Midterm #3: Membrane Transport

Question 1

3 ways in which molecules can cross membrane barriers:

Answer 1

1. Diffusion: small, non-polar molecules can spontaneoulsy pass through the bilayer
2. Passive Transport: larger or more polar molecules can pass through channels down a **concentration gradient. ** May be tightly regulated
3. **Active transport: **solute can also be pumped by transporters against a concentration gradient, in an energy-dependent process

Question 2

Diffusion Across Membranes

Answer 2

• Very small nonpolar substance such as O_2, N₂, CO₂ and NO can diffuse through a lipid bilayer
• The diffusion rate (“flux”) depends on:
  
  • the **solubility **of the molecule in lipid
  • the concentration gradient across the membrane
• flow continues until an equilibrium is achieved
  
  • Larger and polar molecules cannot easily or quickly diffuse across a bilayer and their transport across biological membranes must be mediated by two types of proteins: channels and transporters

Question 3

Channels and Passive Transport

Answer 3

• Hole in membrane where it can diffuse down concentration gradient
  
  • facilitated diffusion ​
• No energy (passive transport)
• Cannot establish concentration gradients
• Can be selective
  
  • provides appropriate size and environment
• Display substrate specificity with defined V_max and K_m parameters

Question 4

Two types of channels or pores: Ungated and Gated

Answer 4

• Ungated:
  
  • facilitated diffusion of substrates
  • selective/nonselective
  • Ex: Beta-barrel porins, aquaporins
• Gated or Regulated Channels
  
  • switch between open and closed states​
  • highly selective (usually small charged ions)
  • Regulated by: membrane potential (voltage), ligand binding, mechanical stress or phosphorylation
  • Ex: Voltage Gated Na+ Channel, Voltage Dependent Ca+ Channel, GABA_A Receptor, Nicotinic-Acetylcholine Receptor

Question 5

Two types of Gating for Channels

Answer 5

• Peptide Plug
• Rotational Gating

Question 6

Porins

Answer 6

• AKA: Beta Barrel Channels
• Outer membrane of gram-negative bacteria and of mitochondria
• Form aqeuous channels through membrane
• Trimers
  
  • Each subunit forming a 16- or 18-strand membrane spanning beta barrel
  • Center is lined with charged side chains and is filled with water, forming a passageway for the movement of small molecules
  • Depending on the nature of the side chains, the porin may be selective for ions, amino acids, or sugar

Question 7

Aquaporins

Answer 7

• alpha-type channels
  
  • several subunits with multiple transmembrane alpha-helices
• Very sensitive, allowing only the passage of water
• Narrow channel lined with charged residues and two specifically placed Asn residues at the center of the pore
  
  • Important role in reabsorption of water in the kidneys, and in water transport in the roots of plants

Question 8

Voltage-Gated Sodium and Potassium Channels

Answer 8

• Diffusion of ions across membranes depends on concentration gradient and membrane potential
  
  • called the electrochemical gradient
• In the resting state, neurons maintain a negative membrane potential with excess extracellular Na+ and intracellular K+
• Signals are transmitted along nerve cell axons in the form of action potentials mediated by Na/K voltage gated channels.
  
  • Inital stimulus causes Na channels to open, Na flows in and depolarizes cell membrane
  • K channels open and K flows out to restore membrane potential

Question 9

Name this structure

Answer 9

• Tetrodotoxin: found in certain pufferfish, inhibits voltage-gated Na+ channels and causes paralysis

Question 10

Voltage-Dependent Calcium Channels (VDCC)

Answer 10

• found in muscle cells and neurons
• closed at resting membrane potential but activated by depolarization of the membrane
• Ca+ entry into the cell, cauing contraction/excitiation
• Open in milliseconds, but stay open longer giving a more sustained action potential

Question 11

Calcium Channel Blockers

Answer 11

• Ex: amlodipine
• Treat HTN
• Inhibit VDCC in heart muscle, decreasing cardiac contractility
  
  • Decreases cardiac output
• Inhibit VDCC in arterial smooth muscle causing increase in arterial diameter (vasodialation)
  
  • Decreases total peripheral resistance

Question 12

Name This Structure

Answer 12

Amlodipine

Question 13

Ligand-gated Channels

Answer 13

• GABA_A Receptor is a Cl- channel found in CNS
• Activation allow Cl into cell, causing hyperpolarization
  
