Transcript 1. dia - univie.ac.at

Unequally distributed electrolytes
Role of alkaline and alkaline earth
metal ions
Unequally distributed electrolytes: Function and
transport of alkaline and alkaline earth metal cations
Concentration gradient between the cells and their environments:
This differs from their thermodynamic equilibrium state
This gradient can be maintained only by significant energy consumption
Average extracellular and intracellular ion concentrations
of mammals in mM
Inside the cell
Outside the cell
Na+
K+
Mg2+
Ca2+
Cl-
HCO3
150
100
5
140
2
2.5
2
0.0001
110
10
30
10
-
HPO42-/
H2PO44
4
Chemical characteristics of the ions
Physical data of the biologically important alkaline and alkaline-earth metal ions
Na+
Ionic radius (Å, N=6)
Relative surface charge density
Coordination number
Coordination geometry
Preferred donor atom
Mobility in biological systems
Water exchange rate (s-1)
1.02
0.25
6-8
various
O
highly mobile
10-10
K+
1.38
0.72
0.14
1.00
6-8
6
various
octahedron
O
O-,N
highly mobile slightly mobile
10-10
10-6
Do not form strong (corrdinative) bonds.
They are present in hydrated form - diffusion.
They are present in high concentration.
Mg2+
Ca2+
1.00
0.52
6-8
various
O,Omobile
10-9
Functions – alkaline metal ions
(i) Form the necessary osmotic pressure, providing stability for the cells and cell
organells.
(ii) Provide charge compensation and stabilisation of structure. Most of the
bio(macro)molecules in the cells have negatively charged groups (phosphate,
carboxylate, sulphonate), which should be neutralised. Furthermore, as the
counter ions of the polyelectrolyte macromolecules (proteins, DNA, RNA,
polysacharydes), or constituents of cells (e.g. phospholipid membranes), actively
participate in stabilisation of their structure. It can be understand easily, if we think
that a single DNA strain should have a few thousand negative charge, and thus a
double helical strain should not be formed without the presence of the counter
ions.
(iii) Information transfer among the cells (action potential/nerve conduction). The
most basic form of the biological control is the electrolytic nerve conduction.
Regulation of the different functions within the cells, even the communication
among the different organs may happen via ion currents, action
potentials/currents.
Physical and chemical characteristics of
Mg2+ and Ca2+ ions
The Mg2+ ion is smaller, occurs in regular octahedral geometry,
surrounded by water or negatively charged O donors.
The Ca2+ ion is larger, occurs in irregular geometry with high
coordination number, and prefers interactions also with neutral O donors.
Due to their two positive charge and higher surface charge density,
these ions form mostly ionic, medium strong interactions with bioligands.
The strengths of the coordinative bonds of Mg2+ are about the same as
that of Ca2+ ion, but the rate of its reactions is about 3 orders of magnitude
slower.
Among the essential divalent metal ions Ca2+-ion is the most mobile.
Functions of Mg2+ and Ca2+ ions
The extracellular Ca2+ stabilises the cell wall and tissue.
In the cell, because of the very low concentration of calcium, Mg2+ ion
have a similar role.
Because of the dipositive charge and their relatively high surface charge
density they form mostly ionic, medium strong interactions with
biomolecules.
Ca2+ - information transfer regulated by coordinative bonds.
Functions of Mg2+ and Ca2+ ions
Mg2+ ion is essential for thermodynamic stabilization of RNA tertiary structures
Selective recognition of the Na+, K+, Ca2+ and Mg2+ ions
The Mg2+ ion is small and prefers the regular octahedral environment,
while the Ca2+ ion is larger and forms easily irregular, fluctuating
coordination sphere with the participation of six-eight donor atoms.
This difference in geometry may results in even 4-5 orders of magnitude
difference in the metal binding strength of thne individual sites, resulting in
complete selectivity between the two metal ions.
However, Na+ and K+ show much higher similarities.
The crown ethers and the cryptands can bind alkaline metal ions
strongly and selectively
(C.J. PEDERSEN and J. M. LEHN - Nobel-prise in 1987)
[15]crown-5
(Na+)
[18]crown-6
(K+)
[21]crown-7
(Cs+)
[2,1,1]cryptand
(Li+)
[2,2,1]cryptand
(Na+)
[2,2,2]cryptand
(K+)
Structure of valynomicin and its potassium complex
Stability of the K+ complex is ~ 17000 x higher than that of its Na+ complex.
