with reversible points of interaction.
• Desensitization/Adaptation: Receptor removal/inactivation occurs due to overwhelming signals.
• Integration: Multiple signals lead to unified outcomes (e.g., concentration of a second messenger or membrane potential).
• Divergence: One signaling event may lead to multiple downstream pathways.
• Localized Response: Signal processing and response is localized to avoid diffusion to distal parts of the cell.

General Types of Signal Transducers

• G protein-coupled receptor (GPCR): External ligand binds to the receptor, triggering a GTP-bound protein that regulates an enzyme to generate secondary messengers.
• Receptor enzyme (tyrosine kinase): Ligand binding activates tyrosine kinase activity via autophosphorylation.
• Gated ion channel: Channel opens or closes in response to signal ligand or membrane potential changes.
• Nuclear receptor: Hormone binding allows the receptor to regulate the expression of specific genes.

Extracellular Chemical Signaling

• Most extracellular ligands interact with membrane receptors as they are unable to directly pass through the membrane.
• Ligands vary in size, charge, hydrophobicity, etc. (e.g., small molecules, gases like nitric oxide, soluble proteins like hormones).

Extracellular Chemical Signaling (continued)

• Small and hydrophobic ligands cross the cell membrane and bind to intracellular receptors in the nucleus or cytoplasm.
• Polar and charged ligands bind to the extracellular domains of cell-surface receptors (e.g., peptide ligands like growth factors, insulin, neurotransmitters.)

Cell-Surface Receptors

• Composed of three domains: extracellular ligand-binding domain, hydrophobic transmembrane domain, and intracellular domain.
• Examples: ligand-gated ion channels, G protein-coupled receptors, receptor tyrosine kinases.

Ligand-gated Ion Channels

• Ligand binding opens ion channels allowing ions to bypass the membrane's hydrophobic core.
• Protein structural changes dictate whether the channel stays open or closed.

Receptor Tyrosine Kinases

• Enzyme-linked receptors located on the cell surface, and their intracellular domains are associated with an enzyme.
• Receptor tyrosine kinases transfer phosphate groups to tyrosine amino acids.

GPCRs

• Large family of receptors reacting to various extracellular signals, regulating cell activities.
• Activate trimeric G proteins (members of guanine nucleotide-binding proteins) functioning as intracellular signaling pathway switches.
• Activate/inactivate ion channels or effector enzymes generating second messenger molecules.

Signal Transduction through GPCRs

• Plasma membrane receptor with 7 transmembrane helical segments.
• G protein that cycles between active (GTP-bound) and inactive (GDP-bound) states.
• Effector enzyme (or ion channels) in the plasma membrane that is regulated by the activated G protein.

G-proteins

• G-protein-coupled receptor (GPCR) structure has 7 membrane-spanning regions, a C-terminal loop and tail for binding, and three different ligand-binding families.
• G proteins bind GTP and hydrolyze it; they are anchored in the membrane.
• Monomeric G-proteins are usually involved in gene expression and have an ON-OFF conformational change.
• Trimeric G proteins (α, β, γ subunits) are critical to receptor signaling in numerous pathways and can dissociate from each other during signaling.

G-protein Activation/Signaling Mechanism (from initial hormone binding)

• Hormone binding induces a conformational change in the receptor.
• Activated receptor binds to the G protein’s α subunit.
• Activated receptor GEF (guanine nucleotide exchange factor) activity stimulates the α subunit to release GDP and bind GTP.
• The α subunit dissociates from the βγ subunits and activates downstream effectors/enzymes.
• Hydrolysis of GTP to GDP causes the α subunit to reassociate with the βγ subunits.
• Gβγ may act as a signaling molecule instead of the α subunit.