262

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

phosphorylation.

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

O

H

H

I

l

I

L

LDLDLDLDLDLDL}

H—C—N—Val— G1 y—Ala—Leu—Ala—Va 1

— Val— Val —Trp—Leu—Trp—Leu—Trp—Leu—Trp—N—C ^ —C ^ —OH

(a)

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).]