Membrane orientation

Figure 22.3 The basic construction of phosphodiglyceride molecules within lipid bilayers. The fatty acid chains are embedded in the hydrophobic inner region of the membrane, oriented at an angle to the plane of the membrane surface. The hydrophilic head group, including the phosphate portion, points out toward the hydrophilic aqueous environment.

FIGURE 4.6 Isolated dark adapted chloroplast imaged with (a) MPF, (b) SHG, and (c) THG. The chloroplast is edge aligned with thylakoid membranes oriented perpendicular to the image plane. The scale bar is 3 pm. 

Most of the above membrane-oriented studies were carried out for peptides in multilayer systems that were collapsed or transferred onto a sample cell surface. An alternative and very interesting way to study membrane systems is by IRRAS (infrared reflection absorption spectroscopy) at the air-water interface. In this way, unilamellar systems can be studied as a function of surface pressure and under the influence of various membrane proteins and peptides added. Mendelsohn et al.[136] have studied a model series of peptides, [K2(LA) ] (n = 6, 8, 10, 12), in nonaqueous (solution), multilamellar (lipid), and unilamellar (peptide-IRRAS) conditions. In the multilamellar vesicles these peptides are predominantly helical in conformation, but as peptide only monolayers on a D20 subphase the conformation is (1-sheet like, at least initially. For different lengths, the peptides show variable surface pressure sensitivity to development of some helical component. These authors further use their IR data to hypothesize the existence of the less-usual parallel (i-sheet conformation in these peptides. A critical comparison is available for different secondary structures as detected using the IRRAS data for peptides on H20 and D20 subphasesJ137 ... 

Fig. 16. Models for the membrane topology and orientation for the FetSp/Ftrlp and Fet5p/Fthlp complexes in yeast membranes. (Top) FetSp/Ftrlp are pictured in the yeast plasma membrane oriented to the extracellular space. The (R)EXXE motifs that may be involved in iron uptake and the transmembrane domain required for correct assembly are indicated. These roles have been suggested by mutagenesis studies (Stearman et al., 1996). (Bottom) Model of Fet5p/Fthlp complex proposed for the membrane of the yeast vacuole oriented to the lumen (inside) of the vacuole. In this model, the EXXE motifs in Fthlp are not oriented toward the ferroxidase site on Fet5p (Urbanowski and Piper, 1999).

The SAXS patterns at higher k-values display asymptotic linear slopes indicating a power law relation. For a sharp interfece with a smooth geometry, the intensity is expected to foUow a k power law in this Porod region. The observed slope varies systematically with pore size, membrane orientation and thickness. In some cases, exponents more negative than 4 are observed. This may arise from either angulosity or a non-sharp interface, and is discussed in more detail elsewhere [15]. 

ESR on aligned membranes. A simple method to record spectra 33. from different membrane orientations in the magnetic field. J. Magnet. Reson. 2006 179 273-277. 34. 

Significantly, the membrane orientation of CF]-CFq is reversed compared with that of the mitochondrial ATP synthase (Figure 19.25). Thus, protons flow out of the thylakoid lumen through ATP synthase into the stroma. Because CF] is on the stromal surface of the thylakoid membrane, the newly synthesized ATP is released directly into the stromal space. Recall that NADPH formed through the action of photosystem I and ferredoxin-NADP+ reductase also is released into the stromal space. Thus, ATP and NADPH, the products of the light reactions ofphotosynthesis, are appropriately positioned for the subsequent dark reactions, in which CO is converted into carbohydrate. 

Hunziker, W., Spiess, M., Semenza, C., and Lodish, H. F. (1986). The sucrase-isomaltase compje) I rimary structure, membrane orientation, and evolution of a stalked, intrinsic brush border protein. Ceff 46,227-234. 

A FIGURE 8-2 Membrane orientation and the direction of proton movement during chemiosmotically coupled ATP synthesis in bacteria, mitochondria, and chloroplasts. The... 

