Transcript Chapter 11B Lecture

Chap. 11B. Biological Membranes &
Transport
• The Composition and Architecture of Membranes
• Membrane Dynamics
• Solute Transport Across Membranes
Fig. 11-3. Fluid mosaic model for plasma
membrane structure.
Overview of Transporter Types
While nonpolar compounds such as
O2 and CO2 can pass though
bilayers spontaneously by what is
called simple diffusion, all polar
compounds and ions require a
membrane protein to cross a
membrane. Movement of the solute
down its concentration gradient
from a high concentration on one
side to a low concentration on the
other side does not require the
investment of energy. If movement
is against an electrochemical
gradient (defined next slide), then
energy must be invested to drive
transport. A variety of names are
applied to membrane transporters.
The types of transporters found in
nature, and discussed in this
chapter, are illustrated in Fig. 1126.
Solute Movement Across a Permeable
Membrane
The direction in which a charged solute moves spontaneously across
a permeable membrane depends on both the chemical gradient (the
difference in solute concentrations) and the electrical gradient (Vm)
across the membrane. Together these two factors are called the
electrochemical gradient or the electrochemical potential. As shown
in Fig. 11-27a (left), an electrically neutral solute moves from the
side of high concentration to the lower concentration, and net
movement stops when the concentrations on both sides of the
membrane become equal. An charged solute’s movement (right) is
governed by both the electrical gradient (Vm) and the ratio of
concentrations (C2/C1). Net ion movement continues until the
electrochemical gradient (potential) reaches zero.
Transporters Lower the ∆G‡ for Passage of
Polar Solutes Through Membranes
The rate of spontaneous passage of a polar
solute through a lipid bilayer is very slow
due to the activation energy barrier for the
process. Both the removal of the hydration
shell around the solute, and the
transmembrane passage itself require
energy that makes the process
thermodynamically unfeasible (Fig. 11-28).
The activation energy barrier for this
process is akin to the activation energy
barrier for an chemical reaction. Similar to
enzymes, membrane transporters reduce
the activation energy barrier for movement
of polar solutes across a bilayer. By binding
to the solute specifically, released binding
energy compensates for the endergonic
process of desolvation. Further, the pathway through the
membrane protein is lined with hydrophilic amino acid residues
unlike the fatty acyl chains of membrane lipids. As a result, the
rate of transmembrane passage typically is increased by many
orders of magnitude. The movement of an uncharged solute, such
as glucose, through a membrane is known as facilitated diffusion,
or passive transport. Proteins that perform facilitated diffusion are
often called transporters or permeases.
Ion Channels vs Transporters (Pumps) (I)
Ion channels allow movement of their solutes down their
concentration gradients (“downhill”) only. Movement is dictated by
the ion’s charge and the electrochemical gradient across the
membrane. To allow solute movement, a single gate (structural
domain) opens allowing the ion to enter the channel and move
across the membrane (Fig. 11-29a). Ion channels transport their
solutes much faster than transporters (next slide) and rates of
movement can approach the limit of unhindered diffusion (tens of
millions of ions per second per channel). Ion channels typically are
specific for a given ion. However, because there is no binding site
for the ion inside the channel, transport does not display
saturation kinetics, as does transport by transporters. Kinetic
curves for transport by ion channels show a liner dependence of
transport rate vs solute concentration.
Ion Channels vs Transporters (Pumps) (II)
In contrast to ion channels, transporters (pumps) bind their solutes
with high affinity, are saturable in the the same sense as enzymes,
and transport solutes at rates well below the limits set by free
diffusion. Structurally, transporters can be viewed as having two
gates, one facing each side of the membrane (Fig. 11-29b). The
rate of transport is slowed due to the added time required for two
gates to open and close. Although transport may be several orders
of magnitude slower than that of ion channels, pumps can move
solutes against an electrochemical gradient with the investment of
energy. As discussed above, passive transporters move solutes only
down their concentration gradients. Active transporters can move
ions against the electrochemical gradient. So-called primary active
transporters use energy derived from a chemical reaction such as
ATP hydrolysis to drive transport. Secondary active transporters
capitalize on the energy stored in the electrochemical gradient by
coupling the transport of one solute uphill and paying the energetic
cost by transporting a second solute downhill.
