OE

Week 2 ELM 4: Water, Ions, and Membranes

Structure of Water

  • Water: the molecular key to life.

Polarity

  • Basic meaning: ends/sides are different.

  • Epithelial cell: structural polarity.

  • Magnet: magnetic polarity.

  • Membrane: electrical polarity.

  • Diagram of water molecule showing partial positive charges (δ+) on hydrogen atoms and partial negative charge (δ-) on the oxygen atom.

  • Representation of hydrogen bonds between water molecules due to attraction between partial positive and negative charges.

Water is Unique

  • Water exists in liquid and solid (ice) forms.

Ions

  • An ION is any atom or molecule that has gained or lost one or more electrons.
  • Ions are, by definition, CHARGED.
  • Sodium atom Na 2, 8, 1 becomes Sodium ion Na [2, 8]+

Why are ions important?

  • Carry signals in the body, such as action potentials.
  • Act as an energy store, such as in secondary active transport.
  • Interact biochemically with proteins and other molecules.
    • Example: Ca^{2+}/troponin C in muscle contraction.
    • Example: Mg^{2+}/ATP.

Biologically Important Ions

  • Ions which are physiologically useful:

    • Na^+
    • K^+
    • Cl^-
    • Ca^{2+}

  • Ions which are biochemically useful:

    • Mg^{2+}
    • Trace metals e.g. Fe^{3+}, Zn^{2+}

  • Both physiologically and biochemically Useful

  • Ions in aqueous solution are surrounded by water molecules.

  • Depiction of Na^+ surrounded by water molecules, illustrating the concept of ions in aqueous solution.

  • Depiction of Cl^- surrounded by water molecules

Ionic Size

  • Ionic "size" = ionic radius?
  • Hydration Shell
    • Li^+
    • Na^+
    • K^+
    • Rb^+
  • Hydration shell affects mobility in solution
  • Hydration shell is the effective “size” of ion
  • Hydration shell affects interactions with proteins

Membranes and Ion Gradients

Membranes

  • Hydrophilic polar head
  • Hydrophobic tail
  • All biological membranes are lipid bilayers
  • Amphipathic nature drives formation of bilayers

Membrane Permeability

  • Bilayer sheet permeability to different molecules (cm s-1):
    • Na^+: 10^{-12}
    • K^+: 10^{-11}
    • Cl^-: 10^{-10}
    • Glucose: 10^{-9}
    • Ethanol: 10^{-8}
    • Water: 10^{-3} - 10^{-2}
  • Membranes are essentially impermeable to ions

Membranes and Concentration Gradients

  • Ions are at different concentrations inside and outside of cells and organelles, maintained by membranes.
  • Example of calcium ion (
    Ca++
    ) concentration differences.

Pumps and Transporters

Membrane Proteins

  • Allow cells to establish ion gradients and use them
  • Components: Glycolipid, Phospholipid, Globular protein, Hydrophobic segment of alpha-helix protein, Alpha-helix protein, Oligosaccharide side chain, Cholesterol

Pumps

  • Concentration of ions against gradient needs energy.
  • Cells get this energy from hydrolysis of ATP.
  • Cells use special proteins termed pumps.
  • Pumps perform PRIMARY ACTIVE TRANSPORT.

Basic Features of Pumps

  • Live in membranes.
  • Move ions “Uphill”.
  • Couple to ATP (usually).
  • Fairly slow.
  • Nearly always move cations.
  • e.g. Calcium pump

Pumps: Primary Active Transport

  • Sodium-potassium ATPase (sodium pump)
    • 3Na^+ out, 2K^+ in.
    • Generates a Na^+ and K^+ gradient
      • Na^+ low in cytoplasm
      • K^+ high in cytoplasm
    • Electrogenic (2+ in, 3+ out).
    • Extremely important pump – cells expend 25% of ATP keeping it going (more in neurons).

Ion Gradients as "Batteries"

  • Gradients represent a source of energy.
  • Can be used to transmit information.
    • e.g. signaling via ion channels.
  • Can be used to power cellular processes.
    • e.g. transport of other ions via cotransporter (secondary active transport)
      • Antiporter/exchanger
      • Symporter

Cotransporters: Secondary Active Transport

  • Sodium-calcium exchanger: an antiporter
  • Carriers can be very effective: Na^+-Ca^{2+} exchanger extrudes 2000 Ca^{2+}/sec, while Ca^{2+} pump extrudes 30 Ca^{2+}/sec.

