Uncoupled Active Transport Mechanisms Accounting for Low Selectivity in Multidrug Carriers: P-Glycoprotein and SMR Antiporters

Abstract

The extraordinarily low substrate specificity of P-glycoprotein conflicts with the notion that specific substrate interactions are required in the control of the reaction path in an active transport system. The difficulty is shown to be overcome by a half-coupled mechanism in which the ATP reaction is linked to carrier transformations, as in a fully coupled system, but in which the transported substrate plays a passive role. The mechanism, which requires no specific interaction with the substrate, brings about uphill transport. A half-coupled mechanism is directly supported by two observations: (i) almost completely uncoupled ATPase activity in purified P-glycoprotein, and (ii) a pattern of substrate specificity like that of passive systems, where maximum rates for different substrates vary little (unlike active systems, where maximum rates vary greatly). The mechanism accommodates other findings: partial inhibition of ATPase activity by an actively transported substrate; simultaneous binding and translocation of more than one substrate molecule; and stimulation or inhibition of the transport of one substrate molecule by another. A half-coupled system associated with an internal competitive inhibitor should behave as if tightly coupled, in agreement with the effects of the synthetic peptide, polytryptophan. The degree of coupling in the intact system is yet to be determined, however. A half-coupled ATPase mechanism could originally have evolved in a flippase, where immersion of the carrier in its substrate, the membrane lipid, precludes uncoupled ATP hydrolysis. These concepts may have wider application. An uncoupled antiport mechanism, driven by a proton gradient rather than ATP, can explain low selectivity in the SMR multidrug carriers of bacteria, and a half-coupled mechanism for the ion-driven cotransport of water (the substrate in which the carrier site is immersed) can explain a recently proposed uphill flow of water.

Human Multidrug Resistance ABCB and ABCG Transporters: Participation in a Chemoimmunity Defense System

“…Most probably both mechanisms may occur, differently for different substrates of variable size and hydrophobicity. In fact, it has been proposed that MDR1/Pgp evolved through mutations that expanded its substrate specificity from lipids to lipidsoluble toxic chemicals (194). Flippase activity has been attributed to other related transporters (see sect.…”

“…A partially uncoupled power source may be ideally suited for a transporter whose task is to handle compounds newly presented to the organism (194). As a prominent multidrug transporter, MDR1/Pgp has to retain a very low selectivity to substrates.…”

Structure and mechanism of ATP-dependent phospholipid transporters

“…However, it has been shown that some of these transporters can be partially uncoupled in reconstituted systems [81,82]. A quest to understand the uncoupling process has led to extensive evidence that the two NBDs bind and hydrolyze ATP in a cooperative manner [83][84][85][86].…”

The Role of ABC Multidrug Transporters in Resistance to Targeted Anticancer Kinase Inhibitors

“…This relatively high basal ABCG2 ATPase activity had previously been believed to represent either an uncoupled state of the transporter or its ATPase stimulation by an unknown substrate resident in the insect membrane [ 94 ]. An uncoupled ATPase activity was also suggested to be important for the promiscuous drug recognition capability of multidrug transporters [ 102 ]. Studies performed on purifi ed and reconstituted functional ABCG2 proposed that the basal ABCG2 ATPase is dependent on the lipid environment and especially on the presence of cholesterol but may not be coupled to any transport activity [ 97 ].…”