High Pressure Bioscience by Kazuyuki Akasaka & Hitoshi Matsuki

Author:Kazuyuki Akasaka & Hitoshi Matsuki
Language: eng
Format: epub
Publisher: Springer Netherlands, Dordrecht

17.8 Reconstituted and Biological Membranes

Cellular membranes are complex heterogeneous aggregate structures, held together by the hydrophobic effect. The integrity and functionality of these membranes are vital for the cell, e.g., for energy production, transport, signaling processes, and maintenance of osmotic pressure and intracellular pH. Although some ion transporters are unaffected or even activated upon mild compression, certain other channels and pumps are inactivated at moderate to high pressures. It has often been observed that at pressures of several hundred MPa, membrane protein function ceases, and integral and peripheral proteins may even become detached from the membrane when its bilayer is sufficiently ordered by pressure, and depolymerization of attached cytoskeletal proteins may be involved as well. For example, the influence of hydrostatic pressure on the activity of Na+, K+-ATPase enriched in the plasma membrane from rabbit kidney outer medulla was studied using a kinetic assay. The data shown in Fig. 17.10 reveal that the activity, k, of the Na+, K+-ATPase is retarded by pressures below 200 MPa. The plot of lnk vs. p revealed an apparent activation volume of the pressure-induced inhibition reaction which amounts to ΔV ≠ = 47 mL mol−1. At higher pressures, exceeding 200 MPa, the enzyme is inactivated irreversibly (De Smedt et al. 1979; Chong et al. 1985). Kato et al. (2002) suggested that the activity of the enzyme shows at least three step changes induced by pressure: at pressures below and around 100 MPa, a decrease in the fluidity of the lipid bilayer and a reversible conformational change in the transmembrane protein is induced, leading to functional disorder of the membrane-associated ATPase activity. Pressures of 100–200 MPa cause a reversible phase transition and the dissociation or conformational changes in the protein subunits, and pressures exceeding 220 MPa irreversibly degrade the lipid-protein assembly due to protein unfolding and separation.

Fig. 17.10Activity k (in arbitrary units) of Na+, K+-ATPase – as measured using an enzymatic assay – at selected pressures and T = 37 °C. The free energy of hydrolysis of one ATP molecule is converted to up-hill transport by actively transporting 3 Na+ ions out of and 2 K+ into the cell (Modified from Winter and Jeworrek 2009; Meersmann et al. 2013)

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