Topic 6.4 Kinetic Analysis of Multiple Transporter Systems
Studies of Km values have shown a feedback inhibition (see Chapter 2 on this website) of carrier-mediated transport that allows the cell to maintain stable cytoplasmic ion concentrations over a wide range of external concentrations. For example, the apparent Km value of the high-affinity K+ carrier in barley roots changes with the plant's demand for K+. Barley plants starved for K+ have high-affinity Km values in the range of 0.02 to 0.03 mM, whereas well-fertilized plants have Km values that are four- to fivefold greater (Web Figure 6.4.A). As explained in textbook Chapter 6, an increase in Km reflects a decrease in the carrier's affinity for K+ (Epstein 1972).
Web Figure 6.4.A The transport of potassium into barley roots shows two different phases. The biphasic kinetics of potassium uptake, accentuated in this figure by the change of scale at around 1 mM, suggests the presence of different types of transport systems for potassium. The high-affinity transport system, having a Km value of 0.02 to 0.03 mM, is attributed to active transport by symporters; the low-affinity system (which may or may not show saturation) is attributed to diffusion through K+ channels. (After Epstein 1972.)
The change in apparent Km probably reflects the expression of different members of a gene family, with their gene products thereby matching cellular needs to environmental ionic conditions (Schachtman 2000; Maser et al. 2001). If the plants’ adjustments were merely a matter of adding more transporters to the plasma membrane, there would be change in Vmax without a change in Km. This is not what is observed for K+. However, plants that have been starved and then resupplied with sulfate or phosphate often respond by an increase in Vmax. As the internal sulfate and phosphate levels recover, the Vmax values for their uptake drop to the usual values as excess carriers are removed from the membrane (Glass 1983).