BIOCHEM 2.5 - CELL SIGNALING INTRODUCTION: SIGNALING BASICS AND RECEPTORS

• Exposure to carbon monoxide (CO) reduces hemoglobin's ability to hold oxygen (O2).
• CO exposure also impairs the ability of hemoglobin to reach full saturation.
• The right kind of oxygen is not effectively utilized when exposed to CO.
• CO acts as a competitive inhibitor of oxygen (O2).
• Oxygen's positive cooperativity is inhibited by CO.
• The body cannot achieve its maximum O2 saturation of hemoglobin when exposed to CO.
• Mitochondrial function is affected by CO, especially in skeletal muscle, cardiac muscle, and vascular tissue, leading to neurological signaling changes.
• CO reduces the ability of mitochondria to function properly in the outside environment, impairing O2 uptake.
• CO exposure results in inflammation and injury.
• CO exposure leads to problems in cardiomyocytes, blood vessels, and neurons.
• CO exposure can lead to increased Reactive Oxygen Species (ROS).
• ROS leads to lipid peroxidation.
• Lipid peroxidation affects lipids in cell membranes.
• This results in damage at all levels.
• CO exposure can make mitochondria leaky and less functional.

Cell Signaling Introduction

• Cell signaling involves a biochemical activity and how alterations in structure, binding, or regulation affect function.
• Cell signaling can be thought of as a conversation, where cells send, receive, and respond to signals.
• Post-translational modifications of proteins are critical to the function and activity of signaling pathways.
• Each step in a pathway can serve to regulate the pathway.
• Enzymes are a key way to control the rate and how a reaction progresses.
• Amplification can only occur during the propagation stage of a signal.
• Much of signal propagation occurs through regulating, modifying, or controlling enzymes.

Signal Receiving and Responding

• Receiving a signal is distinct from responding to it.
• Responding to a signal can extend all the way down to gene expression.
• Failure to propagate or respond to a signal produces the same consequence as if the signal was never received or sent.
• Every major pathway must receive, propagate, and respond to a signal in a regulated manner.
• Signaling pathways consist of initiation, propagation, and termination/reset.

Signaling Basics

• A conversation in cell signaling requires two parts: the signal must be sent, and the signal must be received.
• There are structural, physical, and physiological results when there is a failure to either generate/send or receive/respond to a signal.

Major Components of All Pathways

• The major components of all pathways are:
  • Signal reception from the environment or internal production.
  • Signal propagation through mediators that trigger a stepwise reaction, possibly involving amplification.
  • Generation of a cellular response by activating cellular activities and transcription factors to turn on genes.

Types of Signaling

• There are four types of signaling: autocrine, juxtacrine, paracrine, and endocrine
• Autocrine: A cell targets itself.
• Juxtacrine: Requires direct contact with the target cell.
• Paracrine: Localized signaling to neighboring cells.
• Endocrine: Signal must travel to responsive cells; hormones are produced away from the site of action.
• Cancer cells sometimes do weird things during autocrine signaling.

Normal Specification of Signals

• Signal specificity is conferred by the receptor through differential expression, recognition of certain ligands, and location inside or outside of a cell.
• Location of the receptor is key in signal specification.
• Vesicular trafficking, rough endoplasmic reticulum (ER), and Golgi components are tied to the membrane in which the signal takes place
• Termination is dictated by regulation.
• Specificity is built into receptors, and location is key.
• Receptors for permeable materials are intracellular (steroid hormones).

Normal Signaling

• Without a ligand, no activity will take place.
• The receptor activates enzymes or channels, and activates a signaling casacade to be started.

Receptor Tyrosine Kinases

• Receptor tyrosine kinases autophosphorylate and phosphorylate others and thereore are directly involved enzymatic activity.

Ligands and Ligand-Binding

• Ligands are molecules form complexes with other molecules.
• There are 3 basic protein-binding ligand models:
  • Lock and Key: Involves rigid binding interfaces, or a perfect match.
  • This is an entropy-dominated process.
  • Induced Fit: Assumes flexibility in the binding site and is an enthalpy-dominated process.
  • Conformational selection: Takes into account the inherent energy of protein conformations.
    • It sequentially allows structural changes to occur so enxymes have their capacity of function.
• The majority of biochemical reactions involve conformational selection, which is more common.
• Yet all 3 are generally expressed in all processes.

