Drug Targets 2: Ion channels, enzymes and transporters

Question 1

What is an ion channel ?

Answer 1

1. Ions (e.g. Na+, K+, Cl-) unable to penetrate the lipid bilayer of the cell membrane
   - polar and hydrophilic(water-loving) whereas the interior of the cell membrane is hydrophobic(water-hating)
2. Ions can only get across with the help of specific proteins which span the cell membrane
   - ion channels or transporters
   - plasma and intracellular membranes eg ER

Question 2

What makes the ions move ?

Answer 2

The electrochemical gradient

Question 3

Explain Ion channels and recap and some of its complexities ?

Answer 3

1. Ligand or voltage gated are most common channel types
   - Some are gated/modulated by both eg. Ca2+ activated K channels
2. Channel activation tends to be short lasting
   - Even when stimulus remains – remember inactivation
3. Ionic selectivity can be high or low
   - Single ion - Na+, K+, Ca+, Cl- , H+
   - Non-selective cation channels
   - Different ions can travel in opposite directions eg. Na in, K out simultaneously
4. Ion flow and direction dependent upon electrochemical gradient

Question 4

At rest cells are commonly dominated by ?

Answer 4

by K so resting potential is negative – ions can change membrane potential and also ion concentration

Question 5

Example of excitable cells ?

Answer 5

Excitable cells – cardiac/skeletal muscle, nerves

Question 6

What is Ca and what does it mobilise ?

Answer 6

Ca is a second messenger – remember Ca mobilising GPCRs

Question 7

Briefly explain Ligand gated ion-channels ?

Answer 7

• Activated by binding of a chemical ligand to a receptor site on the channel molecule
  usually extracellular on the outside of the cell membrane e.g. “fast neurotransmitters” ACh, GABA, glutamate
• Fast responding -

Question 8

Explain the structure of Ligand gated ion-channels: the targets of neurotransmitters ?

Answer 8

Multi-subunit channels

• form a central ‘pore’ fluid filled pore for ions to flow through
• Homo- and hetero-multimers depending upon subtype eg. GABA

Question 9

Explain the GABA receptor ?

Answer 9

• GABA = g-aminobutyric acid
  predominant inhibitory neurotransmitter in the brain
• GABA receptors are ligand gated Cl- channels
• Chloride ion influx hyperpolarises nerve
  1. membrane potential becomes more negative
  2. makes depolarisation more difficult
  3. stabilises nerves by preventing reducing action potentials

Question 10

Now explain Benzodiazepine (agonist) ?

Answer 10

Benzodiazepines are positive allosteric modulators of GABAA channels

1. Do NOT bind to the GABA ligand binding site.
2. Enhance the opening of the channel in the presence of GABA
3. Used as sedatives, anxiolytics, anti-convulsants & muscle relaxants
4. Diazepam (Valium), midazolam (Versed)

Question 11

Explain the structure of voltage-gated cation channels?

Answer 11

Single subunit channels

• Large polypeptide, 4 domains and each domain has 6 membrane spanning regions. The 4 domains assemble around a central conduction pore.
• Multiple family members eg. NaV1 - 9

Question 12

Explain Voltage-gated Na+ channel blockers(Anti-epileptics) ?

Answer 12

• Blockers commonly used as anti-epileptics
  1. Phenytoin(Dilantin)
  2. Carbamazepine
  3. Valproate
• Acts to suppress the abnormal brain activity seen in seizure and reduce spread

Question 13

Explain Voltage-gated Na+ channel blockers(Toxins) - what is Tetrodotoxin (TTX) ?

Answer 13

• This is produced by a marine bacterium and accumulated in the Puffer fish
  1. potent neurotoxin with no antidote
• Known as Zombie powder
  1. sublethal doses can leave a person in a state of near-death for several days, while the person continues to be conscious.
  2. Linked to voodooism and zombieism

Question 14

When do symptoms develop ?

Answer 14

• Symptoms develop within 30 minutes,
  1. paresthesias of the lips and tongue; sweating, headache, weakness, incoordination, tremor, paralysis, gastroenteric symptoms, seizures, bronchospasm, respiratory failure, coma, and hypotension, cardiac collapse and death.
• Puffer fish are eaten raw as a delicacy in Japan !

Question 15

Explain Biological Catalysts ?

Answer 15

• Increase the rate of chemical reactions within the body
  1. reacting with substrates forming an ES complex which breaks down to form enzyme plus product
• Enzymes are generally specific for a particular substrate, or a closely related family of substrates

Question 16

Describe competitive inhibition?

Answer 16

• Do NOT compete with substrate for active site; commonly bind at a distinct site
• Can NOT be overcome by increasing substrate concentration

Question 17

Describe non-competitive inhibition?

Answer 17

• Commonly compete with substrate for active site

- not permanent – can be overcome by increasing substrate concentration

Question 18

What is the purpose of Constitutive Cyclo-oxygenases 1 (COX-1) ?

Answer 18

Physiological PG2s that regulate and protect stomach, kidney and intestinal function

Question 19

What is the purpose of Inducible Cyclo-oxygenases 2 (COX-2) ?

Answer 19

PG2 produced by inflammatory cells resulting in pain, fever and swelling during inflammation

Question 20

What does Aspirin block ?

Answer 20

Aspirin (acetylsalicylic acid) blocks the synthesis of prostaglandins by COX-1 and COX-2
- fills and blocking the tunnel, preventing entry of arachidonic acid to the active site.

Question 21

What does Inhibition of COX-1 reduce ?

Answer 21

The protection of the lining of the stomach causing stomach upset, ulceration and bleeding

Question 22

Primary active vs Secondary active ?

Answer 22

1. Primary active
   - Consumes ATP
   - ‘Pumps’ against gradient
   - e.g. the Na+/K+ ATPase which sets up the gradient of these two ions across the membrane.
2. Secondary active
   - Does not use ATP
   - Energy comes from co-movement of an ion down its gradient.

Question 23

Explain the transporters in Depression ?

Answer 23

Low levels of neurotransmitters implicated in the pathogenesis of depression
- Noradrenaline & serotonin (5-HT).

Question 24

Transmission of information across a synapse:

Answer 24

Neurotransmitter, e.g. serotonin, is released from pre-synaptic terminal into synaptic cleft and binds to receptors on the post-synaptic neuron. The neurotransmitter is taken back up into pre-synaptic terminal by a transporter

Question 25

What do SSRI’s block ?

Answer 25

Selective serotonin reuptake inhibitors (SSRIs) block the transporter that takes up neurotransmitter from the synaptic cleft and thereby increases the duration of the response at the synapse.