Unit 5: Transporters in the Biological system - Subjective Questions

Ion Channels are integral membrane proteins that form water-filled pores allowing selective, passive movement of ions across the membrane down their electrochemical gradient.

Transporters (Carriers) are membrane proteins that bind specific solutes and undergo conformational changes to translocate them across the membrane.

Key Differences:

Summary: Channels prioritize speed via open pores, while transporters offer versatility including active transport but operate slower.

Transporters are classified based on the direction and number of solutes moved:

1. Uniporters:

• Transport a single type of solute in one direction.
• Driven by the concentration gradient of that solute (facilitated diffusion).
• Example: GLUT1 glucose transporter in red blood cells.

2. Symporters (Co-transporters):

• Transport two or more different solutes in the same direction simultaneously.
• Coupled transport; energy from one solute's gradient drives the other.
• Example: /glucose symporter (SGLT1) in intestinal epithelium.

3. Antiporters (Exchangers):

• Transport two solutes in opposite directions across the membrane.
• Example: / exchanger, and / antiporter.

Note: Symporters and antiporters together constitute cotransporters, which perform secondary active transport, using the electrochemical gradient of one ion (usually ) to drive uphill transport of another.

Voltage-gated ion channels (VGICs) open or close in response to changes in the membrane potential.

Key Functional Properties:

• Voltage Sensing:

  • Contain a voltage-sensor domain rich in positively charged residues (arginine/lysine) in the S4 transmembrane segment.
  • Changes in membrane potential cause the S4 segment to move, triggering channel opening.

• Gating States: Channels exist in three functional states:

  1. Closed (resting): Pore closed but activatable.
  2. Open (activated): Pore open, ions flow.
  3. Inactivated: Pore blocked despite depolarization (refractory).

• Selectivity Filter:

  • A narrow region determining ion specificity (e.g., vs ).

• Activation and Inactivation Kinetics:

  • Activation is fast upon depolarization.
  • Inactivation follows, e.g., the ball-and-chain mechanism in channels.

Physiological Role: These channels underlie the action potential in excitable cells (neurons, muscle) by generating rapid, regenerative changes in membrane potential.

Structure:

• Voltage-gated channels are typically tetramers — four identical subunits arranged around a central pore.
• Each subunit has six transmembrane segments (S1–S6).
  • S1–S4: Voltage-sensing domain (S4 carries positive charges).
  • S5–S6: Pore-forming domain with the P-loop (selectivity filter).

Selectivity Filter:

• The signature sequence is TVGYG (Thr-Val-Gly-Tyr-Gly).
• The backbone carbonyl oxygen atoms point into the pore, mimicking the hydration shell of .

Basis of Selectivity:

• A dehydrated ion (radius ~1.33 Å) fits perfectly, so the carbonyl oxygens replace its water shell — energetically favorable.
• (radius ~0.95 Å) is smaller and cannot interact optimally with the carbonyl oxygens at the correct distance, so its dehydration is not compensated.
• Therefore passes ~10,000 times more readily than .

Conclusion: Selectivity arises from precise geometric and electrostatic matching in the selectivity filter, not merely pore size.

The -ATPase is a P-type ATPase that actively transports ions against their gradients using ATP hydrolysis.

Stoichiometry: For each ATP hydrolyzed:

• 3 ions are pumped out of the cell.
• 2 ions are pumped into the cell.

Mechanism (Post-Albers Cycle):

1. Pump in E1 state binds 3 from cytoplasm.
2. ATP binds and phosphorylates the pump → E1-P.
3. Conformational change to E2-P; released outside.
4. 2 bind from outside.
5. Dephosphorylation → returns to E1.
6. released into cytoplasm; cycle repeats.

Consequences:

• Maintains high intracellular and low intracellular .
• Electrogenic: net export of one positive charge per cycle → contributes to membrane potential.
• Establishes the gradient used for secondary active transport.

Inhibition: Blocked by ouabain and cardiac glycosides like digoxin.

