Final answer:
ABC importers that use a periplasmic binding protein work through a series of conformational changes powered by ATP hydrolysis, making the transport essentially irreversible due to the energy dependency and the consequent low affinity for the solute once transported inside the cell.
Step-by-step explanation:
The action mechanism of ABC importers that utilize a periplasmic binding protein involves several steps. First, the periplasmic binding protein captures the molecule to be transported (solute) in the periplasmic space. This binding induces a conformational change in the protein, which presents the solute to the transporter. The ABC transporter, powered by ATP hydrolysis, then undergoes a conformational change that translocates the solute into the cytoplasm. This process is essentially irreversible because the hydrolysis of ATP leads to a conformational change in the carrier protein, which reduces its affinity for the solute, causing the solute to be released inside the cell. The transporter then returns to its original state, ready for another cycle, but since ATP has been converted to ADP and inorganic phosphate, the process cannot readily reverse.
These transporters are considered irreversible due to the energy input from ATP hydrolysis, which drives the solute transport against a concentration gradient and prevents the solute from moving back in the opposite direction. This is akin to irreversible enzyme inhibition, where a covalent bond forms at the active site and permanently inactivates the enzyme. Similarly, the energy input from ATP in ABC importers creates a 'one-way' system for solute movement.