Channel Proteins Usually Dissipate Solute Gradients

In contrast to transport proteins that form (Chapter 5) or propagate (Chapter 6) solute gradients by coupling transport to conspicuous sources of free energy, channel proteins appear to form pores through which ions pass by diffusion. The unusual characteristics of the proposed diffusion have been attributed to the narrowness of the channels and the effects on diffusion that such physical constraints produce. As the reader is by now aware, however, we view the transport catalyzed by channels as involving intimate contact between the transport protein and its substrate, as is the case for proteins that produce and propagate solute gradients. Moreover, it is likely to us that during their transport cycles, all of these proteins undergo at least small changes in their conformations that resemble such changes in enzymes and other transport proteins (e.g., Marban and Tomaselli, 1997 ). 1 This view also is supported by computer simulation of water migration across membranes via voltage-gated Na + channels. In this simulation, water and presumably Na + migration can occur when the channel protein is allowed to have the motion that it would have at 300°K but not when its motion is virtually frozen (Jakobsson, 1997). While the magnitudes of such protein motions that are needed for transport may vary widely among the proteins, we propose that virtually all transport proteins need to move to some extent in order to catalyze solute or solvent migration across biomembranes (see also Section III below).