RH421 binds into the ATP-binding site on the Na+/K+-ATPase

Na,K-ATPase: A murzyme facilitating thermodynamic equilibriums at the membrane-interface

The redox metabolic paradigm of murburn concept advocates that diffusible reactive species (DRS, particularly oxygen-centric radicals) are mainstays of physiology, and not mere pathological manifestations. The murburn purview of cellular function also integrates the essential principles of bioenergetics, thermogenesis, homeostasis, electrophysiology, and coherence. In this context, any enzyme that generates/modulates/utilizes/sustains DRS functionality is called a murzyme. We have demonstrated that several water-soluble (peroxidases, lactate dehydrogenase, hemogoblin, etc.) and membrane-embedded (Complexes I-V in mitochondria, Photosystems I/II in chloroplasts, rhodopsin/transducin in rod cells, etc.) proteins serve as murzymes. The membrane protein of Na,K-ATPase (NKA, also known as sodium-potassium pump) is the focus of this article, owing to its centrality in neuro-cardio-musculo electrophysiology. Herein, via a series of critical queries starting from the geometric/spatio-temporal considerations of diffusion/mass transfer of solutes in cells to an update on structural/distributional features of NKA in diverse cellular systems, and from various mechanistic aspects of ion-transport (thermodynamics, osmoregulation, evolutionary dictates, etc.) to assays/explanations of inhibitory principles like cardiotonic steroids (CTS), we first highlight some unresolved problems in the field. Thereafter, we propose and apply a minimalist murburn model of trans-membrane ion-differentiation by NKA to address the physiological inhibitory effects of trans-dermal peptide, lithium ion, volatile anesthetics, confirmed interfacial DRS + proton modulators like nitrophenolics and unsaturated fatty acid, and the diverse classes of molecules like CTS, arginine, oximes, etc. These explanations find a pan-systemic connectivity with the inhibitions/uncouplings of other membrane proteins in cells.

Biochemical Characterization and Allosteric Modulation by Magnesium of (Na+, K+)-ATPase Activity in the Gills of the Red Mangrove Crab Goniopsis cruentata (Brachyura, Grapsidae)

We provide a kinetic characterization of (Na⁺, K⁺)-ATPase activity in a posterior gill microsomal fraction from the grapsid crab Goniopsis cruentata. (Na⁺, K⁺)-ATPase activity constitutes 95% of total ATPase activity, and sucrose density centrifugation reveals an ATPase activity peak between 25 and 35% sucrose, distributed into two, partially separated protein fractions. The (Na⁺, K⁺)-ATPase α-subunit is localized throughout the ionocyte cytoplasm and has an M_r of ≈ 10 kDa and hydrolyzes ATP obeying cooperative kinetics. Low (V_M = 186.0 ± 9.3 nmol Pi min^-1 mg^-1 protein and K_0.5 = 0.085 ± 0.004 mmol L^-1) and high (V_M = 153.4 ± 7.7 nmol Pi min^-1 mg^-1 protein and K_0.5 = 0.013 ± 0.0006 mmol L^-1) affinity ATP binding sites were characterized. At low ATP concentrations, excess Mg²⁺ stimulates the enzyme, triggering exposure of a high-affinity binding site that accounts for 50% of (Na⁺, K⁺)-ATPase activity. Stimulation by Mg²⁺ (V_M = 425.9 ± 25.5 nmol Pi min^-1 mg^-1 protein, K_0.5 = 0.16 ± 0.01 mmol L^-1), K⁺ (V_M = 485.3 ± 24.3 nmol Pi min^-1 mg^-1 protein, K_0.5 = 0.9 ± 0.05 mmol L^-1), Na⁺ (V_M = 425.0 ± 23.4 nmol Pi min^-1 mg^-1 protein, K_0.5 = 5.1 ± 0.3 mmol L^-1) and NH₄⁺ (V_M = 497.9 ± 24.9 nmol Pi min^-1 mg^-1 protein, K_0.5 = 9.7 ± 0.5 mmol L^-1) obeys cooperative kinetics. Ouabain inhibits up to 95% of ATPase activity with K_I = 196.6 ± 9.8 µmol L^-1. This first kinetic characterization of the gill (Na⁺, K⁺)-ATPase in Goniopsis cruentata enables better comprehension of the biochemical underpinnings of osmoregulatory ability in this semi-terrestrial mangrove crab.