Passive Transport: the RBC Glucose
Transporter (I)
A family of 12 passive transporters for glucose (GLUT) proteins is
encoded in the human genome. These transporters only catalyze
downhill transport of the sugar into, and sometimes, out of cells.
The GLUT1 transporter of erythrocytes is a 45 kDa type III
integral membrane protein that has 12 hydrophobic transmembrane
segments that are thought to adopt the  helical conformation (Fig.
11-30a). Erythrocytes rely on blood glucose for their energy
production. Blood glucose is maintained at about 5 mM between
meals, and at this concentration glucose constantly flows downhill
into RBCs due to consumption of glucose for energy production via
glycolysis. Nine of the 12 helices contain a few polar and charged
amino acid residues which are thought to line the path taken by
glucose through the transporter (Fig. 11-30a, b & c).
Passive Transport: the RBC Glucose
Transporter (II)
The process of solute transport by a transporter such as GLUT1,
is analogous to an enzymatic reaction. The “enzyme” is the
transporter, T, and the substrate is the solute outside the cell,
Sout, and the product is the solute inside, Sin. When the initial
rate (v0) of solute transport is measured as a function of external
solute concentration the resulting kinetic plot is hyperbolic (shows
saturation kinetics) and approaches a Vmax at high external solute
concentrations (Fig. 11-31). The kinetic curves for solute
transport are described by a kinetic equation similar to the
Michaelis-Menten equation for an enzyme, namely
V0 = Vmax [S]out/(Kt +[S]out)
where Kt is analogous to the Km and is equivalent to the solute
concentration at which the velocity of transport is half-maximal.
Passive Transport: the RBC Glucose
Transporter (III)
Solute transport by a transporter such as GLUT1 can be
described by the kinetic scheme shown below (left). T1 and T2
refer to transporter states where the solute binding site is facing
out and in, respectively. A structural model for conformational
changes in transporters is illustrated for GLUT1 in Fig. 11-32.
Note the presence of two gates in the transporter and how the
binding of glucose to the T1 structure triggers a conformational
change that exposes the glucose binding site to the inside of the
cell in the T2 structure. Although not shown in the kinetic
equations, the figure shows that it is possible for the transporter
to convert between T1 and T2 conformations without being bound
to the ligand. GLUT1 is highly specific for transport of D-glucose.
Kt values for transport of D-mannose and D-galactose are 3-to5-fold higher, and are 500-fold higher for L-glucose.
Kinetic Properties of Glucose Transporters
The kinetic properties of GLUT proteins are matched to the
physiological functions of the tissues in which they are expressed
(Table 11-3). Two examples, GLUT3 in the CNS and GLUT2 in the
liver & pancreas, are described below. Note, that the average
blood glucose level is 5.5 mM and typically ranges (red arrows in
the graphs) from 4.4-6.6 mM in humans during the day.
GLUT3: High affinity (Kt = ~1 mM)


= 1 mM
Transport near maximal under all
physiological conditions.
GLUT2: Low affinity (Kt = 17 mM)

= 17 mM
Allows cells to respond to blood glucose
level. Liver only takes up glucose rapidly in
the fed state. Pancreatic ß cells can sense
an increase in blood glucose and release
insulin.
Insulin-Activated Glucose Transport
Skeletal muscle and adipose
tissue synthesize an “insulinrecruitable” transporter
(GLUT4) that is shuttled
from intracellular vesicles to
the cytoplasmic membrane in
response to insulin binding to
these cells. This increases
the number of GLUT4
proteins in the membrane and
increases the rate of uptake
of glucose into these tissues
by 15-fold or more (Vmax
effect). In uncontrolled
diabetes, glucose is not
efficiently transported into
these large tissue beds,
resulting in elevated blood
glucose level. In insulin
overdose, transport becomes
excessive depriving the CNS
of glucose.
