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cytoplasmic membrane phospholipids
R groups are fatty acids linked to glycerol via ester bond
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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
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What stabilizes the membrane and makes it less fluid?
- rigid planar molecules
- cholesterol in eukaryotes
- hopanoids in Bacteria
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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