TABLE OF CONTENTS All of the bioenergetic enzymes described in this book can couple an exergonic or free-energy yielding reaction to the electrogenic movement of charged species across the membrane, generating a protonmotive force. In the case of bacteriorhodopsin, the driving reaction is the absorption of a photon, for the bc 1 complex, the oxidation of ubiquinol by cytochrome c is the driving force, and for the respiratory oxidases, the reduction of O 2 to H 2O provides the impetus. In this book, the principles utilized by a number of these systems are detailed with an emphasis on recent structural studies.
It is convenient to classify two classes of mechanisms used to generate a trans-membrane voltage:
Mechanisms utilizing an oxidoreduction loop.
True ion (proton) pumps.
The principle of coupling different chemical reactions is central to biology and is accomplished in a number of ways. Many of the systems that generate a protonmotive force can be understood in terms of Mitchell's chemiosmotic oxidoreduction loop. [1] This is illustrated by the example shown in Figure 1, which shows a redox loop formed from the anaerobic respiratory system comprised of formate dehydrogenase and nitrate reductase enzymes from E. coli. Recently, the structures of each of these two enzymes were determined. [2], [3] The topology of the catalytic active sites assures that the net...
