Explorations of Sodium‐Potassium Adenosine Triphosphatase, Na+K+ ATPase, and the Inhibitory Cardiac Glycoside, Ouabain
Authors: Kywe Christopher , Kambhampati Lasya , Mackenzie Emerson , Meyer Mackenzie , Seeley Lucas , Smuckler Hannah , Smalley Daniel , Kessler Eric 
Published: June 2021

Sodium‐potassium adenosine triphosphatase (Na+K+ ATPase) resides in the plasma membranes of most animal cells, where it is involved in actively maintaining the cell's resting membrane potential. Though most commonly associated with its function in priming neurons for actions potentials, it is also essential for facilitating the transport of other molecules, regulating cell size, the transduction of particular intracellular signals, and for its role in cardiac muscle contraction. Ouabain, a plant derived cardiac glycoside has been found to interact with the cardiac specific isoform of Na+K+ ATPase with the indirect result of inducing stronger than normal muscle contractions. This property has led to ouabains use as a treatment for congestive heart failure. The Blue Valley North 2018–19 MSOE Center for BioMolecular Modeling SMART Team modeled the Na+K+ ATPase (PDB: 3KDP) using JMol with the goal of further exploring this protein's unique properties and its inhibition by ouabain and other toxins. Na+K+ ATPase is heterotrimeric complex formed from a, b, and g‐subunits. The a‐subunit contains the catalytic cytoplasmic and membrane domains. The catalytic domain is further subdivided into a nucleotide (N) binding domain responsible for binding ATP, the phosphorylation (P) domain that becomes phosphorylated upon ATP hydrolysis, and the actuator (A) domain responsible for translating conformational changes from the cytoplasmic portion of the protein to the membrane domain which controls the opening and occlusion of alternating half‐channels. Phosphorylation of the Na+K+ ATPase occurs at a highly conserved Asp369 residue found among P‐type pumps, while a conserved Glu found in the A domain is essential for dephosphorylation. Changes in the interactions between the three catalytic domains during phosphorylation and dephosphorylation are translocated into conformational changes in the membrane domain. The membrane domain is composed of 10 transmembrane helices (M1–M10), and contains the sites for ion binding and transport. The core membrane helices, M1–M6, contain two binding sites that are shared, asynchronously, by Na+ and K+. The third Na+ site is resides within the M7–M10 helices. Na+K+ ATPase cycles between two enzymatic states E1 and E2. The E1 state is characterized by a conformation with a half‐channel opened cytoplasmically, ATP and Na+ binding, and subsequent protein phosphorylation. In contrast, during the E2 state a half‐channel opens extracellularly, K+ binding occurs, and the protein is autodephosphoryled. Na+ export occurs during the E2 state while K+ import occurs during the E1 state, as these ions are exchanged with their counterpart. Overall, for each ATP hydrolyzed, 3 Na+ ions and 2 K+ ions are exchanged. Furthermore, the b‐subunit is facilitates appropriate membrane trafficking of the Na+K+ ATPase complex, while the g‐subunit is implicated in the regulation of transport.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.