A Maxwell's Demon Type of Membrane Transport: Possibility for Active Transport by ABC-Type Transporters?

✍ Scribed by ULRICH HOPFER

• Publisher: Elsevier Science
• Year: 2002
• Tongue: English
• Weight: 188 KB
• Volume: 214
• Category: Article
• ISSN: 0022-5193
• DOI: 10.1006/jtbi.2001.2479

✦ Synopsis

ATP-binding-cassette (or ABC)-type transporters constitute one of the largest family of membrane transporters in nature. Many of its members move substrates &&actively'', i.e. in an ATP-dependent manner against an electrochemical gradient. No consensus is available about the mechanism. Therefore, a novel class of transport mechanisms is proposed based on Maxwell's demon idea. This transport mechanism consists of a gated pore that selectively opens for substrates from one, but not the other side. Thermoenergy (Brownian motion) would su$ce for substrate translocation across the membrane; energy for synchronizing gate opening with substrate arrival would come from ATP hydrolysis. Simulations demonstrate that such a mechanism would be thermodynamically and kinetically feasible. It exhibits &&active'', unidirectional transport, saturation, and other typical features of protein-catalysed reactions. It also shows pore behavior with charged substrates moving under the in#uence of electrical potentials. Its e$ciency depends on a di!usion time constant of the substrate in solution that is slower than the transit time through the membrane, a situation that can realistically be achieved at millimolar or lower substrate concentrations. Features of the novel mechanism that di!er signi"cantly from P-or F-type ATPases are: (1) transport cannot be run in &&reverse'' to synthesize ATP even if su$cient energy is available in the gradient of the transported solute and (2) unidirectional and net substrate #uxes through the transporter diverge with increasing substrate concentration.