Ouabain binding site in a functioning Na+/K+ ATPase.

Area of Science:

• Biochemistry
• Molecular Biology
• Cardiovascular Physiology

Background:

• The Na(+)/K(+) ATPase (sodium-potassium adenosine triphosphatase) is a vital integral membrane protein.
• Cardiotonic steroid derivatives, like ouabain, are crucial inhibitors targeting this enzyme in heart failure treatment.
• Previous functional and structural studies indicated widespread ouabain-sensitive sites, but precise binding locations remained debated.

Purpose of the Study:

• To precisely map the binding sites of ouabain on the Na(+)/K(+) ATPase.
• To elucidate the binding mechanism of cardiotonic steroids.
• To reconcile functional observations with structural data regarding ouabain interaction.

Main Methods:

• Utilized a spectroscopic approach to measure distances between fluorescent ouabain and lanthanide binding tags (LBTs).
• Engineered five functional LBT-Na(+)/K(+) ATPase constructs expressed in Xenopus laevis oocytes.
• Employed homology modeling to determine approximate ouabain positions based on spectroscopic data.

Main Results:

• Spectroscopic data revealed two distinct distances between ouabain and LBTs, suggesting two binding sites.
• Ouabain binds to an external site (low affinity) and a deeper, intracellular-facing site (high affinity).
• The lactone ring of ouabain consistently faces outward in both binding configurations.

Conclusions:

• Ouabain interacts with the Na(+)/K(+) ATPase at two distinct locations within the ion permeation pathway.
• A sequential binding mechanism is proposed, explaining ouabain's interaction with the enzyme.
• Findings provide a refined structural understanding of cardiotonic steroid action on the Na(+)/K(+) ATPase.