Amino pore formation

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. 

Structural changes in staphylococcal alpha-hemolysin (alpha HL) that occurred during oligomerization and pore formation on membranes have been examined with the hydrophilic sulfhydryl reagent, 4-acetamido-4 -((iodoacetyl)amino)stUbene-2,2 -... 

Facial amphiphilic peptides are another class of facial amphiphiles, which play an important rote in many biological processes involving lipid bilayer membranes. Because of the large surface area of the amphiphilic domains, they are prone to interact with the hydrophiUc/hydrophobic interface of lipid bilayers, which is necessary to assist in membrane fusion or transmembrane pore formation. In the case of pore-forming antibiotics, the peptides are often relatively small (between 25 and 100 amino acids) and the entire peptide becomes facially amphiphilic on folding into the secondary structure. In the case of membrane fusion or curvature-inducing proteins only the peptide fragment, which interacts with the bilayer membrane, is facially amphiphilic. 

A proposed mechanism for SNARE-mediated membrane fusion is shown in Fig. 6.4. The zipping of SNARE complexes from the amino-terminal end draws the membranes into proximity and then increases interfacial curvature and lateral tension. This leads to hemifusion, followed by pore formation to eliminate the unfavourable void space. This may be mediated by Ca + ions, which are a trigger for many membrane transport steps because they cause significant changes in membrane tension. Vesicle exocytosis may occur via a related process. 

Later articles dealt with further elaboration of ideas on the driving forces which would have led to the formation of higher aggregates from RNA and amino acids. As had been suggested 20 years earlier, these processes could have taken place in rock pores and could have been driven by hydration and dehydration phases (Kuhn and Waser, 1994). The tiny pores in rocks act as minute test tubes, so optimal compositions could have been determined and replicated using many millions of systems. According to this model, none of the synthetic processes taking place would have required the presence of protein enzymes (see also Lahav, 1999). Just as other... 

Hie transmembrane domain may consist of one or several transmembrane elements (see also Fig. 5.2). In the latter case, these are arranged in the form of bundles, as shown in Figure 5.4 for bacteriorhodopsin. In the case of ion channels, in which several subunits are involved in formation of the transmembrane domain (see acetylcholine receptor, Fig.16.12), prediction of the structure of the membrane portion is very difficult. The different transmembrane elements are no longer equivalent in these cases. Part of the element is involved in formation of the irmer wall of the pore, other structural elements form the surface to the hydrophobic irmer of the phospholipid bilayer. It is evident that the polarity requirements for the amino acid side chains vary according to the position of the transmembrane elements (see Chapter 16). 

---