The mitochondrial ADP/ATP carrier is essential for cellular energy production by exchanging ADP and ATP across the mitochondrial inner membrane. However, a fundamental question has long puzzled researchers: How does this carrier import negatively charged ADP against a strong membrane potential while exporting ATP?

Mitochondria maintain a membrane potential of about 180 mV, which is essential for ATP synthesis. This potential creates a significant barrier for the import of negatively charged molecules like ADP. Understanding how the ADP/ATP carrier overcomes this barrier is crucial for our comprehension of cellular bioenergetics.

In a recent study, a team led by Edmund R. S. Kunji demonstrated that the carrier uses a clever mechanism involving positive charge movements to facilitate both ADP import and ATP export. Their key findings include:

1. The carrier translocates 3.3 positive charges along with each negatively charged substrate.
2. These positive charges are associated with three key residues in the central substrate-binding site and two asparagine/arginine pairs.
3. This mechanism results in a net positive charge movement (+0.3) during ADP import and a net negative charge movement (-0.7) during ATP export.
4. In this way, the membrane potential stimulates ADP import and ATP export while inhibiting ADP export and ATP import.

To study these charge movements, the researchers employed solid-supported membrane electrophysiology (SSME), a powerful technique for investigating membrane transporters:

1. The ADP/ATP carrier was reconstituted into liposomes and adsorbed onto a solid-supported membrane.
2. Capacitive currents generated by substrate transport were measured.
3. These currents were integrated to quantify charge movements during transport.

SSME allowed the researchers to directly measure and compare charge movements for different substrates and carrier variants, providing crucial insights into the transport mechanism.

In conclusion, this work resolves a long-standing puzzle in mitochondrial bioenergetics. By demonstrating how positive charge movements facilitate both ADP import and ATP export, the study provides a mechanistic explanation for the carrier’s ability to maintain high, directional adenine nucleotide exchange rates across the mitochondrial inner membrane, supporting the energy demands of eukaryotic cells.

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Find the full article here: Membrane potential stimulates ADP import and ATP export by the mitochondrial ADP/ATP carrier due to its positively charged binding site

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