Page 68 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

P1: GTQ/GUU P2: GLM Final Pages
Encyclopedia of Physical Science and Technology EN008K-353 June 29, 2001 12:41

Ion Transport Across Biological Membranes 101

controlled movement of inorganic ions through the cell
membrane and the resulting change in transmembrane
voltage, V m .

II. MECHANISM OF TRANSMEMBRANE
INORGANIC ION FLUX

FIGURE 2 Minimum mechanism to account for the rates of a neu- Here, the emphasis is on kinetic techniques used to obtain
rotransmitter (acetylcholine) receptor-mediated cation transloca- the information needed to understand the mechanism of
tion and for receptor inactivation and reactivation as a function of protein-mediated reactions that allow the transport of in-
acetylcholine concentration. The active (A) and inactive (I) forms
organic ions across biological membranes. A combination
of the receptor bind neurotransmitter (L) in rapidly achieved equi-
libria denoted by the microscopic equilibrium constants (K). Active of structural, thermodynamic, and kinetic information is
receptor with two bound ligand molecules (AL 2 ) converts rapidly required to achieve this understanding. The use of X-ray
(1 to 2 msec) to an open channel (AL 2 ) with an equilibrium con- crystallography, NMR measurements, and electron mi-
stant for channel opening (1/ · = k cl /k op where k op and k cl are croscopy, to obtain structural information about proteins
the rate constants for channel opening and closing respectively ). is described in detail elsewhere in this Encyclopedia.
+
+
AL 2 permits the movement of inorganic Na and K ions through
the membrane, where J m , is the observed rate constant for the In 1976, Neher and Sakmann developed the single-
+
flux of inorganic ions (Na ,K ) through the open receptor-formed channel current-recording technique. It is simple, conve-
+
transmembrane channel (see Eq. 3). In the continued presence of nient, and widely used for measuring the properties of a
neurotransmitter, the receptors reversibly form inactive forms I in single, open receptor channel, such as its conductance,
the 10- to 200-msec time region, depending on the receptor and lifetime, and ion specificity. In brief, a glass pipet with an
the concentration of neurotransmitter. This process is called re-
ceptor desensitization. (Reproduced with permission from Cash, internal diameter of 1 to 2 µ is attached to the surface of
D. J., Aoshima, H., and Hess, G. P. (1981) Proc. Natl. Acad. Sci. a cell membrane (Fig. 3A, left). Gentle suction is applied
USA 78, 3381–3322.) to isolate a small membrane patch within the glass pipet
from the rest of the membrane (Fig. 3A, right). A silver
chloride wire running from near the tip of the glass pipet
transmembrane voltage? In the nervous system, signal to electrical recording equipment allows one to record the
transmission is regulated by the binding of chemical current flowing through single receptor-formed channels
signals, neurotransmitters, to membrane-bound proteins, within the membrane patch (Fig. 3B). In the illustration,
called receptors. Commonly, when two molecules of a thedeviationofthecurrentfromthebaselinerepresentsthe
neurotransmitter have bound to the receptor, the protein current flowing through a single nicotinic acetylcholine
forms a transmembrane channel that remains open for a receptor-channel in the presence of 20 µM acetylcholine,
few milliseconds, allowing the receptor-specific passage which activates this channel. The Gaussian distribution of
of sodium, potassium, or chloride ions. The chemical re- the current amplitude gave a peak centered at 3 pA. The
action for many neurotransmitter receptors can be written exponential distribution of the time the channel remains
as shown in Fig. 2; in this case the kinetic mechanism of open (the lifetime distribution) gave a value of 2.4 msec
the nicotinic acetylcholine receptor is used as an example. for the mean lifetime of the open channel, τ op . This life-
This receptor plays an important role in signal transmis- time is a measure of the rate constant for channel closing
sion between nerve cells in the brain and between nerve 1/τ op = k cl (Fig. 2). Additionally, the technique allows one
and muscle cells (Fig. 1). to determine conveniently the ion specificity of the chan-
The specific reaction rate for the transmembrane flux of nel from measurements of the current amplitude.
inorganic cations controlled by the nicotinic acetylcholine In the case of transmembrane channels that open upon
¯
7
receptor, J m , has a value of about 5 × 10 M −1 sec −1 at binding a specific ligand, how does one determine the
◦
14 C. The relationship between the permeability coeffi- other parameters of the channel-opening reactions? These
cient P for a specific inorganic ion M (Eq. 2) and the are the dissociation constant of the neurotransmitter from
±
¯
specific reaction rate J m is given by: the site controlling channel opening, the rate constant for
¯
P M = J M R 0 (AL 2 ). (3) channel opening k op and, therefore, the equilibrium con-
±
±
stant for channel opening, 1/ = k op /k cl . In principle,
R 0 represents the moles of specific receptors in the cell many of these constants can be determined from the life-
membrane and (AL 2 ) the fraction of the receptors that are timeoftheclosedstate,thetimeintervalbetweentheopen-
in the open-channel form. Equations 2 and 3, therefore, ings of single receptor-channels (Fig. 3B). From the mech-
establish the important relationship between the receptor- anism in Fig. 2, a plot of the number of closures observed