RBC Chloride-bicarbonate Exchanger
The formation of bicarbonate from dissolved CO2 by the RBC
enzyme, carbonic anhydrase, increases the blood’s carrying capacity
for CO2. Carbonic anhydrase activity is closely coordinated with
that of a membrane transporter in the RBC membrane known as the
chloride-bicarbonate exchanger. In respiring tissues, CO2 produced
by catabolism enters the blood and spontaneously diffuses through
the RBC membrane. There carbonic anhydrase converts it to
bicarbonate (via carbonic acid), and the exchanger transfers it to
the blood while bringing in one chloride ion. In the lungs the process
reverses, and the CO2 that leaves the RBC is exhaled.
The chloride-bicarbonate
exchanger is an integral membrane
protein that is structurally similar
to GLUT proteins. It increases the
rate of bicarbonate transport
across the RBC membrane by more
than a million fold. Because both
ions are negatively charged, the
exchange is electroneutral. The
coupling of chloride and bicarbonate
ion movement is obligatory. In the
absence of one ion, transport of
the other ceases.
Three General Classes of Transporters
Transporters differ in the number of solutes transported and in the
direction each solute moves. Transporters that carry only one
solute, such as the GLUT proteins, are called uniporters.
Transporters such as the chloride-bicarbonate exchanger carry out
cotransport. Because the exchanger’s two solutes move in opposite
directions it is considered an antiporter. When the two solutes
move in the same direction across the membrane the transporter is
called a symporter. Note that this classification system does not
indicate whether transport is active or passive.
Intro. to Active Transport
Unlike passive transport, active transport can move solutes against
a concentration or electrochemical gradient. Such transporters
actually establish solute gradients across cells. The movement of a
solute “uphill” requires energy. Two types of energy transduction
systems are widely used in nature (Fig. 11-35). In primary active
transport, energy is obtained by absorption of sunlight, an
oxidation reaction, or the breakdown of ATP. The energy released
from hydrolysis of ATP, for example, drives conformational changes
in the transporter that allow it to move a solute against its
concentration gradient. In secondary active transport, the energy
released from the movement of one solute down it concentration
gradient is used to move a second solute against its concentration
gradient. Typically, the concentration gradient for the first solute
has been established by a primary active transport system. Sodium
gradients often serve in this function.
Energy Cost of Active Transport
For uncharged solutes the free energy change for transport is
given by the equation
∆Gt = 2.303 RT log (C2/C1)
where C2 is the concentration of the solute in the final
compartment, and C1 is its concentration in the initial
compartment. For a 10-fold difference in concentration (i.e.,C2 =
10C1), the energy cost is 5.7 kJ/mol. When the solute is an ion,
transport becomes electrogenic due to the separation of positive
and negative charges. This creates an electrical potential across
the membrane. Cells maintain electrical potentials across their
membrane. The typical plasma membrane electrical potential is
about 0.05 V (with the inside negative). When charged solutes are
transported across a membrane the free energy equation for
transport requires another term, namely
∆Gt = 2.303 RT log (C2/C1) + ZF∆
In this equation, Z is the charge of the ion, F is the Faraday
constant (96,480 J/V.mol), and ∆ is the transmembrane
electrical potential in volts. The use of these two equations in
calculating the energetic cost of active transport is illustrated in
the next two slides.
P-type ATPases (I)
The P-type ATPases are a family of cation active transporters
that drive transport using energy derived from ATP hydrolysis.
They are all type III integral membrane proteins, and have 8 or
10 transmembrane segments. They are also inhibited by the
phosphate analog vanadate. During the transport cycle they are
reversibly autophosphorylated on a conserved Asp residue located
within their phosphorylation (P) domain (Fig. 11-36a). The kinase
function of the enzyme resides in the nucleotide-binding (N)
domain. Phosphorylation causes large conformational changes that
allow the transport (T) domain to transport ions across the
membrane. The actuator (A) domain communicates movements of
the N and P domains to the ion-binding sites, and also has a
phosphatase activity that removes the phosphate attached to the
phosphorylation domain at the end of each pumping cycle.
