Cell Biology Quiz on Enzymes and Ion Channels

Enzyme Catalysis

• Enzymes are highly specific protein catalysts
• Enzymes increase the rate of chemical reactions
• Enzymes have an active site where substrates fit
• This binding induces a fit
• Substrates are converted into products (e.g., sucrose to glucose and fructose)
• Products are released and enzyme returns to original shape

Kinases

• Kinases are proteins that regulate other proteins through phosphorylation
• Phosphorylation adds a phosphate group to a protein, changing its activity
• Kinases use ATP (energy)
• Kinase adds phosphate to target protein

cAMP Activates Protein Kinase A (PKA)

• cAMP (cyclic AMP) binds to regulatory subunits of PKA
• This binding releases catalytic subunits
• The released catalytic subunits then carry out their function, regulating other cellular processes

Acetylcholinesterase

• Acetylcholinesterase is an enzyme responsible for "turning off" the neurotransmitter signal
• It breaks down acetylcholine, ending the signal
• Acetylcholine is broken down into acetate and choline

Acetylcholinesterase Cleaves Acetylcholine

• Acetylcholinesterase cleaves acetylcholine into acetic acid and choline
• This cleavage stops the signal caused by acetylcholine

Ion Channels

• Ion channels are membrane proteins that allow ions to pass through the membrane
• Voltage-gated channels: sodium, potassium, and calcium channels
• Ligand-gated channels: nicotinic acetylcholine receptor, TRPV1, and others
• Ligand-gated (intracellular ligand) channels: CNG channel, cAMP, TRPM5, intracellular calcium

Voltage-gated Potassium Channels

• Usually large multimeric integral membrane proteins
• Complex activation and regulation
• Multiple conductance states
• Filter selectivity can be regulated Very rapid responses (~milliseconds)

Distribution of Ions Inside and Outside a Neuron

• Ions are concentrated differently inside and outside the neuron -High sodium (Na+) on outside, potassium (K+) on inside
  • Ions have a charge

Changes in Conformation Cause Changes in Ion Channel Conductance

• Changes in a protein's shape affect how easily ions can pass through it
• Different stages of opening and closing have different conductances

Ion Channel Agonists

• Ion channel agonists stabilize the open state and increase its frequency, enhancing signaling
• Patch clamp measure conductance of ion channel

Voltage-gated Ion Channels Change Conformation

• Voltage-gated channels change shape (conformation) in response to changes in membrane potential
• This shape change affects whether the channel is open or closed

The Nicotinic Acetylcholine Receptor

• A ligand-gated cation channel
• Acetylcholine binding opens the channel
• Ions (Na+ and K+) flow through

Nicotinic Receptors

• Nicotinic receptors are expressed on the postsynaptic side of the neuromuscular junction
• The receptor is a target for acetylcholine

Transporters

• Transporters are integral membrane proteins that move neurotransmitters and hormones across barriers
• DAT is dopamine transporter (example)

Role of Transporters in Drug Response

• Large membrane proteins that transport substances in and out of cells
• Affect drug A-D-M-E (absorption, distribution, metabolism, and excretion)
• P-gp (P-glycoprotein) is an efflux transporter

Active Transport

• Primary active transport: requires ATP to move molecules against their concentration gradient
• Secondary active transport: couples movement of one molecule against gradient to movement of another with gradient

Facilitated Diffusion

• Passive movement driven by concentration gradient
• Proteins provide a pathway for molecules
• Saturable (limited capacity)

Major Transporter Families

• ABC (ATP binding cassette): primary active, vectorial (one direction)
• SLC (solute carrier): facilitated or secondary active transport (bidirectional)

P-gp Mechanism of Action

• Substrate enters internal drug-binding pocket through open portal
• ATP binding causes a large conformational change, exposing the drug to the extracellular side (exocytosis)

SLC Transporters

• Operate by facilitated diffusion and secondary active transport
• Uniporter, symport, and antiport are three types
  • Uniporters move one substrate down concentration gradient
  • Symporters move substrates in the same direction
  • Antiporters move substrates in opposite directions

Drug Targets

• Some drugs, such as antacids, bulk laxatives, and chelating agents, do not need specific targets to be active
• They work through different mechanisms, like neutralizing acid or physically removing substances
• Volatile gases, used as anesthetics, also act without specific targets