chapter 14
Electron Transport and Oxidative Phosphorylation
F IG U R E 1 4-18
Structure of nigericin (a) and its complex with K+ (b). Nigericin is a mobile or channel-forming ionophore; at one side
of the membrane, the anionic form complexes with K+ and then migrates to the other side, where K+ is discharged and
the anionic form is protonated. The protonated form migrates to the opposite side and discharges H+, and the anionic
species formed initiates the next cycle. Thus, nigericin exchanges H+ for K+ across the membrane and uncouples
oxidative phosphorylation. Nigericin, a polycyclic ether carboxylate like valinomycin, forms a complex with K+ that
diffuses through the membrane.
carboxylate group interacts with the cation to form an
electroneutral complex. In the exchange of H+ for K+,
although the membrane potential is not altered, the pH
differential is abolished, thereby uncoupling oxidative
Gramicidin A (Figure 14-19), a linear peptide, forms a
head-to-head dimer that creates an aqueous helical channel
which allows transport of a variety of monovalent cations
(e.g., Na+, K+, and H+). The channel is a helix formed
by the coiling of two /3-pleated sheets and is lined with
H—C—N—Val— G1 y—Ala—Leu—Ala—Va 1
— Val— Val —Trp—Leu—Trp—Leu—Trp—Leu—Trp—N—C ^ —C ^ —OH
F IG U R E 1 4 -1 9
Structure of gramicidin A (a) and the formation of a helical pore through a lipid bilayer by assembly of two
gramicidin A molecules via their formyl groups at the N termini (b). A variety of monovalent cations are transported
through the static pore. [Structure (b) is reproduced with permission from Y.A. Ovchinnikov; Physico-chemical basis
of ion transport through biological membranes: Ionophores and ion channels.
Eur. J. B iochem .
94, 321 (1979).]
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