9.Transport Membrane Systems

R groups are fatty acids linked to glycerol via ester bond

• R groups are linked to glycerol via ether linkages
• R groups are not fatty acids, but repeats of isoprene

• rigid planar molecules
• cholesterol in eukaryotes
• hopanoids in Bacteria

• prokaryotic contain more different proteins than eukaryotic
• 10-20% of total cell protein resides in the cytoplasmic membrane
• proteins are able to move laterally

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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• protein secretion across the cytoplasmic membrane
• SecY, SecE and SecG form a protein translocating channel in the membrane
• two paths lead to this channel

• 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

• 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