Transcript H +

Nutrient Transport
across Membranes
Shixue Yin (Prof Dr) Yangzhou University
Membranes as barriers
Except for H2O, most polar molecules
do NOT move across the lipid bilayers
Shixue Yin (Prof Dr) Yangzhou University
Relative speed of nutrient movement
across bacterial membranes
substance
permeability
water
glycerol
Tryptophan (色氨酸)
glucose
ClK+
Na+
100
0.1
0.001
0.001
10-6
10-7
10-8
Note that many nutrients are polar
Shixue Yin (Prof Dr) Yangzhou University
Membrane transport systems
are the systems to move nutrients and
waste products across membranes
Passive Passive diffusion
Channel proteins
Facilitated diffusion
Uniporter transport
Active
Antiport
Symport
ABC system
group translocation
Shixue Yin (Prof Dr) Yangzhou University
What is diffusion?
1. Molecules move along a concentration
gradient (from region of higher
concentration to lower concentration)
2. Movement is driven by random thermal
action - no energy output by organism
3. Concentration reaches equilibrium
4. Water, gasses, lipids, small uncharged
polar molecules
5. Not primary mode forShixue
hydrophilic
ions
Yin (Prof Dr) Yangzhou University
Passive 1 Passive diffusion
Examples of gases that cross membranes
by passive diffusion include N2, O2, and
CO2; examples of small polar molecules
include ethanol, H2O, and urea.
Shixue Yin (Prof Dr) Yangzhou University
Passive 2. Channel proteins
Channel proteins蛋白通道 transport water or certain
ions down either a concentration gradient, in the case
of water, or an electric potential gradient, in the case of
certain ions from an area of higher concentration to
lower concentration.
While water molecules
can directly cross the
membrane by simple
diffusion, as mentioned
above, their transport
can be enhanced by
channel proteins called
aquaporins(运水蛋白).
Shixue Yin (Prof Dr) Yangzhou University
Passive 3. facilitated diffusion
What is facilitated diffusion?
1. Diffusion aided by a carrier protein permease - in cell membrane
2. Carrier provides specificity and
increases rate to equilibrium
3. is powered by the potential energy
of a concentration gradient and
does not require the expenditure of
metabolic energy.
4. Not highly important in prokaryotes
Shixue Yin (Prof Dr) Yangzhou University
Passive 3 Facilitated diffusion
Diffusion facilitator
protein
Nutrient molec in high conc
Specific binding of nutrient
to facilitator protein
Protein conformation
(change shape)
Release of nutrients into
the cytoplasm of the cell
Shixue Yin (Prof Dr) Yangzhou University
Important points about
facilitated diffusion
1. Most transport proteins are
specific for a single nutrient.
2. Over time facilitated diffusion
results in an equal
concentration of nutrient inside
and outside the cell
Shixue Yin (Prof Dr) Yangzhou University
Passive 4 Uniport
Uniporters are transport proteins that
transport a substance across a
membrane down a concentration
gradient from an area of greater
concentration to lesser concentration.
Uniporter transport is powered by the
potential energy of a concentration
gradient and does not require
metabolic energy.
Shixue Yin (Prof Dr) Yangzhou University
Passive 4 Uniport
Transport
protein
Nutrient
in high conc
outside
inside
e.g. potassium
uniporter K+
Nutrient
in low conc
Shixue Yin (Prof Dr) Yangzhou University
Active transport
What is active transport?
1. Movement against a concentration
gradient. It can produce and
intracellular nutrient concentration
1000x greater than that of the same
nutrient outside the cell
2. Aided by a carrier protein
3. Requires energy from cell (ATP primary or PMF-secondary/simple)
4. Many amino acids and sugars
accumulated by this method
Shixue Yin (Prof Dr) Yangzhou University
Transporters
on
membrane
Shixue Yin (Prof Dr) Yangzhou University
Transport proteins include:
1). uniport单运体
2). Symport共运体
3). Antiport反运体
In prokaryotic environments,
nutrients are often scarce.
Shixue Yin (Prof Dr) Yangzhou University
Active 1 Antiport
Antiporters are transport proteins that transport
one substance across the membrane in one
direction while simultaneously transporting a
second substance across the membrane in the
opposite direction.
Antiporters in bacteria generally use the potential
energy of electrochemical gradients from protons
(H+), that is, proton motive force to co-transport ions,
glucose, and amino acids against their concentration
gradient. Sodium ions (Na+) and protons (H+), for
example, are co-transported across bacterial
membranes by antiporters.
Shixue Yin (Prof Dr) Yangzhou University
3. Antiport
compound 1
(nutrient)
transport
protein
H+
in low conc;
+
+
H
H
glucose, and amino
+
H
+
+
H
H
acids
H+
compound 2
Na+, H+ as anti-anion
outside
inside
H+
Nutrient in high conc
H+
Shixue Yin (Prof Dr) Yangzhou University
Alternative way to look at
Antiport
Shixue Yin (Prof Dr) Yangzhou University
ATP-Binding Cassette (ABC) transport
ABC system is an example of an ATP-dependent active
transport found in various gram-negative bacteria. It
involves substrate-specific binding proteins located in the
bacterial periplasm, the gel-like substance between the
bacterial cell wall and cytoplasmic membrane. The
periplasmic-binding protein picks up the substance to be
transported and carries it to a membrane-spanning
transport protein. Meanwhile, an ATP-hydrolyzing protein
breaks ATP down into ADP, phosphate, and energy. It is
this energy that powers the transport of the substrate, by
way of the membrane-binding transporter, across the
membrane and into the cytoplasm. Examples of active
transport include the transport of certain sugars and
amino acids. Over 200 different ABC transport systems
have been found in bacteria.
