Which of the following shows the correct order of steps for the mechanism of the sodium-potassium ATPase?
I. 2 K+ Ions bind.
II. Phosphorylation of an Asp residue.
III. Conformational change releasing 3 Na+ ions outside the cell.
IV. 3 Na+ ions bind.
V. Release of the phosphate group.
VI. Conformational change releasing 2 K+ ions inside the cell.

• A. II, I, V, III, IV, VI.
• B. IV, II, III, I, V, VI.
• C. I, II, III, IV, V, VI.
• D. IV, II, I, III, V, VI.
• E. IV, III, I, II, VI, V.

Answer: (B)

Verified step by step guidance

1. Understand the function of sodium-potassium ATPase: It is an enzyme that pumps sodium (Na+) out of cells and potassium (K+) into cells, using energy from ATP hydrolysis.
2. Identify the initial step: The process begins with the binding of 3 Na+ ions from the cytoplasm to the enzyme.
3. Next, ATP is used to phosphorylate the enzyme, specifically at an aspartate residue, which causes a conformational change.
4. This conformational change releases the 3 Na+ ions outside the cell and allows 2 K+ ions to bind from the extracellular space.
5. Finally, the phosphate group is released, causing another conformational change that releases the 2 K+ ions inside the cell, resetting the enzyme to its original state.