Chapters
Lodish 4th edition: Chapter 21 pages 921 - 924
Moyes and Schulte: Chapter 3
Before we discuss cotransporters we will take some time to explain the
basis for the membrane potential in animal cells.
Membrane potential
All animal cells are more negative on the inside that on the outside
(usually around -80 to -70 mV when measured).
This is due almost entirely to the distribution of K+ ions (more on the inside
and less on the outside) created by the Na+/K+ ATPase pump. All animal cells
have a K+ leak channel in their membranes. In addition there are a number of
anions inside the cell that can not leak across the membrane, think proteins,
DNA etc.
If you remember back to our theoretical membrane from the second lecture once
K+ flows down the concentration gradient an equilibrium is reached at the
point where one K+ ion flows down the concentration gradient and then one
flows back down the electrical gradient (into the negatively charged side of
the chamber). This is true for real cells as well as the K+ flows out of the
cell down its concentration gradient an equilibrium is reached as the inside
of the cell becomes more negative. This of course is described by the Nernst
potential equation or the equilibrium potential and in most animal cells the
Nernst potential for K+ is -80 to -70 mV.
The diagrams below are models of the pore of the leak K+ channel showing how
the lining of the channel is composed of charged amino acids to allow for the
passage of the K+ ion. In addition the other membrane spanning regions would
be composed of hydrophobic amino acids to allow the protein to remain in the
lipid bilayer.
So why is this important for our discussion of co-transporters. It has
everything to do with the favourable transport of Na+ down both the chemical
gradient (RTln [Na+]i/[Na+]o) and the electrical gradient (zFEm). This doubles
the favourable free energy change associated with the transport of Na+ and
makes it a powerful co-transporter.
Co-transporters
While ATPase pumps harness the favourable free energy change of the
breaking of ATP there is another family of proteins that will harness a
favourable free energy of the cotransport of ions.
For instance, the transport of Na+ into the cell has both a favourable
chemical gradient but also a favourable electrical gradient.
Therefore many cotransporters have evolved to harness this to drive the
transport of chemicals up a strong concentration gradient.
Na+/glucose cotransporter
This transporter is used by some cells especially those in the intestine to
transport glucose up a large concentration gradient
In the intestine this allows glucose to be brought into the body even if
glucose is lower in concentration than in the epithelial cells that form the
lining of the intestine.
In these circumstances the concentration of glucose inside the cell can be
3000 times greater than in the intestine. This cotransporter uses 2 Na+
molecules per one glucose and thus can cotransporter glucose up a very steep
gradient. The free energy of transporting 2 Na+ is = -6 kcal/mol so this
allows for the transport of glucose up a gradient of at least 26,000 fold.
This type of transporter is called a symporter as both molecules are
transported in the same direction
The above model is based on the sequence of the transporter showing 14
transmembrane domains. The last half is necessary for the movement of glucose
across the membrane while the first half may couple Na+ transport to the
glucose.
3Na+/Ca+2 antiporter
A vitally important transporter in muscle cells is the 3Na+/Ca+2 antiporter.
This maintains the low intracellular concentration of Ca+2 and plays a large
role in cardiac muscle cells as we will see later.
Remember the concentration gradient of Ca+2 across the membrane is very large
[0.0002 mM]inside versus [2 mM] on the outside for instance. This generates a:
DG = RTln (2/0.0002) = 5.5 Kcal/mol PLUS
DG = zFEm = +2(23,062)(.070V) = 3.3 Kcal/mol
Total DG = 8.8 Kcal/mol
Therefore it is necessary to utilize the transport of 3 Na+ ions into the cell
to transport one Ca+2 out of the cell.
Carbonate and other transporters that regulate pH
HCO3-/Cl- antitransporter
Another important aspect of transporters is to maintain close to a normal
pH within the cytoplasm (i.e. pH 7).
In concert with this function is the need to remove carbonate from cells. As
respiration proceeds CO2 will build up and normally diffuses
rapidly into cells where it converted to HCO3- by the enzyme
carbonic anhydrase. The reaction is:
H2O + CO2 <-> HCO3- + H+
About 80% of the CO2 in blood is actually transported as HCO3-
in the blood and this is generated by red blood cells, erythrocytes.
To rid themselves of the build up of HCO3- cells and in particular
red blood cells have a protein (AE1) which is a HCO3-/Cl-
antiporter. This is abundant in red blood cells that 1 X 109 HCO3-
ions are pumped out of the cell every 10 milliseconds (msec). This is
necessary to clear the CO2 to prevent build up to toxic levels and
the Cl- transport is to ensure that there isn't a build up of electrical
potential across the membrane or a change in pH within the cell.
The reverse reaction can occur once the levels of CO2 drop (such as
occurs in the lungs). HCO3- is imported into the cell using the
same transporter driven by the reversal of the concentration gradient in HCO3-.
Cl- is now exported from the cell to ensure a pH balance.
Other transporters to regulate pH
It is essential that cells be able to control the pH close to optimal
operating conditions for the proteins which for most is in the range of ph 7.
Some exceptions are proteins that function in lysosomes where the pH is in the
range of 4 to 5.
Most cells transporters to maintain the cytosolic pH (pHi), at about 7.2.
In addition to the HCO3-/Cl- transporter discussed above cells also have:
Na+/H+ antiporters: Na+ transporters to use the energy stored
in the Na+ gradient to pump out excess H+ when a cell becomes too acidic
Na+ HCO3-/Cl- cotransporter: In this case HCO3-
is brought into the cell to neutralize H+ in the cytosol (HCO3- +
H+ -> H2O + CO2 through the actions of carbonic
anhyrdrase). This is a Na+ driven Cl-/HCO3 exchanger that couples
the influx of Na+ and HCO3- to an efflux of Cl-. H+ is neutralized
to water once converted by carbonic anhydrase.
These exchangers are regulated by internal pH and increase their activity as
the pH in the cytosol falls.
