Transcript Lecture-01-2013-Bi

H2O
carbonyl
K+ ion
Bi/CNS 150 Lecture 1
Monday, September 30, 2013
The ionic basis of neuroscience;
Introduction to the course.
Henry Lester
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Who are the Bi/CNS 150 students?
Preliminary numbers
Total undergraduate enrollment, 38
12 seniors, 20 juniors, 5 sophomores, 1 freshman
Majors:
20 Biology,
3 CNS,
6 BE,
2 Ch,
3 ChE
2 Ph
2 CS
3 graduate students
Fields:
2 Bi
1 CNS
4 CCE
1 BE
1 ME
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What is the most abundant molecule in an organism?
Molecule
Class Vote
Comments
water
3
Water is the most abundant molecule in an organism
H2O MW = 18
Density ~ 1 kg/l
Therefore the concentration of water in an aqueous solution is
~ (1000 g/liter )/(18 g/mol) = 55 mol/liter or 55 M.
All other molecules in the body are at least 100 times less concentrated.
Therefore we need to understand the properties of water.
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Typical extracellular and cytosolic ion concentrations (mammalian cell)
Na+
major
monovalent K+
Ions
Cl-
divalent
cations
Other ions
Extracellular Intracellular
conc
(Cytosol)
145 mM
15 mM
4 mM
150 mM
110 mM
10 mM
Ca2+
2 mM
10-8 M
Mg2+
2 mM
0.5 mM
Pi-2
2 mM
40 mM
H+
10-7 M
10-7 M
Protein
0.2 mM
4 mM
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One clue to a cell’s ionic concentrations:
Sea Water
Na+
major
monovalent K+
Ions
Cl-
divalent
cations
Other ions
Sea Water Extracellular Intracellular
conc
(Cytosol)
457 mM
145 mM
15 mM
9.7 mM
4 mM
150 mM
536 mM
110 mM
10 mM
Ca2+
10 mM
2 mM
10-8 M
Mg2+
56 mM
2 mM
0.5 mM
Pi-2
0.7 mM
2 mM
40 mM
10-7 M
10-7 M
10-7 M
0.2 mM
4 mM
H+
Protein
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Membranes provide a barrier to diffusion around cells,
forming compartments
nicotine
Alberts 4th 2-22
© Garland
Alberts 4th 11-1
© Garland
. . . But specialized proteins
(channels and transporters)
control the permeation of
many molecules
natural or synthetic
lipid bilayer
Little Alberts 12-1
© Garland
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A Cell that Lacks Concentration Gradients
External
Monovalent cations:
High Na+
Low K+
Na+
Na+
Na+ +
Na
Internal:
same as
External
Na+
Na+
Na+
Na+
Na+
K+
Na+
Na+
Na+
Na+
K+
8
Storing energy in a concentration gradient
without osmotic stress:
Simply reverse the ratio of Na+ and K+
External
Monovalent cations:
High Na+
Low K+
Na+
Na+
K+
K+
Internal:
Low Na+
High K+
Na+
Na+
Na+
Na+
K+
K+
K+
Na+
Na+
K+
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The “Na+ pump” splits ATP to make a Na+ and K+ concentration gradient
3
Alberts 4th 11-8 © Garland
2
Alberts 4th 11-8 © Garland
From Kandel 6-5
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Converting a concentration gradient
to an electrical potential:
Create permeability to one ionic species (K+)
Na+
Na+
Na+
K+
Na+
Lost positive charge
leads to net negative
interior potential
K+
K+
K+
K+
Na+
Na+
K+ channels
Na+
Na+
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The Nernst potential:
the energy of discharging the concentration gradient for K+ ions
balances
the energy of moving the K+ ions through the potential difference
K+
K+
K+
K+
K+
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Hundreds or thousands of ions
flow through a channel protein
for each opening
A transporter (or pump)
protein moves a few ions for
each conformational change
Kandel 5-19
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Chem 1 textbook (OGC)
Figure 12-10
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Deriving the Nernst potential (chemistry units)
G  RT ln
Ki
 zFV ; at equilibrium G  0 ; therefore
Ko
V 
RT  K i 

ln
zF  K o 
(we’ll assume that z = +1)
An e-fold ratio of K+ concentration ( Ki  Ko )
therefore leads to a potential difference of 
RT
.
