Transporters: Uptake and Efflux

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Transporters Uptake and Efflux

• Main function of cytoplasmic membrane is is as a
  selective permeability barrier that regulates the
  passage of substances into and out of the cell.
  The bacterial membrane allows passage of water
  and uncharged molecules up to M.W. of about 100
  Da, but does not allow passage of larger
  molecules or any charged substances except by
  means special membrane transport processes and
  transport systems

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Transport Processes

• Transport systems operate by one of three
  transport processes
• Uniporter a solute passes through the membrane
  unidirectionally
• Symporter two solutes must be transported in the
  same direction at the same time
• Antiporter one solute is transported in one
  direction simultaneously as a second solute is
  transported in the opposite direction

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Types of Transport Systems for Nutrient Uptake

• Four types of carrier-mediated transport systems
• The carrier is a protein (or group of proteins)
  that functions in the passage of a small molecule
  from one side of a membrane to the other side. A
  transport system may be a single transmembranous
  protein or it may be a coordinated system of
  proteins
• Transport systems have the property of
  specificity for the solute transported. Some
  transport systems transport a single solute with
  the same specificity and kinetics as an enzyme.
  Some transport systems will transport
  (structurally) related molecules, although at
  reduced efficiency compared to their primary
  substrate.
• Most transport systems transport specific sugars,
  amino acids, anions or cations that are of
  nutritional value to the bacterium.

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Types of Transport Systems

• Facilitated diffusion systems (FD) the least
  common type of transport system in bacteria. The
  glycerol uniporter in E. coli is the only well
  known facilitated diffusion system. FD involves
  the passage of a specific solute through a
  carrier that forms a channel in the membrane. The
  solute can move in either direction through the
  membrane to the point of of equilibrium on both
  sides of the membrane. Although the system is
  carrier-mediated and specific, no energy is
  expended in the transport process. For this
  reason the glycerol molecule cannot be
  accumulated against the concentration gradient

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Types of Transport Systems

• Ion driven transport systems (IDT) used for
  accumulation of many ions and amino acids IDT is
  a symport or antiport process that uses a
  hydrogen ion (H) i.e., proton motive force
  (pmf), to drive the transport process. Example
  the lactose permease of E. coli. The lactose
  permease is a single transmembranous polypeptide
  that spans the membrane seven times forming a
  channel that specifically admits lactose.

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Types of Transport Systems

• Binding-protein dependent transport systems
  (BPDT) frequently used for uptake of sugars and
  amino acids. Examples iron siderophores
  transporters of E.coli. The transport systems are
  composed of four proteins. Two proteins form a
  membrane channel that allows passage of the
  nutrients. A third protein resides in the
  periplasmic space where it is able to bind
  substrates and pass it into the membrane channel.
  Driving the solute through the channel involves
  the expenditure of energy, which is provided by
  the hydrolysis of ATP by a fourth protein, which
  contains a well-conserved ATP-binding cassettes
  (hence ABC transporter).

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BtuE
BtuC
BtuD
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Types of Transport Systems

• Group translocation systems (GT)
• more commonly known as the phosphotransferase
  system (PTS) in E. coli, are used primarily for
  the transport of sugars.
• Glucose specifically enters the channel from the
  outside, but in order to exit into the cytoplasm,
  it must first be phosphorylated by the
  phosphotransferase system. The PTS derives energy
  from the metabolic intermediate phosphoenol
  pyruvate (PEP). PEP is hydrolyzed to pyruvate and
  glucose is phosphorylated to form
  glucose-phosphate during the process. Thus, by
  the expenditure of a single molecule of high
  energy phosphate, glucose is transported and
  changed to glucose-phosphate

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Facilitated diffusion is a carrier-mediated
system that does not require energy and does not
concentrate solutes against a gradient. Active
transport systems such as Ion-driven transport
and Binding protein-dependent transport, use
energy and concentrate molecules against a
concentration gradient. Group translocation
systems, such as the phosphotransferase (pts)
system in Escherichia coli, use energy during
transport and modify the solute during its
passage across the membrane
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Distinguishing characteristics of bacterial
transport systems
PD passive diffusion FD facilitated
diffusion IDT ion-driven transport BPDT
binding protein dependent transport GT group
translocation
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Efflux Proteins (Multidrug Efflux Pumps)

• A group of proteins found in the cytoplasmic
  membrane of bacteria that mediates the excretion
  of toxic compounds including antibiotics,
  detergents, and dyes
• Common in many bacterial species, plays an
  increasing important role in antibiotic
  resistance for clinically important pathogens
• Required energy, mostly proton motive force
  (PMF), rarely ATP

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Efflux systems of Gram ve Bacteria

• Consists of a single efflux protein in the
  cytoplasmic membrane
• The efflux proteins consist of 12 to14 ? helices
  that traverses the membrane
• Deals with a narrow range of structurally
  related substrates
• Example QacA of S. aureus, Bmr of B. subtilis

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Gram-positive Bacteria
Efflux transporter
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Efflux systems of Gram-ve Bacteria

• Consists of three components a cytoplasmic
  membrane efflux protein, a periplasmic protein,
  and an outer membrane channel, pumps the drugs
  directly into the external media, bypassing the
  periplasm
• Mediates the efflux of a wide range of
  structurally unrelated compounds
• Example AcrA-AcrB-TolC of E. coli,
  MexA-MexB-OprM of P. aeruginosa

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Gram-negative Bacteria
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AcrA-AcrB-TolC Efflux System
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Structure of AcrB
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Structure of AcrB
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Structure of TolC
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Structure of MexA ( a AcrA homolog)
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