Studies with cell-free translation systems containing ER-derived microsomal membranes have revealed that PrP may exist in different topological forms, including two transmembrane forms with opposite membrane orientation. A specific transmembrane form was found to be increased in transgenic mice expressing the All TV mutation and was detected in brain tissue of patients with GSS A11 TV (Hedge et al, 1998). 

Zhang, J.T. and Ling, V., Study of membrane orientation and glycosylated extracellular loops of mouse P-glycoprotein by in vitro translation, J Biol Chem 266 (1991) 18224-18232. 

There is much current debate about the relevance of such carotenoid repair processes to hydrocarbon carotenoids such as 8-carotene and lycopene in vivo where the parent carotenoid is unhkely to encounter the polar ascorbic acid. However, the cation radical, with a positive charge, maybe sufficiently polar and long-lived for such interactions to be possible. For the carotenoids found in the macula, where an efficient anti-oxidant process is crucial, the hydroxy carotenoids zeaxanthin, meso zeaxanthin and lutein are likely to be in a membrane orientation such that the corresponding cation radicals are efficiently repaired by the vitamin C (cf. vitamin E, below). 

Some MF membranes have a variation in pore size from upper to lower face (see Figure 2.32). We call this "anisotropy". At least one manufacturer intentionally makes an anistropic MF membrane. The idea is to provide a built-in prefilter with the more open pore-size upstream of the finer pore-size. Indeed, an improvement in throughput is achieved, but usually at the expense of retention. When using discs of anisotropic membranes, orientation can have a dramatic effect on throughput. One manufacturer puts a note in every box of membrane "Use this side upstream". 

A very simple and informative description of helices is provided by drawing a wheel. One looks from the NH2 end and displaces neighboring amino acids by exactly 100°, (e.g., Gly 1 and He 2 in melittin) (Fig, 9.2.7) (Juvvadi et al., 1996). One thus obtains an alignment of the amino acids along the circumference of the total helix. In membrane proteins, such as melittin, one often obtains circumferences where one side is hydrophobic or membrane oriented and the other side is hydrophilic or prone to domain and water-filled pore formation in membranes. 

Praxair has also filed patent applications on a syngas reactor incorporating tubular membranes, as shown in Fig. 8.11 [26]. Similar to the Unde AG design of Fig. 8.9, this design uses tubular membranes oriented with the axes of the tubes parallel to the axis of the cylindrical pressure vessel. High pressure syngas flows on the exterior of the ceramic membrane tubes, and low pressure air is fed into... 

Cheng, T. and Lin, C. T. (2004) A study on cross flow ultrafiltration with various membrane orientations. Separation and Purification Technology 39, 13-22. 

FIGURE 33.3 Transport phenomenon and membrane orientation of the FO process using an asymmetric FO membrane, (a) PRO mode and (b) FO mode. (Adapted from Desalination, 309, Tan, C.H. and Ng, H.Y., Revised external and ICP models to improve flux piedietion in FO process, 125-140, Copyright 2013, with permission from Elsevier.)... 

G.T. Gray, J.R. McCutcheon, M. Elimelech, Internal concentration polarization in forward osmosis Role of membrane orientation. Desalination 2006, 197, 1-8. 

Figure 8.9 Orientation of molecules in membranes from NMR. (a) H-cholesterol orientation with respect to the membrane normal, Z, seen from the plane (left) and from above (right), from Aussenac et al. (2003). (b) N-NMR spectra of peptide-containing membranes oriented perpendicularly to the magnetic field, (c) Schematics of N-labelled helices oriented flat on the membrane or embedded normal to the membrane plane, with corresponding spectra on (b), top and middle. The Bottom spectrum is that of nonoriented peptide...

De Strooper, B., Beullens, M., Contreras, B., Levesque, L., Craessaerts, K., CordeU, B., Moechars, D., Bollen, M., Fraser, R, St George-Hyslop, P. and Van Leuven, F. 1997. Phosphorylation, subcellular localization, and membrane orientation of the Alzheimer s disease-associated presenilins. J. Biol. Chem. 272 3590-3598... 

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