F
R = 1.99 cal/mol oK; T = 300o; F = 9.65 x 104 C/mol (C is abbrev for coulomb).
OGN Figure 7-7
1.99 cal
 300
RT
mol


Therefore
=
F
9.965104 C
mol
 6 103 cal/C.
Now, 1 cal = 4.18 J (J is the abbreviation for joule),
and 1 J = 1 V x 1 C.
Therefore
RT
= 6 103 cal/C 4.18 V  C cal  25 mV.
F
Thus an e-fold concentration ratio gives a -25 mV membrane potential.
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Deriving the Nernst potential
(physics units)
R = Nk, where N is Avogadro’s number and k is Boltzmann’s constant;
And F = Ne, where e is the charge on the electron.
23 J
 300
RT kT 1.38  10



 25 mV
Therefore
19
F
e
1.6  10 C
(we are familiar with the statement that kT = 25 meV)
----------------------------------------------
And a 10-fold concentration ratio leads to a membrane potential of
ln 10
RT
 58 mV
F
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What is the selective advantage . . .
that the membrane is permeable at rest to K+ rather than to Na+?
a small inward leak of Na+ would change the internal [Na+]
by fractionally more than
a small outward leakage of K+ would change internal [K+ ]
Na+
Na+
Na+
[K+]I = 140 mM; [Na+]I = 10 mM. A leak of 10 mM:
[Na+] would increase from ~ 10 mM to 20 mM, doubling
[Na+]I and causing a 17 mV change in the Nernst
potential.
But a similar outward leak in K+ would decrease [K+]i
from 140 mM to 130 mM, causing a < 2 mV change in
the Nernst potential for [K+].
Na+
Na+
Conclusion: cell function is more stable when the resting
permeability is to K+ .
Na+
Na+
Na+
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What is the selective advantage . . .
that the membrane is permeable at rest to K+ rather than to Na+?
Conclusion: cell function is more stable when the resting
permeability is to K+ .
Na+
Na+
Na+
Indeed, there are many dozens of K+ channels
in the genome, but only ~ 10 Na+ channels.
K channels are metabolically “free” at rest.
Na+
Important, because the “Na/K pump” splits
~ 2/3 of the brain’s ATP.
Na+
Na+
Na+
Na+
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Other monovalent ions
Under what circumstances do neurons use Cl- fluxes?
Apparently it’s not straightforward to make a permeability pathway that
distinguishes among anions using protein side chains. Therefore there is
no “anion pair” corresponding to K+ / Na+. Few cells use anions to set the
resting potential.
But most postsynaptic inhibitory channels do use anion (mainly Cl-) fluxes.
Could neurons utilize plasma membrane H+ fluxes?
Probably not.
There are not enough protons to make a bulk flow, required for robustly
maintaining the ion concentration gradients.
(but some very small organelles (~ 0.1 mm) and bacteria do indeed store
energy as H+ gradients).
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Divalent Cations
What is the selective advantage that cells maintain Ca2+ at such low levels?
Cells made a commitment, more than a billion yr ago, to use high-energy
phosphate bonds for energy storage.
Therefore cells contain a high internal phosphate concentration.
But Ca phosphate is insoluble near neutral pH.
Therefore cells cannot have appreciable concentration of Ca2+;
they typically maintain Ca2+ at < 10 –8 M.
What is the selective advantage that cells don’t use Mg2+ fluxes?
