Class 2: Electrochemical Potential-driven
transporters (Porters)
A. Porters
(uniporters, symporters, antiporters):
.
• utilize a carrier-mediated process to catalyze uniport, antiport, or
symport
Uniport
= a single species is transported either by facilitated
diffusion or in a membrane potential-dependent
process if the solute is charged
Antiport = two or more species are transported in opposite
directions in a tightly coupled process, not coupled to a
direct form of energy other than chemiosmotic energy
Symport = two or more species are transported together in the
same direction in a tightly coupled process, not
coupled to a direct form of energy other than
chemiosmotic energy
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Example:
2.A.1.5.1
2.A.1.5
2.A.1
Lactose permase / LacY (lactose:H+ symporter)
from E. coli
The Oligosaccharide:H+ Symporter (OHS) Family
Major Facilitator Superfamily (MFS)
MF Superfamily
• very large and diverse superfamily (ca. 4000 sequenced members)
• catalyze uniport, solute:cation (H+ or Na+) symport and/or solute:H+
or solute:solute antiport
• specificity for sugars, polyols, drugs, neurotransmitters, Krebs cycle
metabolites, phosphorylated glycolytic intermediates, amino acids,
peptides, osmolites, siderophores (efflux), iron-siderophores
(uptake), nucleosides, organic anions, inorganic anions, etc.
• Most have about 400-600 a.a., and contain 12, 14 or 24 α-helical
spanners
2
Structure of LacY
Structure of GlpT
2-fold symmetry, 1 substrate binding site
12 helices (417 aa)
Substrate transport presumably occurs via
an “Alternating Access Mechanism”
Glycerol-3-phosphate:P antiporter
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from E. coli (TC# 2.A.1.4.3)
B. Non-ribosomally synthesized porters:
Valinomycin from Streptomyces
fulrissimus (2.B.1.1.1)
• best characterized carrier
ionophore (uniport mechanism)
• antibiotic
• high selectivity for K+ ions (i.e.
disruption of K+ gradients)
• 105 x lower stability
constant for Na+
• very rigid cyclic depsipeptide
• used to create electrochemical
gradients in membrane vesicles
Taken from Voet and Voet (3rd Ed., page 731)
4
Monensin from Streptomyces
cinnamonensis (2.B.2.1.1)
•
•
•
•
polyketide antibiotic
high specificity for Na+ ions
catalyzes Na+/H+ antiport
results in the disruption of
individual gradients without
affecting membrane potentials
• used extensively in cattle feed
• mammalian cells are usually
not affected (exception:
horses)
Taken from Voet and Voet (3rd Ed., page 731)
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C. Ion gradient-driven energizers:
• Poorly understood group of porters
• There is only one single family (with only 3 members) in this group
•
The TonB-ExbB-ExbD/TolA-TolQ-TolR (TonB) Family of Auxiliary Proteins for Energization of
Outer Membrane Receptor (OMR)-mediated Active Transport
• Operative in cases where substrates enter the periplasm of Gramnegative bacteria through the outer membrane, although their
concentration in the periplasm is higher than outside the cell
(substrate accumulation)
• Involves the coupling of proton or sodium ion transport with transport
of a substrate against a gradient
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Class 3: Primary active transporters
• Use of a primary source of energy to drive active transport of a
solute against a concentration gradient (a secondary ion gradient is
not considered a primary energy source because it is created by the
expenditure of another primary energy source)
• Primary energy sources may be chemical, electrical or solar
A. P-P bond hydrolysis-driven transporters
B. Decarboxylation-driven transporters
C. Methyltransfer-driven transporters
D. Oxidoreduction-driven transporters
E. Light absorption-driven transporters
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A. P-P bond hydrolysis-driven transporters:
• Hydrolyzes diphosphate (ester) bonds in inorganic pyrophosphate,
ATP or other nucleoside triphosphates
• The substrate is not being phosphorylated during transport
• The transport protein may be (transiently) phosphorylated during
transport of the substrate
• Members include many ABC-type uptake (bacteria and archaea)
and efflux (bacteria and eukaryotes) systems, but also different
types of ATPases
Example:
3.A.1.13.1
3.A.1.13
3.A.1.
Vitamin B12 porter (from E. coli)
The vitamin B12 uptake transporter family
ATP-binding cassette (ABC) Superfamily
8
Class 4: Group translocators
A. Phosphotransfer-driven group translocators
(phosphoenol pyruvate-dependent)
• Found only in bacteria
• Dual role: Enzymatic catalysis and transport (tightly
coupled process)
• Transport:
Sugar (extracellular) -> sugar phosphate (intracellular)
B. Nicotinamide ribonucleoside uptake permease
(PnuC) family
• Relatively new subclass
• PnuC functions together with NadR (cytoplasmic) to
catalyze the following reaction:
NR (out) + ATP -> NMN (in) + ADP
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PEP-dependent phosphotransferase system (PTS)
Taken from Voet and Voet (3rd Ed., page 745)
10
Class 5: Transport electron carriers
• Catalyze electron flow across a membrane from a donor on one side
of the membrane to an acceptor on the other side
• Contribute to an increase/decrease of membrane potentials
• Two families:
A. Two-electron carriers
B. One-electron carriers
Example:
5.A.1.1.1
5.A.1
Disulfide bond oxidoreductase-D (DsbD) from E. coli
The disulfide bond oxidoreductase (DsbD) familiy
11
The DsbD protein:
• Involved in delivering
electrons from NADPH in
the cytoplasm to periplasmic
dithiol/disulfide-containing
proteins via thioredoxin
pathway
• “overall transport”:
2 e-cytoplasm -> 2 e-periplasm
Direction of electron flow:
NADPH -> thioredoxin reductase (TrxB) -> thioredoxin (TrxA) -> DsbD -> DsbC,
DsbE, DsbG -> proteins
DsbC is involved in proof-reading: reduces incorrectly formed disulfides
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Class 8: Accessory factors involved in transport
• Proteins that in some way facilitate, but do not directly participate in
the transport of substrates
• These proteins always work together with one or more established
transport systems
• Some of these proteins are involved in energy coupling, others play
a role in regulation or stabilizing the transport complex
Class 9: Incompletely characterized transport systems
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