Shixue Yin (Prof Dr) Yangzhou University
ATP-Binding Cassette (ABC) transport
The transporters are a family of
periplasmic binding proteins that
have high affinity for substrate
only in Gram
negative prokaryotes
Transport channel
Supplying energy
Shixue Yin (Prof Dr) Yangzhou University
Active Transport, The "ABC"
System of Transport Step 1
This form of active transport involves
both transporter proteins and the
energy provided by the hydrolysis of
ATP. A specific periplasmic-binding
protein carries the substance to be
transported to a membrane-spanning
Step 2 The molecule to be
transported across the membrane
enters the transporter protein
system
and a molecule of ATP enters the
ATP binding site of the ATPhydrolyzing protein.
Shixue Yin (Prof Dr) Yangzhou University
Step 3
Energy provided by the
hydrolysis of ATP into
ADP, phosphate, and
energy moves the
molecule across the
membrane.
Step 4
The carrier protein
releases the molecule
being transported and the
transporter system is
ready to be used again.
Shixue Yin (Prof Dr) Yangzhou University
4. Symport
Symporters are transport proteins that
simultaneously transport two substances
across the membrane in the same direction.
Symporters use the potential energy of
electrochemical gradients from protons (H+),
that is, proton motive force to co-transport
ions, glucose, and amino acids against their
concentration gradient. Sulfate (HSO4-) and
protons (H+) as well as phosphate (HPO4-)
and protons (H+) are co-transported across
bacterial membranes by symporters.
Shixue Yin (Prof Dr) Yangzhou University
4. Symport
compound 1
(nutrient)
transport
protein
compound 2
outside
inside
lac permease
e.g. lactose
H+
Shixue Yin (Prof Dr) Yangzhou University
Alternative way to look at
symport
Shixue Yin (Prof Dr) Yangzhou University
C. group translocation
The phosphotransferase system (PTS)
The nutrient is chemically altered
during transport.
Phosphoenolpyruvate (磷酸烯醇丙酮酸PEP) (a
"high energy compound") supplies the energy.
Phosphate is transferred to the nutrient by a
series of phosphorylation dephosphorylation
reactions
Materials such glucose; mannose; fructose;
NAG; purines; pyrimidines are transported
through this mechanism
Shixue Yin (Prof Dr) Yangzhou University
The phosphotransferase system (PTS)
1
2
Phosphoenolpyruvate undergoes a series of
phosphorylation and dephosphorylation reactions till
EnzIIc receives the phosphate
Glucose is phosphorylated on EnzIIc and is transported
into the inside of the cell.
Shixue Yin (Prof Dr) Yangzhou University
Glucose uptake by the PTS
Glucose
Enzyme IIc
P
Phosphoenolpyruvate
P PEP
IIb
P
glucose
phosphate
IIa
Hpr
I
pyruvate
Shixue Yin (Prof Dr) Yangzhou University
VI. Proton motive force (PMF): an
energy source for active transport
A. proton pumping
B. proton gradient
C. charge gradient
D. symport with H+
E. symport with Na+
Note: PMF is also a general energy source
Shixue Yin (Prof Dr) Yangzhou University
A. proton pumping
Most cells "pump" protons out.
H+
H+
H+
H+
H+
H+
H+
H+
H+
cytoplasmic
membrane
This creates two sources of energy:
a proton gradient and a charge gradient.
Shixue Yin (Prof Dr) Yangzhou University
B. proton (pH) gradient
The cell membrane is a barrier that holds the
protons back the way a dam holds back water.
H+
H+
H+
H+
H+
H+
H+
H+
H+
cytoplasmic
membrane
The controlled movement of protons back into the
cell can be used as energy for nutrient concentration.
Shixue Yin (Prof Dr) Yangzhou University
C. charge gradient
When the cell membrane holds back
protons it also holds back a charge.
+
-
+
+
-
+
+
+
+
-
-
+
cytoplasmic
membrane
+
The controlled movement of charge across the
cell membrane can also provide energy for
nutrient concentration.
Shixue Yin (Prof Dr) Yangzhou University
Thus, proton pumping provides
two sources of energy:
1) a proton (pH) gradient
2) a charge gradient
This dual energy source is called
proton motive force (PMF).
Shixue Yin (Prof Dr) Yangzhou University
D. Symport with H+ allows the
concentration of nutrients using PMF
compound 1 (nutrient)
Transport protein
H+
outside
H+
inside
Usually, one proton is used Shixue
per Yin
nutrient
molecule
(Prof Dr) Yangzhou University
E. Symport with Na+ also allows the
concentration of nutrients.
compound 1 (nutrient)
transport protein
Na+
outside
Na+
inside
Usually, one sodium is usedShixue
perYin
nutrient
molecule
(Prof Dr) Yangzhou University
Symport with H+ uses both the proton
gradient and the charge gradient as
energy, while symport with Na+ uses
only the charge gradient.
During symport, uptake of the nutrient
and the second molecule is "coupled",
that is they must be taken up together.
Shixue Yin (Prof Dr) Yangzhou University
Membrane transport proteins
Shixue Yin (Prof Dr) Yangzhou University
Shixue Yin (Prof Dr) Yangzhou University