Ion Channels: Basic Properties

Movement Across Membranes

  • Carrier/Transporter Protein
    • Maximum rate ~ 10000/s
    • Active or passive
    • Example: Na^+/K^+ ATPase
  • Ion channel
    • Maximum rate ~ 1000000/s
    • Passive transport…

Basic Properties of Ion Channels

  • Transmembrane proteins.
  • Selectively permeable.
  • Opening controlled somehow.
  • Diverse..

Selective Permeability

  • Cations: K^+, Na^+, Ca^{2+}
  • Anions: Cl^-
  • Selectivity filter with rings of charge determines which ions can pass.

Gating

  • Mechanical
  • Second messenger (inhibitory/activating)
  • Phosphorylation
  • Leak
  • Ligand-gated
  • Voltage-gated
  • Proton-gated
  • G protein-gated
  • Temperature-gated..
  • Most channels closed, most of the time.

Ion Channel Structure

  • Diagram showing side view of an ion channel with key structural elements:
    • Extracellular funnel
    • Selectivity filter
    • Central cavity
    • Activation gate

Naming of Ion Channels

  • Ion channels characterised/classified by:
    • Gating
    • Ion selectivity
    • Eg. Voltage-gated potassium channel
  • But, for ligand-gated channels, named after natural ligand
    • Eg. GABAA receptor…

Ionic Gradients of a “Typical” Mammalian Cell

  • Inside vs. Outside concentrations:
    • Na^+: 15 mM inside, 150 mM outside (DEPOLARISATION)
    • K^+: 100 mM inside, 5 mM outside (HYPERPOLARISATION)
    • Cl^-: 13 mM inside, 150 mM outside (HYPERPOLARISATION)
    • Ca^{2+}: 0.002 mM (free) inside, 2 mM outside (DIVERSE..)
  • Change in membrane potential when channel opens.

Ligand Gated Ion Channels: Structure and Function

Ligand-gated Ion Channels

  • nicotinic AChR
  • GABAA
  • 5HT3 receptor
  • inhibitory glycine receptor
  • ionotropic glutamate receptors…
  • Cys loop receptors

All Ligand Gated Channels Have:

  • Pore - Lets ions through
  • Ligand binding site -Tells channel to open in response to ligand binding
  • Coupling mechanism - Couples channel opening to ligand binding
  • Desensitization mechanisms - Close channel if ligand binds for too long
  • Ligand-gated ion channels Open in response to binding of an activating ligand (agonist) e.g. acetylcholine

Complex Structure of Ligand-Gated Channels

  • e.g. Nicotinic acetylcholine receptor
    • Pentamer of five similar subunits
    • SIDE VIEW
    • TOP VIEW
    • Gate
    • Pore

Characterisation of the nAChR

  • Energy release triggered by “molecular switch” (nicotinic acetylcholine receptor
  • Muscle stores energy in electrical gradients (Sodium pump)
  • Electric organ is similar to muscle

Structure of Voltage-Gated Ion Channels

Voltage-Gated Ion Channels

  • Closed state.
  • Open state enabling permeant ion flow.

Types of Voltage-Gated Ion Channels

  • Calcium channels (Ca_V)
  • Sodium channels (Na_V)
  • Potassium channels (K_V)

Complex Structures of Voltage-Gated Channels

  • e.g. “Simple” potassium channel
    • Tetramer of four equivalent subunits

Voltage-Gated Potassium Channels

  • Human genome has 40 K_V channel genes
  • Subdivided into 12 families
  • K_V channels appear early in evolution (present in prokaryotes)

Voltage-Gated Channels - Superfamily

  • 143 genes in superfamily
  • Includes K2P, CNG, HCN, Cav, Nav, Kv, Kca, Kir, TRP

Evolution of CaV and NaV Channels

  • KV gene -> KV gene -> KV gene Two pore channels (TPC)
  • KV gene -> KV gene -> KV gene -> KV gene -> NaV and CaV

Structure of CaV and NaV Channels

  • "Pseudosubunit"
  • α subunit
  • Extracellular
  • Intracellular

Subunits of CaV and NaV Channels

  • Calcium channel subunits
    • α Ca_V 1.1-1.4
    • α Ca_V 2.1-2.3
    • α Ca_V 3.1-3.3
    • 4β
    • 4α2δ
    • 8γ subunits
    • Native channel possibly 1α:1β:1α2δ ?1γ
  • Sodium channel subunits
    • α Na_V 1.1-1.9
    • 4β subunits
    • Native channel possibly 1α:1/2β α2δ β α β α γ

Formation of α2δ Subunits

  1. Gene produces a single polypeptide.
  2. Disulphide bond formed in extracellular domain.
  3. Extracellular domain cleaved to yield two linked peptides α2δ subunits. SH SH SS SS