Na⁺ Translocation Dominates over H⁺-Translocation in the Membrane Pyrophosphatase with Dual Transport Specificity

Area of Science:

• Biochemistry
• Molecular Biology
• Membrane Transport

Background:

• Cation-pumping membrane pyrophosphatases (mPPases) are crucial membrane proteins involved in ion transport across cellular membranes.
• These enzymes exhibit diverse transport specificities, ranging from obligatory proton (H+) transporters to sodium/proton (Na+/H+) co-transporters.
• Existing models suggest a direct-coupling mechanism for H+ transport and a 'billiard' mechanism for Na+ transport, but these do not fully explain simultaneous Na+/H+ transport.

Purpose of the Study:

• To investigate the transport stoichiometry and mechanism of the Na⁺,H⁺-co-transporter from *Bacteroides vulgatus*.
• To explore the relationship between sodium (Na+) and proton (H+) transport reactions catalyzed by this enzyme.
• To elucidate the underlying mechanism of simultaneous cation transport in Na⁺,H⁺-PPases.

Main Methods:

• Utilized a pyranine-based fluorescent assay to monitor cation translocation across membrane vesicles.
• Employed a stopped-flow technique for real-time kinetic measurements.
• Manipulated experimental conditions, including the use of an H+ ionophore, to selectively measure Na+ or H+ flux.

Main Results:

• Demonstrated that the *Bacteroides vulgatus* Na⁺,H⁺-PPase transports Na⁺ and H⁺ in a fixed ratio of approximately 1:8.
• Observed that this transport ratio is independent of the external Na⁺ concentration.
• The findings support an 'extended billiard' model, indicating kinetic limitations in Na+ delivery to the enzyme's loading site.

Conclusions:

• The Na⁺,H⁺-PPase exhibits a distinct transport stoichiometry, with a preference for Na+ over H+.
• The 'extended billiard' model provides a framework for understanding the observed transport mechanism.
• Kinetic factors, specifically the rate of Na+ delivery, appear to be rate-limiting in the overall transport process.