P-type ATPases (II)
The P-type ATPases are widespread and perform critical
processes in many organisms (Fig. 11-36). For example the Na+K+
ATPase of animal cells creates electrochemical gradients across
many cells including neurons. These gradients are then exploited in
nervous system signaling. The sarcoplasmic/endoplasmic reticulum
Ca2+ ATPase (SERCA) pump and the plasma membrane Ca2+ ATPase
pump maintain cytosolic calcium levels at about 1 M. This is
important for signal transduction systems that are dependent on
intracellular calcium levels. In the mammalian stomach, parietal
cells have a H+K+ ATPase that pumps protons into the lumen of
the stomach acidifying it. Lastly, lipids flippases are structurally
and functionally related to P-type ATPases.
Mechanism of the SERCA Pump
The SERCA pump (Fig. 11-37) moves
two Ca2+ ions across the membrane
and hydrolyzes one ATP per catalytic
cycle. When ATP binds to the E1
conformation of the pump two Ca2+
ions bind to their sites which face the
cytoplasm. On phosphorylation of the
P domain the conformation of the
pump converts to the E2 form, in
which the Ca2+ binding sites now face
the lumen of the ER or sarcoplasmic
reticulum. In this conformation, the
binding sites have lower affinity for
these ions which are released from
the pump. After dephosphorylation of
the P domain the enzyme reverts to
its E1 state and another cycle can
begin.
The Na+K+ ATPase of Animal Cells
The Na+K+ ATPase of the cytoplasmic
membranes of animal cells is
primarily responsible for setting and
maintaining the intracellular
concentrations of Na+ and K+ inside
and outside cells. This pump is
thought to act analogously to the
SERCA pump, except that 3 Na+ ions
are pumped out of the cell while 2 K+
ions are pumped in per ATP
hydrolyzed. Both ions move against
their concentration gradients which
actually are maintained by this pump.
Due to the difference in the number
of charges moved across the
membrane, transport is electrogenic
and is responsible for the creation of
a -50 to -70 mV electrical potential across the membrane (inside
negative). The potential established across the plasma membrane
is central to electrical signaling in neurons, and the gradient of
sodium ions is used to drive the uphill cotransport of different
solutes in many cell types. The consumption of energy by this
ATPase accounts for 25% of the energy expenditure of a human
at rest.
V-type & F-type ATPases
V-type (vacuolar) and F-type (mitochondrial inner membrane)
ATPases have elaborate structures consisting of an integral
membrane domain through which protons are transported uphill, and
a globular peripheral domain which hydrolyzes ATP to provide
energy for proton transport (Fig. 11-39). V-type ATPases are
responsible for acidifying intracellular compartments such as
lysosomes and the vacuoles of fungi and higher plants. Although
bacteria like E. coli can use an F-type ATPase to create a proton
gradient across the inner membrane, in mitochondria the enzymes
are used to synthesize ATP. The reactions of the mitochondrial
electron transport chain lead to pumping of protons from the
matrix to the intermembrane space and the build up of a large
proton gradient. Protons then flow downhill into the matrix through
the ATPase resulting in synthesis of ATP. Note that the cytosol
and matrix sides of the membrane are mislabeled in Fig. 11-39b.
ABC Transporters
ABC transporters are a large family of
ATP-dependent active transporters that
have evolved to recognize many different
solutes. They all contain cytoplasmic
nucleotide binding domains (NBDs) where
ATP is bound and hydrolyzed, and multiple
transmembrane segments through which
solutes are pumped (Fig. 11-39). The
name ABC transporters stems from the
fact that the NBDs are also called ATPbinding cassettes. Some ABC transporters
are highly specific for a single solute
(e.g., the E. coli vitamin B12 importer,
Panel b), whereas others are more
promiscuous (e.g., the mammalian
multidrug transporter (MDR1 or P
glycoprotein, Panel a). MDR1 is responsible
for the resistance of many tumors to
chemotherapeutic drugs like adriamycin,
doxorubicin, and vinblastine, all of which
are pumped out of malignant cells by
MDR1. In non-malignant cells, MDRs pump
toxic compounds out of cells.
The Cystic Fibrosis Transmembrane
Conductance Regulator (CFTR) (I)
The CFTR transporter is a member of the ABC transporter family
that is important for keeping the mucus on the surfaces of
epithelial cells moist and of the right viscosity so that
microorganisms can be trapped and cleared from cell surfaces by
ciliary motion. The transporter is very important in epithelial cells
lining the lung airway, the digestive tract, and exocrine glands
such as the pancreas. Hereditary defects in the CFTR gene in
homozygotes cause the serious disease known as cystic fibrosis.