Membrane transport processes
Outside of cell
Cholester
ol
Polysaccharid
e
Glycoprotein
Phospholipid bilayer
thickness 30-50 nm
Peripheral
protein
Transmembrane
protein
Inside of cell
Schematic picture of the cell membrane
Membrane transport processes
Two basic membrane transport processes can be distinguished:
Passive transport – in the direction of a concentration gradient
Via ionophores, or channel forming transport proteins
Active transport – opposite to the direction of the concentration gradient
Via pumps (special transport proteins)
Ionophores
Mobile ionophores (siderophores, valynomicine)
Channel forming ionophores (gramicidin A)
Ionophores
Gramicidin A:
HCONH-(L)-Val-Gly-(L)-Ala-(D)-Leu-(L)-Ala-(D)-Val-(L)-Val-(D)-Val-(L)Trp-(D)-Leu-(L)-Trp-(D)-Leu-(L)-Trp-CONHCH2-CH2-OH
Channel forming proteins
The differences in the size is differentiated further by the charge,
as the charge/ion radius ratio determines the extent of hydration (the
higher this ratio the larger the number the strongly bound water molecule
surrounded the metal ion). This relatively simple control mechanism
works fairly well. Differentiation between anions and cations is made by
charged groups located at the openings of the channel, electrostatic
interactions between the given ions perform this task perfectly. These
channels are usually in a closed state, their openings are covered by a
part of the protein. Opening of the channel is initiated by some stimulus,
which results in conformation change. Depending on the type of the
stimulus channels can be classified as potential or receptor dependent
ion channels.
K-channel forming protein
a
b
c
K-channel forming protein
In the open state of the channel, the opposite direction flow of the Na+ ions
is prevented by a filter. As in its inner hole the dehydrated Na+ ion may be
bound only „loosely”, through several carbonyl oxygens; this energy is not
enough to cover the dehydration energy of the sodium ion. Besides, the
hydrated Na+ ion is too large to go across the filter.
és
ProteinFoszforiláció
Phosphorylation
konformáció változás
and conformational
change
A nátriumionok
Binding
of Na+ ion
megkötése
A nátriumionok
távozása
Release
of Na+ outside
sejten
of athe
cellkívüli térbe
I.
A káliumionok
+ ion
Binding
of
K
megkötése
A káliumionok
Release
of K+ to
távozása
sejt
the inside
ofathe
belsejébe
cell
Defoszforiláció
és
Dephosphorylation
and
konformáció
változás
structral change
in protein
During a cycle of the Na+-K+ATPase 3 Na+ ion is transported out of the cell and
2 K+ ion into the cell. Both ions are transported against the concentration
gradient; the energy need of these processes is covered by the hydrolysis of an
ATP molecule.
Nerve conduction/Action potential
In resting state number of the positive and negative ions little differs on the
two sides of the nerve membrane (inside the cell there is a little excess of
the negative charge), and this results in some ptential in the two sides of
the membrane, this is the so called membrane potential. As the membrane
is permeable for the K+ ions, the membrane potential in the resting state is
close to the equilibrium potential of K+ (-70mV).
Nerve conduction, i.e. spreading the action potential along the axon, is
caused by the molecular work of the potential-dependent Na+ and K+
channels located in the nerve membranes.
Membrane potential (mV)
Action potential
Time (s)
Phase
K+-channel
Na+-channel
1.
Resting state
A few are opened
All are closed
2.
Stimulation
A few are opened
All become open
3.
Depolarization
All become open
open
4.
Max depolarization
Open
All become inactive
5.
Repolarization
Open
All are inactivated
6.
Refractory period
Most of them close inactive - closed transition
7.
Resting state
A few are opened
All are closed
Na+ ions flow in
the
cell, K+ ions flow
out.
Nerve conduction
When this wave reaches the end of the axon the so called
synapse, due to depolarisation of the membrane through the potentialdependent Ca2+ channel Ca2+ ionok flow in the synapse, and
neurotransmitter molecules (e.g. acethyl-cholin) are released. These
cross the synaptic cleft and initiate a new action potential in a
neighbouring nerve by chemical stimulation. This new action potential
goes on its way through the other neuron. The neurotransmitter
decomposes soon and the stimulus ceased. The stimulus is conducted
at high speed, it can reach even 100 m/s (360 km/h) value.
Muscle contraction
Izom
Muscle
Muscle fibre
Izomrost
Myofibrill
Miofibrillum
Izomrostok
Fascicle
kötege
Components of
thin filament and
the
their connection
myosine
to
Troponin
Actin
Aktin
Ca2+
Troponin
Tropomyosin
Tropomiozin
AMyosin
miozinhead
fejrésze
Miozin
Myosin
Muscle contraction
I. In the resting state of
the muscle cell actin and
myosine
are
not
connected.
Muscle contraction
II. Ca2+ ion is bound to one
component to the troponin
complex (troponin C), and
undergoes
a
conformational change; a
phosphate dissociate from
the myosin binding site
and the actin binds at the
head space of myosin.
Muscle contraction
III. As a result of this,
the Mg2+-ADP adduct
cleaved of the head
space of myosin, its
conformation changes
and the actin can move
as compared to myosin.
Muscle contraction
IV. In the next step Mg2+ATP binds to the head
space
of
myosin.
Meanwhile concentration of
Ca2+ decreases in the
sarcoplasma due to the
function of the Ca2+ pump.
Zoltán Kerényi