Structural changes in the catalytic cycle of the Na+,K+-ATPase studied by infrared spectroscopy

Na+/K+-ATPase Revisited: On Its Mechanism of Action, Role in Cancer, and Activity Modulation

Maintenance of Na⁺ and K⁺ gradients across the cell plasma membrane is an essential process for mammalian cell survival. An enzyme responsible for this process, sodium-potassium ATPase (NKA), has been currently extensively studied as a potential anticancer target, especially in lung cancer and glioblastoma. To date, many NKA inhibitors, mainly of natural origin from the family of cardiac steroids (CSs), have been reported and extensively studied. Interestingly, upon CS binding to NKA at nontoxic doses, the role of NKA as a receptor is activated and intracellular signaling is triggered, upon which cancer cell death occurs, which lies in the expression of different NKA isoforms than in healthy cells. Two major CSs, digoxin and digitoxin, originally used for the treatment of cardiac arrhythmias, are also being tested for another indication—cancer. Such drug repositioning has a big advantage in smoother approval processes. Besides this, novel CS derivatives with improved performance are being developed and evaluated in combination therapy. This article deals with the NKA structure, mechanism of action, activity modulation, and its most important inhibitors, some of which could serve not only as a powerful tool to combat cancer, but also help to decipher the so-far poorly understood NKA regulation.

Probing the activity of a recombinant Zn2+ -transporting P-type ATPase

P-type ATPase proteins maintain cellular homeostasis and uphold critical concentration gradients by ATP-driven ion transport across biological membranes. Characterization of single-cycle dynamics by time-resolved X-ray scattering techniques in solution could resolve structural intermediates not amendable to for example crystallization or cryo-electron microscopy sample preparation. To pave way for such time-resolved experiments, we used biochemical activity measurements, Attenuated Total Reflectance (ATR) and time-dependent Fourier-Transform Infra-Red (FTIR) spectroscopy to identify optimal conditions for activating a Zn²⁺ -transporting Type-I ATPase from Shigella sonnei (ssZntA) at high protein concentration using caged ATP. The highest total activity was observed at a protein concentration of 25 mg/mL, at 310 K, pH 7, and required the presence of 20% (v/v) glycerol as stabilizing agent. Neither the presence of caged ATP nor increasing lipid-to-protein ratio affected the hydrolysis activity significantly. This work also paves way for characterization of recombinant metal-transporting (Type-I) ATPase mutants with medical relevance.

Time-resolved Fourier transform infrared spectroscopy of the nucleotide-binding domain from the ATP-binding Cassette transporter MsbA: ATP hydrolysis is the rate-limiting step in the catalytic cycle

MsbA is an essential Escherichia coli ATP-binding cassette (ABC) transporter involved in the flipping of lipid A across the cytoplasmic membrane. It is a close homologue of human P-glycoprotein involved in multidrug resistance, and it similarly accepts a variety of small hydrophobic xenobiotics as transport substrates. X-ray structures of three full-length ABC multidrug exporters (including MsbA) have been published recently and reveal large conformational changes during the transport cycle. However, how ATP hydrolysis couples to these conformational changes and finally the transport is still an open question. We employed time-resolved FTIR spectroscopy, a powerful method to elucidate molecular reaction mechanisms of soluble and membrane proteins, to address this question with high spatiotemporal resolution. Here, we monitored the hydrolysis reaction in the nucleotide-binding domain of MsbA at the atomic level. The isolated MsbA nucleotide-binding domain hydrolyzed ATP with V(max) = 45 nmol mg(-1) min(-1), similar to the full-length transporter. A Hill coefficient of 1.49 demonstrates positive cooperativity between the two catalytic sites formed upon dimerization. Global fit analysis of time-resolved FTIR data revealed two apparent rate constants of ~1 and 0.01 s(-1), which were assigned to formation of the catalytic site and hydrolysis, respectively. Using isotopically labeled ATP, we identified specific marker bands for protein-bound ATP (1245 cm(-1)), ADP (1101 and 1205 cm(-1)), and free phosphate (1078 cm(-1)). Cleavage of the β-phosphate-γ-phosphate bond was found to be the rate-limiting step; no protein-bound phosphate intermediate was resolved.