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)
- 3 $Na^+$ out, 2 $K^+$ 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 $Ca<em>V$ and $Na</em>V$ Channels

KV gene -> KV gene -> KV gene Two pore channels (TPC)
KV gene -> KV gene -> KV gene -> KV gene -> $Na<em>V$ and $Ca</em>V$

Structure of $Ca<em>V$ and $Na</em>V$ Channels

"Pseudosubunit"
α subunit
Extracellular
Intracellular

Subunits of $Ca<em>V$ and $Na</em>V$ 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

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