Membranes extrinsic effects

A closer look at the properties of the bilayer shows that certain characteristics of the solution bathing a membrane are likely to influence its physical state and, thereby, affect its sensitivity to temperature. Although these extrinsic effects remain much less studied than effects involving changes in lipid composition, there is increasing evidence that temperature-dependent changes in the intra- and extracellular fluids affect membranes as well as cytosolic proteins. 

In view of the partially nonpolar properties of the phenylpyruvate and caprylate anions and of the lipid solubility of their protonation products which are in equilibrium with the anions, it is likely that these organic salts not only release extrinsic hydrophilic proteins which are attached to the membranes, but also affect hydrophobic bonding within the membrane structure. Such effects cannot be seen in protein release experiments because apolar proteins remain insoluble. 

Membrane proteins have a profound influence on the thermotropic properties of lipid bilayers. Moreover, the effects of intrinsic (or integral) proteins are very different from those of extrinsic (or peripheral) proteins, reflecting their different modes of interaction, respectively, from the hydrophobic and hydrophyllic moieties of lipids. Some proteins can also influence, even at very low proteindipid ratios, the lamellar to inverted hexagonal transitions of certain lipids. 

Solvent polarity and the local environment have profound effects on the emission spectra of polar fluorophores. These effects are the origin of the Stokes shift, which is one of the earliest observations in fluorescence. Emission spectra are easily measured, and as a result, there are num ous publications on emission spectra of fluoropho-res in different solvents and when bound to proteins, membranes, and nucleic acids. One common use of solvent effects is to determine the polarity of the probe binding site on the macromolecule. This is accomplished by comparison of the emission Spectra and/or quantum yields of the fluorophore when it is bound to the macromolecule and when it is dissolved in solvents of different polarity. However, there are many additional instances where solvent effects are used. Suppose a fluorescent ligand binds to a protein. Binding is usually accompanied by a spectral shift due to the different environment for the bound ligand. Alternatively, the ligand may induce a spectral shift in the intrinsic or extrinsic protein fluorescence. Additionally, fluorophores often display spectral shifts when they bind to membranes. 

Hydroxylamine can destroy the oxygen evolving activity of PSII by reduction of Mn (4) and the subsequent release of the ion from PSII membrane preparations. Table 1 records anion effects on NH2OH inhibition of PSII. In intact PSII preparations, NH2OH inactivation of PSII is prevented by a set of anions F , Cl , Br , C104 ,and S04 where F provides maximum protection. After salt washing to remove the extrinsic 23 and 17 kDa polypeptides, however, the anions are unable to prevent NH2OH inhibition of activity. Since anions such as F and are inhibitory with... 

In contrast to the anions shown in Table 1, Ca confers strong protection against NH2OH attack, but only in salt-washed PSII membranes (Table 2). This result confirms our earlier observation that calcium cannot cross the barrier imposed by extrinsic 23 and 17 kDa polypeptides in native PSII membranes (5). At higher concentrations, Sr 2 can also prevent NH2OH inactivation in salt-washed PSII (Table 2), while other divalent cations, such as Mg and Mn" 2, have no effect on NH2OH inhibition. Therefore, this protective effect is Ca specific, a result which is consistent with a model in which Ca binding in PSII is required to produce a stable conformation of the Mn... 

Fig.3 Effect of Incubation at various pH values on the removal of the extrinsic 23 kD protein from PS2 membranes. PS2 enriched membrane preparations were isolated from pea...

Fig.4 Effect of Incubation at different pH values on the removal of the extrinsic 33 kD protein from PS2 membranes. PS2 enriched membrane preparations were isolated from control pea plants (—), pea plants frozen at between -3 C (—and -18 C (- -) overnight. The amount of the 33 kD protein In the supernatant was assayed by ELISA.

An additional method to model biomembranes was recently described by Leaver et al. Polymerizable diacetylene fatty acids were biosynthetically incorporated into Acholeplasma laidlawii cells and their polymerization via UV-irradiation could be realized. In order to determine the effect of polymerization on the properties of the membrane, the activity of intrinsic and extrinsic membrane-bound enzymes, NADH oxidase and ribonuclease were studied. The NADH oxidase activity decreased rapidly upon polymerization of the lipid environment whereas the ribonuclease activity was unaffected. 

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