Rate limitation of the Na(+),K(+)-ATPase pump cycle

Area of Science:

• Biochemistry
• Enzyme kinetics
• Membrane transport

Background:

• The Na(+),K(+)-ATPase (sodium-potassium pump) is crucial for maintaining cellular ion gradients.
• Its function involves a cycle of conformational changes and phosphorylation.
• Understanding the rate-limiting steps is key to comprehending its overall efficiency.

Purpose of the Study:

• To investigate the kinetics of Na(+)-dependent phosphorylation of the Na(+),K(+)-ATPase.
• To determine the role of the E(2) to E(1) conformational transition in the pump cycle.
• To identify the rate-determining step in the Na(+),K(+)-ATPase enzymatic cycle.

Main Methods:

• Stopped-flow technique with the fluorescent label RH421.
• Investigated Na(+)-dependent ATP phosphorylation kinetics under saturating conditions.
• Manipulated buffer composition to favor specific enzyme conformations (E(2) vs. E(1)).

Main Results:

• The E(2) --> E(1) transition rate was found to limit subsequent phosphorylation reactions.
• Rate constants for the E(2) --> E(1) transition were estimated at ~65 s⁻¹ (pig kidney) and ~90 s⁻¹ (rabbit kidney).
• Computer simulations confirmed these transition rates are consistent with overall enzyme turnover numbers.

Conclusions:

• The E(2) --> E(1) conformational change is the major rate-determining step for the alpha(1) isoform of Na(+),K(+)-ATPase.
• This finding clarifies the mechanism controlling the pump's catalytic cycle efficiency.
• The study provides crucial insights into the regulation of ion transport by the Na(+),K(+)-ATPase.