Due to decreased transport of water along with the transporter’s
solute, chloride ion, the mucus on the surface of the lung airway
becomes dehydrated, thick and excessively sticky. This traps
pathogenic bacteria, notably S. aureus and P. aeruginosa, in the
airway leading to repeated infections
that damage the lungs and reduce
respiratory efficiency (Fig. 2, Box 112). It is not uncommon that death due
to respiratory insufficiency occurs by
age 30. Defective CFTR alleles are
most prevalent in Caucasians, and about
5% of white Americans are carriers of
one defective gene.
The Cystic Fibrosis Transmembrane
Conductance Regulator (CFTR) (II)
The CFTR transporter, although a member of the ABC transporter
family, has some unusual structural features not common in this
family of pumps. Namely, the transporter is actually a channel
protein that is regulated by ATP binding to the cytoplasmic NBDs.
When a regulatory (R) domain is phosphorylated and ATPs are
bound to each NBD, the channel opens and chloride ion and water
molecules flow out to the surface of the epithelial cell. If the R
domain is not phosphorylated or ATP is not bound to the NBDs,
the channel remains closed. The most common mutation that
interferes with CFTR function in CF patients is the deletion of
Phe508 in one of the NBDs. This mutation results in aberrant
protein folding and failed insertion of CFTR into the membrane.
Secondary Active Transport
In secondary active transport, energy stored typically in a Na+ or
H+ gradient across a membrane is exploited for transport of a
second solute. Namely, the transport of Na+ or H+ down its
electrochemical gradient releases energy to the transporter that
drives conformational changes that result in the transport of the
second solute uphill. As long as the net free energy change for
the two processes is negative, transport can occur. Note that
these proteins can be symporters or antiporters. Many examples
of secondary active transporters occur in nature (Table 11-4).
We will discuss two examples--the E. coli lactose permease and
the vertebrate Na+-glucose symporter.
The Lactose Transporter of E. coli (I)
The lactose transporter (lactose permease, or galactoside
permease) of E. coli is a proton-driven symporter (Fig. 11-41).
During each cycle of transport one H+ and one molecule of lactose
enter the cell. The proton is derived from a gradient of protons
that is high outside the inner membrane and low inside that is
created by proton pumps driven by fuel oxidation. The influx of a
proton moving down the electrochemical gradient releases enough
free energy to drive uptake of lactose against its concentration
gradient. When energy yielding reactions of metabolism are blocked
by cyanide (CN-), the lactose permease reverts to a membrane
facilitator merely equilibrating the concentrations of lactose on the
two sides of the membrane. Certain mutations discussed below have
the same effect on the permease.
The Lactose Transporter of E. coli (II)
The lactose permease is a member of the major facilitator
superfamily which contains 28 families of transporters. Most like
the lac permease have 12 transmembrane segments and function as
monomers. The structure of the permease has been determined by
x-ray crystallography (Fig. 11-42). The six N-terminal and six Cterminal transmembrane segments are arranged with a rough twofold symmetry. Loops connecting the transmembrane segments
project out into the periplasm and cytoplasm. The solutes bind to
the permease deep within the interior of the membrane in a large
aqueous cavity. The proposed mechanism for transmembrane passage
of the solutes involves a rocking motion between the two domains.
This “rocking banana” model is similar to that shown for the GLUT1
protein in Fig. 11-32. Extensive site-directed mutagenesis studies
have revealed that only 6 residues are absolutely essential for
cotransport of H+ and lactose. Charge pairing between two of
these, Glu325 and Arg302 (green), which is affected by protonations,
determines the two conformations and to which side of the
membrane the lactose binding site is exposed. Mutations at either
of these residues converts the permease to a simple facilitator.
Intestinal Na+-glucose Symporters
The uptake of glucose and amino acids from the lumen of the small
intestine into intestinal epithelial cells (enterocytes) occurs via
secondary active transport with Na+. The transporter that
catalyzes glucose uptake is known as the Na+-glucose symporter
(Fig. 11-43). The energy used for glucose transport is stored in
the electrochemical gradient created by the Na+K+ ATPase, which
operates from the basal side of the enterocyte. The concentration
of sodium inside these cells is kept lower than in the intestinal
lumen due to the action of the ATPase.
