Flexibility of Na,K-ATPase Secondary Structure upon Drug-Protein Interaction

H.A. Tajmir-Riahi*

Department of Chemistry-Biology, University of Quêbec at Trois-Rivières, C.P. 500, TR (Quêbec) Canada G9A 5H7

The enzyme Na⁺, K⁺-ATPase is an integral membrane protein which transports sodium and potassium cations against an electrochemical gradient. The transport of Na⁺ and K⁺ ions is connected to an oscillation of the enzyme between the two conformational states, the E₁ (Na⁺) and the E₂ (K⁺) conformations. The enzymatic activity of ATPase is largley affected by different ligands complexation. This review reports the effects of several drugs such as AZT (anti-AIDS), cis-Pt (antitumor), aspirin (anti-inflammatory) and vitamin C (antioxidant) on the stability and secondary structure of Na,K-ATPase in vitro. Drug-enzyme binding is mainly through H-bonding to the polypeptide C=O and C-N groups with two binding constants K_1(AZT) = 5.30 × 10⁵ M^-1 and K_2(AZT) = 9.80 × 10³ M^-1 for AZT and one binding constant for K_cis-Pt = 1.93 × 10⁴ M^-1, K_aspirin = 6.45 × 10³ M^-1 and K_ascorbate = 1.04 × 10⁴ M^-1 for cis-Pt, aspirin and ascorbic acid. The enzyme secondary structure was altered from that of α-helix 19.8% (free protein) to almost 22-26% and the b-sheet from 25.6% to 18-22%, upon drug complexation with the order of induced stability AZT > cis-Pt > ascorbate > aspirin.

Keywords: ATPase, Drug-Enzym, Binding mode, Binding constant, Secondary structure, UV-Vis and FTIR