19.7 Secondary Active and Coupled Transport

Secondary Active and Coupled Transport describes the movement of a solute across a synthetic cell boundary against its own unfavorable driving force, powered not by direct energy consumption but by coupling that movement to the favorable, downhill movement of a second solute, typically an ion, along a gradient established by a separate primary active transport process. It covers the coupling patterns transport proteins use, the mechanistic cycle of coupled binding and release, and how the energy stored in a driving gradient is converted into transport of the coupled solute.

Basic Concept and Common Driving Gradients

Synthetic Cell Secondary Active Transport

Synthetic cell secondary active transport is the general concept of solute movement powered indirectly through coupling to a separately established gradient, distinguishing this mechanism from primary active transport, which draws energy directly from a chemical reaction, light, or redox process.

Ion Gradient-Driven Solute Transport

Ion gradient-driven solute transport is secondary active transport in which the downhill movement of an ion supplies the energy driving the coupled uphill movement of a second solute.

Proton-Coupled Solute Transport

Proton-coupled solute transport is a specific instance of ion gradient-driven transport in which the driving ion is a proton, with the proton's gradient supplying the energy for coupled solute movement.

Sodium-Coupled Solute Transport

Sodium-coupled solute transport is a specific instance of ion gradient-driven transport in which the driving ion is sodium, with the sodium gradient supplying the energy for coupled solute movement.

Coupling Patterns

Membrane Transport Symport

Membrane transport symport is a coupling pattern in which the driving ion and the coupled solute move in the same direction across the boundary during a single transport cycle.

Membrane Transport Antiport

Membrane transport antiport is a coupling pattern in which the driving ion and the coupled solute move in opposite directions across the boundary during a single transport cycle.

Quantitative Coupling Characteristics

Coupled Transport Stoichiometry

Coupled transport stoichiometry is the specific numerical ratio between the number of driving ions moved and the number of coupled solute molecules moved per transport cycle, a defining quantitative characteristic of a given coupled transporter.

Coupled Transport Directionality

Coupled transport directionality describes the fixed relationship between driving ion movement and coupled solute movement established by the transporter's symport or antiport mechanism, determining the overall net direction of the coupled transport event.

The Coupled Transport Cycle

Driving Ion Binding

Driving ion binding is the step in which the driving ion attaches to its binding site on the transporter, initiating the coupled transport cycle.

Coupled Solute Binding

Coupled solute binding is the step in which the coupled solute attaches to its own distinct binding site on the transporter, occurring alongside driving ion binding to set up the coupled transport event.

Coupled Substrate Translocation

Coupled substrate translocation is the conformational transition step in which both the driving ion and the coupled solute are moved together across the boundary as a result of the transporter's structural cycle.

Driving Ion Release

Driving ion release is the step in which the driving ion is released from the transporter on the side of the boundary favored by its own gradient, completing its downhill movement.

Coupled Solute Release

Coupled solute release is the step in which the coupled solute is released from the transporter on the opposite or same side as the driving ion depending on the symport or antiport pattern, completing its uphill movement.

Energy Conversion and Persistence

Gradient-Transport Energy Conversion

Gradient-transport energy conversion describes the underlying process by which the free energy stored in the driving ion's gradient is converted into work moving the coupled solute against its own unfavorable gradient.

Secondary Transport Reversal

Secondary transport reversal describes a change in the net direction of coupled transport that occurs if the relative strength of the driving ion gradient and the coupled solute gradient shifts sufficiently, since the coupled mechanism itself does not inherently fix an absolute direction independent of the underlying gradients.

Coupled Transport Slippage

Coupled transport slippage describes an imperfection in the coupling mechanism in which the driving ion moves through the transporter without a corresponding movement of the coupled solute, reducing the efficiency of the coupled transport process.

System-Level Dependence

Gradient Depletion by Transport

Gradient depletion by transport describes the reduction of the driving ion's gradient that occurs as ongoing secondary active transport consumes that gradient, requiring the gradient to be actively maintained or replenished for coupled transport to continue.

Secondary Transport Dependence

Secondary transport dependence describes the overall reliance of secondary active transport on the continued existence of its driving gradient, meaning secondary transport cannot function independently but instead depends entirely on primary active transport or another gradient-establishing process to sustain the driving ion's favorable gradient over time.