Channelrhodopsins (ChRs) are light-gated ion channels found in green algae (e.g., Chlamydomonas reinhardtii), where they act as sensory photoreceptors mediating phototaxis.

Structure:

• Belong to the microbial (type I) rhodopsin family.
• Consist of seven transmembrane helices.
• Contain the chromophore all-trans retinal covalently bound to a lysine residue via a Schiff base.

Mechanism of Light Activation:

1. Absorption of a photon (~470 nm blue light) causes isomerization of retinal from all-trans to 13-cis.
2. This triggers conformational changes in the protein.
3. The channel opens, allowing passive flow of cations (, , , ).
4. Ion influx depolarizes the membrane.
5. Retinal re-isomerizes back to all-trans (photocycle), closing the channel.

Key Feature: Unlike animal (type II) rhodopsins that act via G-proteins, channelrhodopsins directly conduct ions, making them fast and useful in optogenetics.

Optogenetics is a technique that uses light to control the activity of genetically modified cells (particularly neurons) that express light-sensitive proteins.

Tools Used:

• Channelrhodopsin-2 (ChR2):

  • A light-gated cation channel.
  • Activated by blue light (~470 nm).
  • Causes depolarization → excites/activates neurons.

• Halorhodopsin (NpHR):

  • A light-driven inward pump.
  • Activated by yellow light (~590 nm).
  • Causes hyperpolarization → inhibits/silences neurons.

• Archaerhodopsin (Arch):

  • A light-driven outward pump.
  • Also causes hyperpolarization and neuronal silencing.

Applications:

• Precise spatial and temporal control of neural circuits.
• Mapping brain circuits and behavior.
• Studying diseases: Parkinson's, epilepsy, depression.
• Potential therapeutic applications (e.g., restoring vision).

Advantages: Millisecond precision, cell-type specificity, and reversibility make optogenetics a revolutionary neuroscience tool.

Both are microbial (type I) rhodopsins using retinal, but differ fundamentally in transport mechanism.

Bacteriorhodopsin: Pumps out to create a proton motive force for ATP synthesis.

Summary: Pumps move a fixed number of ions uphill per photon; channels allow bulk passive ion flow, making channels faster for depolarization.

Cilia and eukaryotic flagella share a common internal structure called the axoneme.

The 9+2 Arrangement:

• 9 outer doublet microtubules arranged in a ring.
• 2 central single microtubules (the central pair).
• Total: 9 doublets + 2 singlets = 9+2 pattern.

Components:

• Microtubule Doublets:

  • Each doublet has a complete A-tubule (13 protofilaments) and an incomplete B-tubule (10–11 protofilaments).

• Dynein Arms:

  • Outer and inner dynein arms project from A-tubule.
  • Motor proteins that generate sliding force using ATP.

• Nexin Links:

  • Elastic proteins connecting adjacent doublets, converting sliding into bending.

• Radial Spokes:

  • Connect outer doublets to the central pair; regulate movement.

• Central Sheath:

  • Surrounds the central pair.

Basal Body:

• Anchors the axoneme; has a 9+0 (nine triplets) structure like a centriole.

Conclusion: The precise 9+2 architecture and its accessory proteins enable coordinated bending movements.

The sliding filament (sliding microtubule) model explains how cilia and flagella bend.

Key Players:

• Dynein motor proteins anchored to the A-tubule of one doublet.
• The B-tubule of the adjacent doublet.

Mechanism:

1. ATP binds dynein, causing it to detach from the adjacent B-tubule.
2. ATP hydrolysis causes a conformational change (power stroke).
3. Dynein reattaches and walks toward the minus end (base) of the adjacent microtubule.
4. This attempts to make one doublet slide relative to its neighbor.

Sliding → Bending Conversion:

• Nexin links and basal anchoring resist free sliding.
• Because the doublets are cross-linked and constrained, the sliding force is converted into localized bending.
• Coordinated activation of dynein on opposite sides produces the characteristic beating (bending) waves.

Energy: Powered by ATP hydrolysis by dynein ATPase.

Regulation: The central pair and radial spokes coordinate which dyneins are active, ensuring rhythmic, propagated bends rather than uniform sliding.

