BIO464 - Module 10 AI Notes

Facilitated Diffusion and Membrane Transport

Overview of Membrane Permeability

  • Phospholipid bilayers are selectively permeable to small and relatively hydrophobic molecules.

    • Examples of permeable molecules:

    • Gases (e.g., O₂, CO₂)

    • Hydrocarbons

    • Alcohols (e.g., ethanol, methanol)

    • Acetone

  • Barriers to diffusion for:

    • Glucose and other hexoses

    • Carbohydrates

    • Amino acids and proteins

    • ATP and other nucleotides (NTPs)

    • DNA and RNA

    • Ions (e.g., Na⁺, K⁺, Cl⁻, Ca²⁺)

Partition Coefficient (K)

  • The ability of a molecule to diffuse across the lipid bilayer correlates to its partition coefficient (K).

    • Definition: K = oil/water solubility.

    • Example scenario: Placing a substance in a beaker with oil and water; the partition coefficient indicates the fraction of the substance that ends up in oil.

Membrane Transport Proteins

Classes of Membrane Transport Proteins

  1. Channels:

    • Pores in the membrane that can be gated (opened and closed) and are specific to certain substances.

  2. Transporters:

    • Bind to molecules and ions, change shape, and release them on the opposite side of the membrane.

  3. ATPases:

    • Utilize ATP to transport substances against their concentration gradients.

Specific Transport Mechanisms

Transporters: GLUT1

  • GLUT1 Transporter:

    • A transmembrane protein located in the plasma membrane of human cells.

    • Functions: Allows the uptake of glucose from the bloodstream.

    • Classification: Uniporter (transports one type of molecule at a time, i.e., glucose).

    • Characteristics:

      • Two stable conformations (flips between them; one side opens to inside and other side opens to the outside of the cell).

      • Contains a specific binding site for glucose in the middle

      • glucose can enter and leave the site from either side

    • Mechanism:

      • Glucose concentration is typically higher in blood than in cells, facilitating its transport into the cell down the concentration gradient.

      • Hexokinase: Converts glucose to glucose-6-phosphate (keeps intracellular glucose low and prevents it from re-entering the transporter).

      • Does not require ATP; changes in shape are driven by the concentration gradient.

    • Comparison to enzymes: Transporters can speed up transport without altering the physical properties (partition coefficient) or the distance molecules travel but reduce the activation energy required for transport through the lipid bilayer.

    • Importance: Transport via GLUT1 is significantly faster than pure phospholipid bilayer transport.

Satuaration and Kinetics

  • Transport rate is saturable; there is a maximum transport capacity (Vmax) since it is mediated by a finite number of proteins.

  • Transporters exhibit different

    • Km values:

      • Low Km indicates excellent binding to substrate (e.g., GLUT1 has a low Km for glucose, allowing effective transport).

      • High Km for glucose-6-phosphate, which prevents easy escape from the cell.

      • Example: GLUT1 efficiently transports D-glucose but poorly transports mannose and galactose.

Experimental Evidence for Transport Proteins

  • Using erythrocyte membranes and detergent:

    • Proteins were solubilized, mixed with phospholipids, and formed liposomes capable of glucose transport after dialysis of detergent.

Other Transport Types: Symporters and Antiporters

  • Symporters & Antiporters

    • use the energy of a molecule moving down its concentration gradient to drive a different kind of molecule against its concentration gradient

      • also called secondary active transport

    • ex: 2-Na+/1-glucose symporter: used to move low concentrations of glucose from the lumen of the intestines into the cell

      • couples the movement of glucose to the sodium gradient

      • there are binding sites for sodium and glucose in the transporter

      • binding of all three sites induces conformational change, opening the transporter to the inside of the cell

      • transporter resets after all three molecules pop off; relative concentration gradients ensure that more glucose is moved into the cell than is moved out

      • comprised of 14 membrane spanning helices

        • 5 that interact with glucose can act as a glucose channel (pore), if expressed separately

        • other 9 link this activity to the power of the Na+ gradient, ensuring glucose moves through the channel in only one direction

      • Example: once glucose (candy bar) moves from lumen of the intestine into the cell, glucose uniporter (GLUT2) sitting on the bloodstream side of the cell allows glucose into the blood. (note the Na/K pump sits on bloodstream side and keeps the Na+ concentration low in the cell)

Mitochondrial Transport Mechanism

  • The mitochondrial inner membrane has cotransporters driven by a proton gradient, generated by electron transport chains.

    • Various molecules (e.g., pyruvate, phosphate, ATP, ADP) are shuttled across the membrane.

    • The proton gradient provides the driving force due to charge differences, e.g., exchanging ATP (4-) with ADP (3-) in a positively charged environment is energetically favorable.

Active Transport and ATPases

Types of ATPases

  1. P-class pumps:

    • Move ions and are often the simplest type, such as the Ca²⁺ ATPase.

  2. ABC transporters:

    • Complex, force substances across membranes through various mechanisms, such as flippases.

  3. V-class and F-class ATPases:

    • Differ in energy usage; V-class pump H⁺ ions, while F-class uses H⁺ gradients to synthesize ATP.

The Role of Ion Gradients

  • Ion gradients maintained by ATPases contribute significantly to cellular functions:

    • Transporting molecules

    • Synthesizing ATP

    • Facilitating cell signaling and movement.

  • Concentration differences for significant ions inside mammalian cells:

    • High inside: Potassium (K⁺), Proteins (-)

    • High outside: Sodium (Na⁺), Chloride (Cl⁻), Calcium (Ca²⁺)

Ca²⁺ ATPase Mechanism

Steps in Ca²⁺ ATPase Cycle

  1. Binding: High-affinity Ca²⁺ binding sites face the cytosol.

  2. ATP Activation: ATP binds to the ATPase, leading to phosphorylation of a specific Asp residue.

  3. Conformational Change: Alters the binding sites, facilitating transport.

  4. Release: After Ca²⁺ transport, binding sites lose affinity for Ca²⁺.

  5. Dephosphorylation: Removal of phosphate from Asp signals the ATPase to revert to its original conformation.

Mechanistic Insights

  • Important note: The binding site for ATP and phosphorylation site shifts during Ca²⁺ transport, affecting membrane-spanning helices and facilitating ion movement.