Types of Ligands

• A wide range of small molecules are considered ligands.
• Various ligands include heme prosthetic groups in cytochrome c553.
• Ligands can be other proteins, hydrophobic molecules like steroids (cortisol), or water-soluble hormones.
• Water-soluble ligands remain outside the cell.

HER Kinase Receptors

• Human epidermal growth factor receptor’s (HER) family depends on the activity and ligand associated with it

Lipid versus water-soluble hormones

• Lipid-soluble factors:
  • Require a transport vehicle through the blood
  • Mainly use intracellular receptors because they can easily pass through the hydrophobic core of membranes
• Water-soluble:
  • Travel freely in the blood
  • Must be recognizes and generally initiate second messenger casacades on target hormones.

Cell-Surface Receptors

• Cell-surface receptors can induce other molecules, act as ion channels, function as enzymes (like insulin), or act as signaling molecules (like EGFR).
• Outside signals will change the inside of the cell, which:
  • Includes required binding
  • Includes hanges in gene expression
  • Must include intracellular changes.

Receptor classes and ligaments

• G-Protein-Coupled Receptors:
  • Multipass proteins
  • Use G-Proteins and ligands for Glucagon, a-Adrenergic, β-adrenergic (epinephrine), Muscarinic (acetylcholine), rhodopsin (vision), Chemokines (IL-8)
• Ion-Channel Receptors:
  • Multipass proteins
  • Use Ion Channel Activity and ligands for neurotransmitters, Ions, Nucleotides, Inositol trisphosphate (IP 3 )
• Intrinsic Tyrosine Kinase Receptors:
  • Single pass transmembrane proteins
  • Use Tyrosine Kinase Activity and ligands for Insulin, and Peptide Factors like PDGF, FGF, NGF and EGF
• Intrinsic Guanylate Cyclase Receptors:
  • Single Pass Transmembrane Domain
  • Use guanylyl cyclase and ligands for a atrial natriuretic receptor
• Death-Domain Receptors:
  • Use Death domain,
  • None, but some need contain ITAM/TIM

G-Protein Coupled Receptors (GPCRs)

• GPCRs are the largest (+1000) and most diverse form of membrane receptors.
• This indicates a wide range of ligands that target various signaling pathways by using the neurotransmitters and hormones involved.
• There are generally two types of signal transduction pathways involving GPCR: -cAMP
  • Phosphatidylinositol (IP3)
• GPCRs are named for GTP/GDP binding sites.
• GPCRs couple to g-proteins which toggle between GTP and GDP.
• The receptors initiate responses based on activation of downstream effectors by Gα subunits.
• When receptors are activated GDP is exchanged for GTP.
• GProteins toggle between GTP and GDP.
• Signal transduction occurs through either cAMP or IP3.
• Hydrolysis of GTP is a relatively slower process than ATP hydrolysis.
• The alpha subunit is active as long as GTP is bound, which will also activates adenylate cyclase.

Steps for GPCR Regulation

• List of Steps:
  1. Inactive
  2. Ligand Binding
  3. Activation and dissociation
  4. From shift
  5. Leads
  6. Activation
  • GPCRs are transmembrane proteins, each step is critical in expression.

Tyrosine Kinase Receptors

• Critical growth receptors like growth factor receptors generally cause dimerization. -Ligand binding induces dimerization, and autophosphoralation. Binding to the intracellular domain occurs next.

Insulin Signalling

• Insulin is a type of Tryosine Kinase, which will turn on genes by phosphorylating them.
• This phosphorylation is accomplished via serine, threonine, to hold phosphates. Then a transfer of phosphates from the donstream will cause change.
• This will cause the action of downstream actions via signal cascades and translocation.

Extracellular Matrix (ECM)

• ECM has multiple means of signaling between cells and acts as a way for cells to build and respond to the matrix.
• It is a critical component of morphogenesis for cells and provides a way for repairs by different methods such as; Via Cell Receptors, via Proteolysis or Via Mechanotransduction

Cellular Responses

• Receipt of single by a few receptors will lead into amplification, where the cell responses to different signals and receptors which increases the functionality from few number of receptors. This includes, receipt, signals/bunch.
• Integrins, ECM influences cells. These integrins will enable the ECM to influence cellular behavior (downstream) as well as mediation of cells.
• Plays a vital role in wound, growth, and proliferation.