9.Transport Membrane Systems

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9.Transport Membrane Systems
2011-10-05 03:18:50
PMB 112 midterm1

general microbiology midterm 1
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  1. cytoplasmic membrane phospholipids
    R groups are fatty acids linked to glycerol via ester bond
  2. How are phospholipids of Archaea different?
    • R groups are linked to glycerol via ether linkages
    • R groups are not fatty acids, but repeats of isoprene
  3. What stabilizes the membrane and makes it less fluid?
    • rigid planar molecules
    • cholesterol in eukaryotes
    • hopanoids in Bacteria
  4. prokaryotic cytoplasmic membrane proteins
    • prokaryotic contain more different proteins than eukaryotic
    • 10-20% of total cell protein resides in the cytoplasmic membrane
    • proteins are able to move laterally
  5. functions of the cytoplasmic membrane
    permeability barrier - permeable to water, CO2, NH3, medium-chain fatty acids, small hydrophobic molecules

    protein anchor - sites of processes like lipid, PG biosynthesis

    energy conservation - electron transport chains are in the membrane
  6. cytoplasmic membrane proton gradient
    • enzymes of electron transport chain reside in the membrane and create a proton gradient with H+ higher outside
    • proton gradient can be used to power:
    • ATP synthesis
    • transport reactions
    • flagellar rotation
  7. Hallmarks of ALL protein-mediated transport
    • transport rate is saturable - finite number of transport sites exist per cell
    • mutants that do not transport individual substrates can be isolated - inactivation of individual proteins can block transport
  8. active transport systems
    • use the energy of an ion gradient or ATP
    • allow the accumulation of solutes within the cell to concentrations 100-1000x higher than in the surrounding fluid
  9. facilitated diffusion
    • glycerol enters or exit through the GlpF transporter
    • if [glycerol] is higher outside the cell, net entry will occur until inside and outside concentrations are equal
    • usually cells use glycerol as soon as it enters to keep internal concentration low
    • energy independent
  10. simple transport
    • use of energy proton motive force or another existing ion gradient to drive uptake or expulsion of another solute
    • symport = two solutes go the same direction
    • antiport = two solutes go opposite direction
    • lactose - symporter
    • sodium-proton - antiporter
    • phosphate - symporter
    • sulfate - symporter
  11. group transport
    • substrate is modified in an energy-requiring reaction to prevent reverse transport
    • glucose - phosphorylated sugars can't travel back through channel, energy comes from bond in PEP
  12. ATP-binding cassette (ABC) transporters
    • energy of ATP powers transport directly
    • consists of periplasmic binding protein, membrane transporter, and ATP-hydrolyzing proteins
    • some function in reverse to expel antibiotics
    • in gram+ bacteria, the binding protein is a lipoprotein anchored in the cytoplasmic membrane
    • binding proteins determine specificity
    • one channel can interact with family of binding proteins to take up to several substrates
  13. TonB-dependent transporters
    • OM transporters that interact with IM proteins via the TonB box
    • proton motive force across IM provide energy for uptake of substrate across OM via TonB complex
    • after reaching periplasm, ATP transporter brings the substrate across the IM into the cytoplasm
  14. SecYEG
    • protein secretion across the cytoplasmic membrane
    • SecY, SecE and SecG form a protein translocating channel in the membrane
    • two paths lead to this channel
  15. SecB pathway
    • SecB chaperone binds the signal sequence of a fully synthesized protein and keeps it in an unfolded state for export powered by the SecA ATPase
    • used for OM proteins, lipoproteins, periplasmic proteins
  16. SecYEG pathway
    • signal recognition particle (SRP) binds to some signal sequences as they emerge from the ribosome
    • translation pauses and the ribosome/nascent chain/SRP complex binds to the membrane via FtsY, the SRP receptor
    • nascent chain is transferred to SecYEG and continued translation pushes the new protein across the membrane
    • used for integral IM proteins - too hydrophobic to be handled by SecB
    • SRP recycled by GTP hydrolysis