Page 135 - Color Atlas of Biochemistry

P. 135

126 Metabolism

+
Energy conservation at membranes constantly maintained by the enzyme Na /
+
K -ATPase, which consumes ATP.
Metabolic energy can be stored not only in the
form of “energy-rich” bonds (see p. 122), but
also by separating electric charges from each B. Proton motive force
+
other using an insulating layer to prevent Hydronium ions (“H ions”) can also develop
them from redistributing. In the field of tech- electrochemical gradients. Such a proton gra-
nology, this type of system would be called a dient plays a decisive part in cellular ATP syn-
condenser. Using the same principle, energy is thesis (see p. 142). As usual, the energy con-
also stored (“conserved”) at cell membranes. tent of the gradient depends on the concen-
Themembranefunctions as an insulator; tration gradients—i. e., on the pH difference
electrically charged atoms and molecules ∋pH between the two sides of the membrane.
(ions)functionas charges. In addition, the membrane potential ∆ψ also
makes a contribution. Together, these two
values give the proton motive force ∆p, a
A. Electrochemical gradient +
measure for the work that the H gradient
Although artificial lipid membranes are al- can do. The proton gradient across the inner
most impermeable to ions, biological mem- mitochondrial membrane thus delivers ap-
+
branes contain ion channels that selectively proximately 24 kJ per mol H .
allow individual ion types to pass through
(see p. 222). Whether an ion can cross this
type of membrane, and if so in which direc- C. Energy conservation in proton gradients
tion, depends on the electrochemical gra- Proton gradients can be built up in various
dient—i. e., on the concentrations of the ion ways. A very unusual type is represented by
on each side of the membrane (the concen- bacteriorhodopsin (1), a light-driven proton
tration gradient)and on the difference in the pump that various bacteria use to produce
electrical potential between the interior and energy. As with rhodopsin in the eye, the
exterior, the membrane potential. light-sensitive component used here is cova-
The membrane potential of resting cells lently bound retinal (see p. 358). In photosyn-
(resting potential;see p. 350) is –0.05to thesis (see p. 130), reduced plastoquinone
–0.09 V—i. e., there is an excess negative (QH 2 ) transports protons, as well as electrons,
charge on the inner side of the plasma mem- through the membrane (Qcycle, 2). The for-
brane. The main contributors to the resting mation of the proton gradient by the respira-
+
+
potential are the two cations Na and K ,as tory chain is also coupled to redox processes
–
well as Cl and organic anions (1). Data on the (see p. 140). In complex III, a Q cycle is respon-
concentrations of these ions outside and in- sible for proton translocation (not shown). In
+
side animal cells, and permeability coef - cytochrome c oxidase (complex IV, 3), H trans-
cients, are shown in the table (2). port is coupled to electron flow from
The behavior of an ion type is described cytochrome c to O 2 .
+
quantitatively by the Nernst equation (3). In each of these cases, the H gradient is
∆ψ G is themembranepotential (in volts, V) utilized by an ATP synthase (4)to form ATP.
at which there is no net transport of the ion ATP synthases consist of two components—a
concerned across the membrane (equilibrium proton channel (F 0 )and an inwardly directed
potential). The factor R T/F n has a value of protein complex (F 1 ), which conserves the
0.026 V for monovalent ions at 25 °C. Thus, energy of back-flowing protons through ATP
+
for K ,the table (2)gives an equilibrium po- synthesis (see p. 142).
tential of ca. –0.09 V—i. e., a value more or less
thesameas that of the resting potential.By
+
contrast, for Na ions, ∆ψ G is much higher than
the resting potential, at +0.07 V. Na + ions
therefore immediately flow into the cell
+
when Na channels open (see p. 350). The
+
+
disequilibrium between Na and K ions is

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