The answer derives from considering the atomic-scale structure of a K+ selective channel (next slide), which received the 2003 Nobel Chemistry Prize:
(A suitable molecular graphics program, such as Swiss-prot viewer,
must be installed on your computer)
http://www.its.caltech.edu/~lester/Bi-150/kcsa.pdb
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In the “selectivity filter” of most K+ channels,
K+ ions lose their waters of hydration and are co-ordinated by backbone carbonyl groups
H2O
carbonyl
K+ ion
(Like Kandel Figure 5-15)
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Atomic-scale structure of (bacterial) Na+ channels (2011, 2012)
shows that here, too, partial loss of water is important for permeation
(As in Kandel Figure 5-1, Na+ channels select with their side chains)
Views
from the
extracellular
solution
The entire water-like pathway
Views
from the
membrane
plane
Payandeh et al,
Nature 2011;
Zhang et al,
Nature 2012
PDB files
4EKW, 4DXW
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Time required to exchange waters of hydration
Na+ , K+
1 ns
(~ 109/s)
Ca2+
5 ns
(2 x 108/s)
Mg2+
10 ms
(105/s)
Na+ , K+, and Ca2+ can flow through single
channels at rates > 1000-fold greater than Mg2+
As the most charge-dense cation, Mg2+ holds its
waters of hydration most tightly.
The “surface / volume” principle:
We know of several Mg2 transporters,
but Mg2+ channels apparently exist only in
mitochondria & bacteria.
Moomaw & Maguire, Physiologist, 2008
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Indeed, Mg2+ remains in the NMDA receptor channel so long . . .
that it becomes a voltage-dependent blocker
. . . this is crucial for learning and memory
Zigmond et al. (Eds.)
Fundamental Neuroscience,
© Sinauer (1999)
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Primary (ATP-coupled) vs secondary (ion-coupled) pumps / transporters
Kandel 6-5
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Cells have evolved elaborate processes for pumping out intracellular
Na+ and Ca2+
These gradients can be used in two ways:
Next image
1. The gradients are used for uphill “exchange” to control the
concentrations of other small molecules.
2. Transient, local increases in intracellular Ca2+ and Na+
concentrations can now be used for signaling inside cells!
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Ion-coupled transporters in the plasma membrane also
control the levels of neurotransmitters
Antidepressants
(“SSRIs” =
serotonin-selective
reuptake inhibitors):
Prozac, Zoloft, Paxil,
Celexa, Luvox
Drugs of abuse:
MDMA
Attention-deficit
disorder medications:
Trademarks:
Ritalin, Dexedrine,
Adderall,
Strattera (?)
material
that won’t
of abuse:
Presynaptic MarksDrugs
an exam
cocaine
terminals appear on
amphetamine
Na+-coupled
cell membrane
serotonin
transporter
Na+-coupled
cell membrane
dopamine
transporter
cytosol
NH3+
HO
outside
HO
N
H
HO
H2
C
C
H2
NH3+
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The “alternating access” mechanism explains
both ATP-driven (primary) and ion-coupled (secondary) transport
Based on structure
(Ca2+ pump)
Based on biochemistry
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3 classes of proteins that transport ions across membranes:
(transporter)
modified from
Alberts 4th 11-4
© Garland
Ion channels that flux
many ions per event
Ion-coupled
transporters
“Active” transporters
(pumps) that split ATP
These proteins have evolved in a natural—perhaps necessary--way to provide that
•
The resting potential arises via selective permeability to K+
This selective permeability also leads to the Nernst potential.
Transient breakdowns in membrane potential are used as nerve signals.
•
Neuronal and non-neuronal cells also signal via transient influxes of Na+ and Ca2+.
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Transport proteins (transporters, pumps, and channels)
are 5% of the human genome . . .
~ 1250 genes
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The Bi / CNS 150
Home Page
http://www.cns.caltech.edu/bi150/
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Come to class, please. Quizzes occur randomly,
During ~ 1/3 of the lectures,
And count for 10% of your grade.
Exams will cover material in the lectures and the required readings in Kandel.
Don’t consult previous problem sets or exams.
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Coursera
https://www.coursera.org/#course/drugsandbrain
Drugs and the Brain
7 weeks of lectures
Partial overlap with Bi/CNS 150.
Extra credit for Bi/CNS 150 students (~ 1/3 grade)..
Credit will be assigned **after** we make the Bi/BNS 150 curve;
Therefore you won’t be penalized for not taking the MOOC.
You must complete all MOOC work by 19 December 2013
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If you drop the course,
or if you register late,
please email Teagan
(in addition to the Registrar’s cards).
Also, if you want to change sections,
please email Teagan
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Henry Lester’s office hours occur at an unusual
time today: 12:30 -1:15 PM.
At the usual place: Outside the Red Door
End of Lecture 1
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