Although cilia and eukaryotic flagella share the 9+2 axoneme, they differ in several functional aspects.

Note on Bacterial Flagella:

• Bacterial flagella are structurally different — made of flagellin, lack the 9+2 axoneme, and rotate like a propeller driven by proton motive force, not ATP.

Summary: Cilia are short and numerous with oar-like strokes; eukaryotic flagella are long, few, and undulate to propel cells.

The Nernst equation gives the equilibrium (reversal) potential at which the electrical force balances the chemical (concentration) force for a given ion.

Derivation:

At equilibrium, the electrochemical potential of the ion is equal on both sides. The electrochemical potential is:

Setting the electrochemical potential equal inside (i) and outside (o):

Rearranging:

Thus the equilibrium potential is:

where:

• = gas constant (8.314 J/mol·K)
• = absolute temperature (K)
• = ionic valence
• = Faraday constant (96485 C/mol)

At 37°C, converting to :

Significance:

• Predicts the membrane potential at which there is no net flux of that ion.
• Essential for understanding resting potential and action potentials.

The GHK equation calculates the resting membrane potential considering the contribution of multiple permeant ions simultaneously, weighted by their permeabilities.

GHK Voltage Equation:

where:

• = permeability of ion
• Note: terms are inverted due to its negative charge.

Improvement over Nernst Equation:

• The Nernst equation considers only one ion and gives the equilibrium potential for that ion alone.
• The GHK equation accounts for several ions and their relative permeabilities, giving a realistic resting potential.

Key Insights:

• The ion with the highest permeability dominates (usually at rest).
• During an action potential, when increases dramatically, shifts toward .

Conclusion: GHK provides a weighted, more physiological description of membrane potential, whereas Nernst applies only to single-ion equilibrium conditions.

The action potential (AP) is a rapid, transient reversal of membrane potential driven by voltage-gated ion channels.

Phases:

1. Resting State (~ −70 mV):

• Dominated by permeability; and channels closed.

2. Depolarization (Rising Phase):

• Stimulus depolarizes membrane past threshold (~ −55 mV).
• Voltage-gated channels open rapidly.
• rushes in → membrane potential rises toward (~ +40 mV).

3. Repolarization (Falling Phase):

• channels inactivate.
• Voltage-gated channels open (delayed).
• flows out → membrane repolarizes.

4. Hyperpolarization (Undershoot):

• channels close slowly → transient overshoot below resting potential.

5. Return to Rest:

• -ATPase restores gradients over time.

Key Properties:

• All-or-none: Occurs fully once threshold reached.
• Refractory period: Due to channel inactivation, ensures unidirectional propagation.

Conclusion: The coordinated opening and inactivation of voltage-gated channels produces the stereotyped AP waveform.

Active transport moves solutes against their electrochemical gradient, requiring energy.

1. Primary Active Transport:

• Uses direct chemical energy from ATP hydrolysis.
• The transporter itself is an ATPase.
• Examples:
  • -ATPase
  • -ATPase (SERCA)
  • -ATPase (gastric proton pump)

2. Secondary Active Transport (Cotransport):

• Uses the electrochemical gradient of one ion (established by primary transport) to drive the uphill movement of another solute.
• Indirect use of ATP energy.
• Types:
  • Symport: /glucose cotransporter (SGLT1).
  • Antiport: exchanger.

Key Difference:

Link: Secondary transport depends on primary transport to maintain the driving gradient — e.g., the gradient set up by -ATPase powers glucose uptake.

The patch-clamp technique, developed by Neher and Sakmann (Nobel Prize 1991), records ionic currents through single or multiple ion channels.

Principle:

• A fine glass micropipette (tip ~1 µm) is pressed onto the cell membrane.
• Gentle suction forms a high-resistance gigaohm seal (gigaseal).
• This isolates a tiny membrane patch, allowing measurement of picoampere-level currents.