Because glucose uptake is
performed by an active transport
process, small concentrations of
this nutrient can be efficiently
absorbed. Subsequently, absorbed
glucose moves downhill into the
bloodstream through the passive
glucose uniporter, GLUT2, that is
expressed in enterocytes.
Worked Example 11-3 (continued)
Ionophores: Valinomycin
Ionophores are hydrophobic peptides that bind to ions such as K+
and Na+ and transport them across membranes. They carry these
ions down their concentration gradients, dissipating the gradient
and equilibrating the concentration of the ion on both sides of the
membrane. Valinomycin (Fig. 11-44), which carries K+, and
monensin, which carries Na+ are used as antibiotics. Both have a
central hydrophilic cavity to which the ion binds, and a hydrophobic
exterior which makes the complex capable of passing through the
fatty acyl chains of membrane lipids. Ultimately, they kill
microorganisms by disrupting secondary transport processes and
energy-conserving reactions.
Aquaporins (I)
Aquaporins (AQPs) are a large family of integral membrane
proteins that serve as channels for movement of water molecules
across membranes. They are widely distributed in nature, and
eleven are known in mammals (Table 11-5). Aquaporins often
function in the production of fluids by exocrine glands such as
sweat, saliva, and tears. They also are important in urine
production and water retention in the nephrons of the kidneys. For
example AQP2 in the
epithelial cells of the
renal collecting ducts is
regulated by vasopressin
(antidiuretic hormone).
Increased levels of
vasopressin stimulate
reabsorption of water.
In the rare disease,
diabetes insipidus, a
genetic defect in AQP2
leads to impaired
reabsorption of water in
the kidney and excessive
urination (polyuria) (Box
11-1).
Aquaporins (II)
AQPs only transport water molecules in the direction dictated by
the osmotic gradient (i.e., towards the side of the membrane
having the higher solute concentration). Water molecules are known
to flow through a AQP1 channel at a rate of about 109 per
second. AQPs do not allow the passage of protons as hydronium
ions, which would dissipate electrochemical gradients. X-ray
crystallographic studies of AQP1 have revealed how water
molecules pass through the channel and protons are excluded
(Fig.11-45). AQPs occur as tetramers in the membrane where
each subunit contains six transmembrane segments and contains a
transmembrane pore that allows water molecules to pass in single
file. The pore narrows in the middle of the membrane to 2.8Å
which is about the size of a water molecule. The narrow
constriction also is lined
with positively charged
residues which repel
hydronium ions. Also water
molecules are prevented
from orienting in a
hydrogen bonded network
within the pore so that
protons cannot pass
through by “proton
hopping”. AQPs commonly
are gated, and regulated
so water only passes
through when necessary.
Intro. to Ion Channels
The last general type of transporter covered in Chap. 11 is ionselective channels. These are widely dispersed in nature and are
perhaps best known for their roles in generating action potentials
that carry signals from one end of a neuron to another. In
myocytes, rapid opening of Ca2+ channels in the sarcoplasmic
reticulum releases the Ca2+ that triggers muscle contraction. In
many cells, an alternating cycle of events occurs wherein the Na+K+
ATPase generates a membrane potential, and ion channels open and
partially break down that potential. The flux of ions through ion
channels is very high, approaching 107 to 108 ions per second which
is near the diffusion controlled limit. Unlike the other transporters
we have discussed, transport is also not saturable and is linear as
a function of ion concentration. Ions flow only down their
concentration gradients. Lastly, ion channels are gated and open in
response to some cellular event. Gating can be achieved via the
binding of ligands (ligand-gated channels) or by changes in the
transmembrane electric potential (voltage-gated channels). A
channel typically opens in a fraction of a millisecond and remains
open for only milliseconds.
