Helical membrane element

To illustrate the power of PISA wheels and dipolar waves to determine the structure of helical peptides and proteins in uniaxiaUy oriented lipid bilayers. Fig. 6a-c show SIMPSON/SIMMOL-simulated PISEMA spectra of an ideal 18-residue a-helix with a tilt angle of 10°-30° relative to Bq. In these simulations, we have tried to mimic experimental conditions by including a random distribution of the principal components of the chemical shift tensor and the dipolar coupling. The chemical shift distribution is 6 ppm for each principal element and has been established as follows we obtained — 85000 N isotropic chemical shifts reported to the BioMagResBank and selected only the — 31000 located in helical secondary stractures to have a data set independent on secondary chemical shifts. The standard deviation on the N chemical shifts for these resonances was — 6 ppm. With the lack of other statistically reliable experimental methods to establish such results for the individual principal elements of the N CSA tensor, we assumed the above variation of 6 ppm for all three principal elements. The variation of the H- N dipolar coupling was estimated by investigating the structures for a small number of a-helical membrane proteins for which the structures were established by liquid-state NMR spectroscopy. These showed standard deviations... 

The externally wound membranes were developed by Universal Water Corp., San Diego, California. In this design, the membrane element consists of a porous supporting tube on which are simultaneously wound, in helical fashion, a strip of permeable fabric overlaid with a helical wound strip of semlpermeable membrane film. Adjacent turns of the membrane overlap in winding, and these overlaps are sealed by a bonding solvent so that, the membrane Itself... 

Fabrlcatlon/assembly of externally wound tubular membrane elements is accomplished by specially designed equipment that simultaneously and continuously winds, in helical fashion. Infinite lengths of membrane and backing material strips onto prefabricated tubular support structures. Membrane strip winding overlaps are solvent bonded during the winding process. 

A related application for RDCs has also been described based on the sequence-dependent pattern of RDCs along a helical structure, called a dipolar wave by the Opella group [317, 352, 353]. The magnitude and periodicity of the dipolar wave depends on the orientation of the helix, and can allow irregularities in helix structure to be identified. Most commonly, dipolar waves have been used to help determine the location of helices in a protein sequence, allowing these structural elements to be more rigidly restrained over the course of a structure calculation [57,159, 323, 354]. This is particularly useful for larger helical membrane proteins, since a-helices are not well defined by the NOEs available in sparsely protonated samples [262]. 

Shi et al.71 have assigned the backbone and side-chain chemical shifts for 103 of 238 residues of proteorhodopsin using solid state NMR spectroscopy. Analysis of the chemical shifts has allowed determination of protonation states of several carboxylic acids as well as boundaries and distortions of trans-membrane a-helices and secondary structure elements in the loops. It has been shown that internal Asp227, making a part of the counterion, is ionised, while Glul42 located close to the extracellular surface is neutral. 

G-Protein coupled receptors (GPCR) represent the start element in secondary messenger producing systems. They comprise a family of over 1000 structurally-related members. These membrane proteins are also called serpentine or seven-helix receptors due to their seven transmembrane domains with an a-helical conformation. Receptors belonging to this class respond to a variety of hormones and neurotransmitters, and they detect odorant molecules or light [3,4]. 

At present, it is generally assumed that transmembrane receptors span the cell membrane as a-helices. However, it is not known how often other structural elements occur in the transmembrane domains of receptors. Tlius, the presence of P-sheet structures, particularly in the case of receptors with complex structures, cannot be excluded (Hucho et al., 1994). 

The occurrence of seven sequence segments, each made up of 20—25 amino acids, is characteristic for the G-protein coupled receptors. From this, it is assumed that they form transmembrane domains, and span the membrane in the form of a-helices. The transmembrane elements are linked by loops of various sizes on the outer and inner side. 

The system may be regarded as involving a Na+/Mg2+ co-catalysed phosphorylation step and a K+ catalysed dephosphorylation. Each phosphorylation/dephosphorylation step involves a pseudorotation of an Mg2+-stabilised 5-coordinate intermediate, resulting in transport of the alkali metal cations. The cation transport ability of the enzyme is a direct result of the enzymatic reactivity of the protein. There are three binding sites with high Na+ affinity and two with K+ affinity (occupied by Rb+ in the crystal structure determination). The structure (which is of the E2K state of the system) reveals that carboxy end of the a-subunit is held in a pocket in between transmembrane helices and acts as an unusual regulating element that controls sodium affinity and may be influenced by the membrane potential. 

Figure 15. Model of the secondary and tertiary structure of the SR Ca2+-ATPase (MacLennan et al., 1985 Green, 1989). Amino acid residues are indicated by single-letter code. Two types of secondary structure elements are shown a-helices are represented by diagonal rows consisting of three or four residues as seen for instance in the membrane (M) and stalk (S) sectors P-strands are indicated by a ladder-type of arrangement of the symbols.

Curved space elements. Membranes, micelles, helices. Higher structures by curvature of lower structures... 

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