Chapter 10 : Metabolite Transport by Facilitated Diffusion, 1900 to 2000

Abstract:

This chapter on metabolite transport by facilitated diffusion describes some of the history of studying how molecules move across cell membranes, particularly yeast membranes. In discussing how substances both enter and leave yeast cells, it is necessary to consider the structures through which the substances must pass, namely, the cell wall and the plasma membrane, as well as intracellular membranes. Steveninck and Rothstein suggested that while uptake involves sugar phosphorylation, when glycolysis is prevented by iodoacetate, sugar uptake occurs by facilitated diffusion. The results of pulse-labeling experiments indicated that uptake of galactose by baker’s yeast was adaptive and probably involved entry of the free sugar and its accumulation to diffusion equilibrium. The following three independent findings were consistent with this conclusion. First, the high-affinity mode of galactose uptake was found to depend on the presence of galactokinase. Second, from pulse-labeling studies of glucose uptake by a wild-type yeast, via a carrier of low Km, Kotyk published convincing evidence that free sugar in the intracellular pool was labeled first. Finally, the route with a high Km, in a kinaseless yeast strain, conformed to the established pattern of a facilitated diffusion pathway. The results suggest that high-affinity glucose transport is not necessarily dependent on the presence of glucose-phosphorylating enzymes. Apparent low-affinity uptake kinetics can arise as a consequence of an insufficient rate of removal of intracellular free glucose by phosphorylation.