Configurations:

• Cell-attached: Patch remains on intact cell.
• Whole-cell: Membrane ruptured; records total cell current.
• Inside-out: Cytoplasmic face exposed to bath.
• Outside-out: Extracellular face exposed to bath.

Importance:

• Measures single-channel conductance and open/closed kinetics.
• Reveals gating behavior of voltage- and ligand-gated channels.
• Determines ion selectivity and drug effects on channels.
• Confirmed that channels open in discrete, all-or-none steps.

Conclusion: Patch-clamping provided direct evidence for the existence of ion channels and remains the gold standard for electrophysiological study.

Bacterial flagella differ fundamentally from eukaryotic flagella — they are rotary motors, not bending structures.

Structure:

• Filament: Long helical structure made of the protein flagellin.
• Hook: Flexible joint connecting filament to the motor.
• Basal Body: Embedded in the membrane; includes rings (L, P, MS, C rings) acting as bearings and rotor.

Mechanism of Rotation:

1. The motor is driven by the proton motive force (PMF) — the electrochemical gradient across the membrane.
2. Stator proteins (MotA/MotB) form proton channels.
3. Protons flowing through MotA/MotB down their gradient cause conformational changes.
4. This drives rotation of the rotor (C-ring) and hence the flagellum.

Directional Control:

• Counterclockwise (CCW): Flagella bundle together → smooth swimming (run).
• Clockwise (CW): Bundle flies apart → tumbling (reorientation).

Chemotaxis: Switching between run and tumble allows movement toward attractants.

Key Difference from Eukaryotic Flagella: Bacterial flagella use PMF (not ATP) and rotate rather than bend.

The electrochemical gradient is the combined driving force acting on an ion, comprising two components:

1. Chemical (Concentration) Gradient:

• Ions tend to move from high to low concentration.

2. Electrical Gradient (Membrane Potential):

• Charged ions are influenced by the potential difference across the membrane.
• Cations move toward the negative side; anions toward the positive side.

Net Driving Force:

The net movement depends on the difference between the membrane potential () and the ion's equilibrium (Nernst) potential ():

• If : net outward movement of cations.
• If : net inward movement.
• If : no net flux (equilibrium).

Example:

• For : both concentration gradient (high outside) and negative interior favor influx.
• For at rest: concentration favors efflux, but electrical gradient opposes it — near balance.

Significance: The electrochemical gradient is the fundamental determinant of passive ion flow and stores energy used for secondary active transport.

Both are microbial type I rhodopsins found in archaea, using retinal as a chromophore, but they pump different ions.

Bacteriorhodopsin (BR):

• Found in Halobacterium salinarum.
• A light-driven outward proton () pump.
• Absorbs green light (~570 nm).
• Pumps out of the cell, generating a proton motive force used by ATP synthase to make ATP.

Halorhodopsin (HR):

• Also in halophilic archaea.
• A light-driven inward chloride () pump.
• Absorbs yellow light (~578 nm).
• Pumps into the cell, maintaining osmotic balance.

Comparison:

Common Mechanism: Light isomerizes retinal (all-trans → 13-cis), driving vectorial ion transport through a photocycle.

The ball-and-chain model (also called N-type inactivation) explains how voltage-gated channels rapidly become non-conducting even while depolarization persists.

Model Description:

• The channel has an inactivation particle (the 'ball') — a globular domain at the N-terminus of the protein (in channels) or a linker loop (in channels).
• This ball is tethered to the channel by a flexible 'chain' of amino acids.

Mechanism:

1. Upon depolarization, the channel activates (opens).
2. The tethered ball then swings into and physically blocks the open pore from the cytoplasmic side.
3. The channel enters the inactivated (blocked) state — no ion flow despite open gate.
4. On repolarization, the ball is released; the channel returns to closed/resting state.

Physiological Importance:

• Creates the refractory period in neurons.
• Ensures unidirectional propagation of the action potential.
• Limits the frequency and duration of firing.
• Prevents sustained, uncontrolled ion flux that would exhaust gradients.

Conclusion: Inactivation via the ball-and-chain mechanism is essential for the precise timing and directionality of electrical signaling.