  • neuronal inhibition
• **Benzodiazepines **and barbituates enhance GABA_AR activity.
• Nicotinic-Acetylcholine Receptor (nAChR) is a gated channel at neuromuscular junction
  
  • ACh binding to receptor opens the channel, enabling Na+ to enter and K+ to exit
  • Local depolarization at motor end plate, initiating mucle contraction
• Acetylcholinesterase rapidly degrades ACh in the synaptic cleft

Question 14

Name this structure

Answer 14

GABA: gamma-aminobutyric acid

Question 15

Name this structure

Answer 15

acetylcholine

Question 16

nAChR inhibitors

Answer 16

• Non-depolarizing neuromusclar junction agents
  
  • bind to nAChR and competitvely inihibit ACh binding
  • Curare alkaloids like d-tubocurarine
  • Muscle relaxant like **atracurium **
• Depolarizing neuromuscular junction agents
  
  • bind to nAChR and open the channel resulting in membrane depolarization
  • Mimic ACh, first causing contraction and then paralysis
  • Succinylcholine is used via IV. Adjunct to general anesthesia to facilitate tracheal intubation and to provide skeletal muscle relaxation during surgery and mechanical ventilation

Question 17

Name this structure

Answer 17

d-tubocurarine

Question 18

Name this structure

Answer 18

atracurium

Question 19

Name this structure

Answer 19

Succinylcholine

Question 20

Uniport, Symport, Antiport

Answer 20

• Uniport: moves a single substance at a time
• Symport: transports two different substances in the same direction across membrane
• Antiport: move two different substances in opposite directions across the membrane

Question 21

Transporters may also operative passively (“facilitated transport”) or actively, through two modes:

Answer 21

1. Primary active transport: involves the consumption of ATP
2. Secondary active transport: is coupled to an exisiting electrochemical gradient

Question 22

GLUT1

Answer 22

• passive uniporter
• transports glucose down a concentration gradient into RBC and many other tissues

Question 23

Primary Active Transport: The Na/K-ATPase

Answer 23

• Maintains the concentration of sodium (extracellular→intracellular) and potassium (intra→extra)
• Each rxn cycle hydrolyzes 1 ATP, pumps 3 Na out and 2 K in
• Excitable tissue like nerve and muscle have high Na/K ATPase activity to maintain membrane potential
• Separate Ca ATPase transporter maintains the Ca gradient for VDCC activity
• In addition, the concentration gradients can power other co-transport proteins (secondary active transporters)
• Main consumer of ATP in body at rest

Question 24

Primary Active Transport: P-Glycoprotein (P-gp)

Answer 24

• P-gp (aka: MDR1 or ABCB1) pumps a broad array of xenobiotic compounds out of cytoplasm in an ATP-dependent process.
• Key player in drug transport in gut, liver, kidney and blood-brain barrier as well as in cancer cell multidrug resistance
• Many drugs also inhibit P-gp activity, a potential source for drug-drug interaction
• P-gp inhibitors such as quinidine can substantially increase exposure to the P-gp substrate digoxin

Question 25

Secondary Active Transport​

Answer 25

• ​Takes advantage of gradient already established by ATPase pump
• The Na-glucose transporter found in renal epithelial cells of the kidneys is a symport
  
  • Glucose import is powered by the sodium gradient set up by the Na/K-ATPase
• The Na/Ca-exchanger (NCX) in cardiac cells is an antiport also powered by sodium gradient.
  
  • exchanges 3 Na per Ca exported
  • Much faster than Ca-ATPase but has a lower affinity
    
    • cannot maintain as steep a concentration gradient

Question 26

Cardiac Glycosides

Answer 26

• natural products that increase the intensity of heart muscle contraction
• treat congestive heart failure
• ​Digoxin
  
  • Narrow therapeutic index: 0.8-2 ng/mL, toxic is greater than 2.4 ng/mL
• Steroids that inhibit Na/K-ATPase decreasing the Na gradient
  
  • Inhibits NCX, attenuates the rapid removal of Ca from the cell
• Increase in intracellular Ca increases the contractility of the cardiac muscle and the intensity of cardiac contraction
  *

Question 27

Name this structure

Answer 27

Digoxin