Membrane structure hydrophobic interaction

A typical biomembrane consists largely of amphiphilic lipids with small hydrophilic head groups and long hydrophobic fatty acid tails. These amphiphiles are insoluble in water (<10 ° mol L ) and capable of self-organization into uitrathin bilaycr lipid membranes (BLMs). Until 1977 only natural lipids, in particular phospholipids like lecithins, were believed to form spherical and related vesicular membrane structures. Intricate interactions of the head groups were supposed to be necessary for the self-organization of several ten thousands of... 

Soluble proteins exhibit hundreds of distinct localized folded structures, or motifs (see Figure 3-6). In comparison, the repertoire of folded structures in Integral membrane proteins Is quite limited, with the hydrophobic a helix predominating. Integral proteins containing membrane-spanning ct-hellcal domains are embedded in membranes by hydrophobic Interactions with specific lipids and probably also by Ionic Interactions with the polar head groups of the phospholipids. 

Fig. 48.14. A composite diagram indicating a Schwann cell that has wrapped around a portion of an axon, forming the myelin sheath. The expansion represents a portion of the myelin sheath. CNS myelin is shown, although it is similar to PNS myelin except that Po would take the place of PLP (proteolipid protein). Recall that there are multiple layers of membrane surrounding the axon PLP protrudes into the extracellular space and aids in compaction of the membranes through hydrophobic interactions. MBPs help to stabilize the structure from within the membrane.

Since both the free and the bound forms of the lipoprotein are located exclusively in the outer membrane, there are two possible ways in which a superhelical assembly could interact with the outer membrane (1) The interaction could occur through the three fatty acids attached to the amino-terminal amino acid of the lipoprotein, as suggested by Braun. In this case, the hydrocarbon chains of the fatty acids stick out of the assembly and penetrate into the phospholipid bilayer of the outer membrane. Therefore, the protein part of the assembly protrudes from the inside surface of the outer membrane. This model would predict that the peptidoglycan layer should be at least 76 A apart from the outer membrane, which is not likely. (2) Alternatively, the whole assembled structure, with a height of 76 A, penetrates through the 75-A-thick outer membrane with hydrophobic interaction between the surface of the assembly and the lipid bilayer of the outer membrane. This arrangement is further stabilized by the three hydrocarbon chains at the amino-terminal end of the individual molecules, which could be flipped back over the helix and inserted into the bilayer (Fig. 14). In order to arrange the hydrocarbon chains as shown in Fig. 14, the side chains of two serine residues at the amino terminus are made to face upward, which makes the uppermost part of the assembly hydrophilic, as a part of the surface of the outer membrane. 

The L and the M subunits are firmly anchored in the membrane, each by five hydrophobic transmembrane a helices (yellow and red, respectively, in Figure 12.14). The structures of the L and M subunits are quite similar as expected from their sequence similarity they differ only in some of the loop regions. These loops, which connect the membrane-spanning helices, form rather flat hydrophilic regions on either side of the membrane to provide interaction areas with the H subunit (green in Figure 12.14) on the cytoplasmic side and with the cytochrome (blue in Figure 12.14) on the periplasmic side. The H subunit, in addition, has one transmembrane a helix at the car-boxy terminus of its polypeptide chain. The carboxy end of this chain is therefore on the same side of the membrane as the cytochrome. In total, eleven transmembrane a helices attach the L, M, and H subunits to the membrane. 

NHS-ester compounds to study protein interactions. These bis-NHS-PEG compounds may provide a superior crosslinker for studying such interactions due to their water solubility and the fact that the PEG bridge won t get buried in hydrophobic pockets on proteins or within hydrophobic membrane structures. 

Thus, lipoproteins could be injected over the surface of a lipid covered SPR sensor in a detergent free buffer solution and showed spontaneous insertion into the artificial membrane.171 Again two hydro-phobic modifications are necessary for stable insertion into the lipid layer, whereas lipoproteins with a farnesyl group only dissociate significantly faster out of the membrane. Therefore the isoprenylation of a protein is sufficient to allow interaction with membraneous structures, while trapping of the molecule at a particular location requires a second hydrophobic anchor. Interaction between the Ras protein and its effector Raf-kinase depends on complex formation of Ras with GTP (instead of the Ras GDP complex, present in the resting cell). If a synthetically modified Ras protein with a palmi-... 

Membrane proteins (which make up approximately one-third of the total number of known proteins) are responsible for many of the important properties and functions of biological systems. They transport ions and molecules across the membrane they act as receptors and they have roles in the assembly, fusion, and structure of cells and viruses. Presently, investigating membrane proteins is one of the most difficult challenges in the area of structural biology and biophysical chemistry. Our knowledge of membrane proteins is limited, primarily because it is very difficult to crystallize these protein systems due to the extreme hydrophobic interactions between the proteins and the membrane. New methods are needed and current techniques need to be extended to study the structural properties of membrane proteins. 

The final mode of regulating enzymic activity to be discussed is the coupling of an enzyme with a membrane. Several different types of regulation are possible (1) Specific interactions between the protein and phospholipid may be required (2) a general requirement for a hydrophobic type of environment may exist (3) the enzyme can be immobilized by a membrane and can be localized in a particular place where it is needed (4) the function of the enzyme can be coupled with another membrane function, such as transport (this coupling may require a closed membranous structure) and (5) the enzymic activity can be modulated by interaction of the enzyme with other membrane proteins (e.g., by coupling to other enzymes or to receptors). A few examples illustrating these possibilities are now considered. 

As with other multisubunit enzymes (e.g., allosteric enzymes), the structural integrity of a membrane-bound enzyme primarily is maintained by noncovalent interactions such as hydrogen bonding, electrostatics, and hydrophobic interactions. Hydrophobic polypeptides (or hydrophobic portions of polypeptides) apparently are used to anchor the enzymes to the membrane through interactions with phospholipids. Therefore, I would characterize the interaction between the enzyme and membrane as chemical in nature rather than as geometric. ... 

Many biomolecules are amphipathic proteins, pigments, certain vitamins, and the sterols and phospholipids of membranes all have polar and nonpolar surface regions. Structures composed of these molecules are stabilized by hydrophobic interactions among the non-... 

FIGURE 11-3 Fluid mosaic model for membrane structure. The fatty acyl chains in the interior of the membrane form a fluid, hydrophobic region. Integral proteins float in this sea of lipid, held by hydrophobic interactions with their nonpolar amino acid side chains. Both proteins and lipids are free to move laterally in the plane of the... 

Membranes are composed of lipids and proteins in varying combinations particular to each species, cell type, and organelle. The fluid mosaic model describes features common to all biological membranes. The lipid bilayer is the basic structural unit. Fatty acyl chains of phospholipids and the steroid nucleus of sterols are oriented toward the interior of the bilayer their hydrophobic interactions stabilize the bilayer but give it flexibility. 

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