Oct 15th - Small molecule Transport & Electrical Properties of Membranes

movement of an atom/molecule across membranes is governed by its concentration and properties

• the hydrophobic layer of membranes is a barrier to the movement of some molecules

• larger and more polar molecules have more difficult time crossing than smaller & non-polar molecules

  • for practical purposes, ions can’t cross

• an imbalance in the concentration of a solute on either side of the membrane is a driving force for the movement of the solute

transport proteins grant selectivity to lipid bilayers

channels and transporters allow charged and polar solutes across membranes

• proteins that facilitate movement of solutes across membranes belong to two broad classes

• transporters move fixed amounts of solutes at a time via conformational changes; they are slower than channels

  • some transporter participate in facilitated diffusion & others use energy to move solutes against their gradient (active transport)

• channels create open paths through which solutes can rapidly flow

  • some channels can have open and closed states that are regulated

    • eg ligand, voltage and mechanically-gated channels

  • channels can only allow solutes to move according to their gradient (passive transport / facilitated diffusion)

• both channels & transporters are selective with respect to which solute they move

multiple modes of transmembrane movement

• simple diffusion and passive transport both involve movement of a solute down its concentration gradient

• active transport involves solute movement against its concentration gradient

transporters are defined by number & direction of solutes

• uniport — transport single molecules across the membrane

• symporters — transfer two solutes, same direction

• antiporters — transfer two solutes, opposite directions

passive transporters alternate between different conformations independent of solute binding

• passive transporters continuously alternate between different conformations that position the solute binding site on either side of the membrane

• this conformation is independent of solute binding so a solute moved in one direction can be moved in the opposite direction just as easily

active transporter use energy to move solutes against their electrochemical gradient

• movement of a solute against its electrochemical gradient is energetically unfavorable, so additional energy must be applied

  • coupled transporters use the movement of one solute with its gradient to power movement of another against its gradient

  • ATP-driven pumps use the energy of ATP hydrolysis to power solute-power moving conformational changes

  • light-driven pumps use photon-powered conformational changes to move solutes

  • Redox driven pumps use the energy of electron transfers to move solutes

ion movement is governed by both gradient and electrical charge

• unequal distribution of ions results in a charge difference (voltage) across the plasma membrane

  • opposite charges on either side of the membrane collect at the membrane; the cytosol and extracellular fluid are electrically neutral — the voltage exists only at the membrane

• this membrane potential influences the movement of ions

  • ions have both a chemical and an electrical gradient

  • no membrane potential only the concentration of ions matters

  • with a typical membrane potential, inward movement of cations is assisted by the voltage; inward movement of anions is resisted by the voltage

Na+ glucose symporter facilitate glucose uptake by cells of the intestines

• passive glucose transporters would not allow glucose to accumulate in the absorptive epithelial cells

• use Na+ electrochemical gradient to transport glucose

• to maintain a high conc of glucose in the cells, glucose uptake is couple to inflow of sodium, which follows its conc gradient

  • the transporters won’t face the cytosol until both sodium and glucose are bound

• the Na+ gradient is maintained by an ATP-powered sodium pump

  • secondary active transport involves using ATP to establish a gradient that is used to move another solute against its gradient

different membrane domain-restricted glucose transporters in the gut

• the Na+ glucose symporter brings glucose from the intestinal lumen into the absorptive cell

• tight junctions limit transporter diffusion and keep them on the apical side of the cell

• glucose is also low in the extracellular fluid so passive glucose transporters on the basolateral surface are sufficient to move glucose out of the cells

• Na is also moved out the cell by a Na+ K+ pump, which prevents Na accumulation within the cell

• an almost identical mechanism moves amino acids from the gut to the extracellular fluid

ATP-driven pumps take one of three forms

• P-type pump: phosphorylated during pumping cycle | moves ions | pumps via conformational oscillations

• ABC transporter: ATP-Binding Cassette | move small molecules | pumps via conformational oscillations

• V-type proton pump: discovered in plant vacuoles | hydrolyzes ATP to pump protons | pumps via rotating domain | cannot run backwards (too leaky)

• F-type ATP synthase: restricted to mitochondrion, chloroplast, and aerobic prokaryotic membranes | harness proton gradients to synthesize ATP | can run backwards

SERCA: the sarcoplasmic reticulum Ca2+ transporter (P-type pump)

• SR is a Ca2+ rich modified ER that is found in muscle cells

• muscle contraction is governed by rapid increases in cytosolic calcium conc due to opening of Ca2+ channels on the SR membrane

• contraction is terminated by movement of Ca2+ back into the SR by ATP-powered pumps

• SERCA is a family

  • different family members can be found in different cell types

• conformational cycling of SERCA

  • SERCA moves two protons into the cytosol in exchange for moving 2 Ca2+ ions into the SR/ER lumen

  • 1. In the ATP-bound, unphosphorylated state, SERCA is open to the cytosol, where it can bind calcium.

    2. Calcium binding promotes ATP hydrolysis and phosphorylation of a regulatory aspartic acid that converts SERCA to an occluded state.

    3. ADP is exchanged for ATP, but the protein

    remains phosphorylated.

    4. The ATP-bound phosphorylated SERCA

    opens to the SR/ER lumen where calcium is

    released and protons bound.

    5. Bound protons shift SERCA conformation

    and promotes removal of the phosphate.

    6. Phosphate release opens SERCA to the

    cytosol.

Na+ K+ pumps use ATP to maintain gradients of both ions

• P-pump

• Na+ K+ pumps hydrolyze ATP to move both Na+ and K+ against their electrochemical gradients

• cells with these pumps maintain reciprocal gradients of Na (higher outside) and K (higher inside)

ABC transporters

• ABC transporters are the most numerous and diverse

• bind ATP but aren’t phosphorylated

• importers & exporters vary based on whether solute binding precedes or follows ATP binding

• most eukaryotes have only exporters; prokaryotes have both

• alternate between openings regulated by ATP binding

  • ATP bound — faces cytosol

  • ATP unbound — face periplasmic space

gram-negative bacteria have a double membrane with a periplasmic space

• the space between (periplasmic space) contains a porous network of peptidoglycan

• gram-positive bacteria have a single membrane but a thicker peptidoglycan layer that stains dark purple with crystal violet

• gram-negative bacteria use auxiliary transport in conjunction with ABC transporters

  • Movement of materials into gram-negative bacteria requires cooperation between non-selective porins on the outer membrane, ABC transporters on the inner membrane, and a substrate binding protein that moves between them

  • The spacing of peptidoglycan in the periplasmic space is wide enough to allow free diffusion

ABC transporters can contribute to drug resistance by pumping toxins out of the target cell

• multi-drug resistance (MDR) protein is upregulated in many cancer cells and contributes to resistance to chemotherapy