Ion Channel Methods
The movement of ions through ion
channels is measured electrically
using a technique called patchclamping (Fig. 11-46). With this
procedure the flux of ions
through a single channel can be
accurately measured. Changes in
membrane potential typically
occur in the millivolt range, and
the electric current is in the
micro- to picoampere range. The
size and duration of the current
that flows per opening event can
be measured, as can the effects
of membrane potential,
regulatory ligands, and toxins on
ion flow. Patch-clamping studies
have revealed that as many as
104 ions can move through a
single ion channel in 1 ms.
The K+ Channel of S. lividans (I)
Bacteria also contain ion channels, and one of the best studied
channels comes from Streptomyces lividans (Fig. 11-47).
Crystallographic studies have uncovered the structure of the
channel and the basis for its selectivity for K+ ions. This channel
protein consists of four identical subunits each of which contribute
two transmembrane  helices and a third short  helix to the lining
of the channel. The so-called selectivity filter of the channel
allows K+ (radius 1.33 Å) to pass 104 times more readily than Na+
(radius 0.95 Å) and at a rate of 108 ions/s.
The K+ Channel of S. lividans (II)
The mechanism for the rapid and selective movement of K+ ions
through the channel is summarized here. On both sides of the
membrane, the entrances to the channel are wide and contain
several negatively charged residues that attract Na+ and K+ ions.
Selectivity for positive ions is also achieved via the short helices
that project into the channel which have the negatively charged
ends of their helix dipoles facing the path for ion movement.
Initially, ions enter with their bound hydration spheres, but these
are stripped off where the channel narrows and contacts between
backbone carbonyl oxygens and the ions
stabilize the unhydrated K+ ions. Na+ ions
do not move readily through the
constriction because their radii are to
small to interact well with the carbonyl
groups of the selectivity filter. Four K+
interaction sites occur in the selectivity
filter. Two K+ ions move through the
channel together with the ions occupying
alternating sites (i.e., sites 1 and 3,
then sites 2 and 4), then out. It is
thought that charge repulsion between
the two ions in the channel, and only
weak interactions to the carbonyl oxygens
keeps ions moving rapidly through the
channel.
The Mammalian Voltage-gated K+ Channel
The tetrameric mammalian voltage-gated K+ channel, while
structurally more complex than the S. lividans K+ channel, still
exhibits many of the same structural features as the bacterial
channel (Fig. 11-48). Namely, the ion selectivity filter is
constructed from two transmembrane segments from each subunit,
and K+ ions interact with carbonyl oxygens that replace the ion’s
hydration shell as they pass through the interior of the protein.
Gating is achieved due to repositioning of an arginine-containing S4
transmembrane helix in response to changes in membrane potential.
This helix, which serves as a voltage sensor, is pulled toward the
more negatively charged cytoplasmic side of the membrane keeping
the channel closed in resting neurons. However, when the neuron
membrane depolarizes as an action potential passes by, the helix
moves outward opening the channel. K+ then flows out of the cell
repolarizing the cell membrane (refer to Fig. 12-26).
The Nicotinic Acetylcholine Receptor
The nicotinic acetylcholine receptor is an ion channel that functions
in passing an electrical signal from a motor neuron to a muscle fiber
at neuromuscular junctions, signaling the muscle fiber to contract.
This channel is an example of ligand-gated ion channels, and the
ligand is acetylcholine which is released from the motor neuron.
The channel transports Na+, Ca2+, and K+ ions into the myocyte
depolarizing its membrane and signaling it to contract. On
dissociation of acetylcholine from the receptor channel, the channel
closes. The acetylcholine receptor channel is grouped in the same
superfamily as the -aminobutyric acid (GABA), glycine, and
serotonin receptors of neurons.
Diseases Caused by Ion Channel Defects
There are numerous diseases associated with mutations in genes
encoding ion channels such as the acetylcholine receptor channel
and the CFTR channel (Table 11-7). In addition, many channels are
targeted by toxins and poisons that cause paralysis by interfering
with neuromuscular transmission. For example, tetrodotoxin from
the puffer fish binds to and inactivates voltage-gated Na+ channels
of neurons. Dendrotoxin from the venom of the black mamba snake
inhibits voltage-gated K+ channels in neurons. Tubocurarine, the
active component of curare used as arrow poison in the Amazon
region, interferes with the function of the acetylcholine receptor
channel.