Globular proteins transport, membrane

The proteins involved with electron transfer vary in size and shape. Thus the internal membranes of chloroplasts and mitochondria are not uniform and regular. Proteins taking part in ion transport and cell wall synthesis can be embedded in the plasma membrane, and other proteins involved with transport occur in the tonoplast. Many globular proteins in membranes serve a structural role. 

Proteins built up entirely from a-helices include the globin and hanerythrin families and bacterioihodopsin. These are transport proteins and membrane-bound proteins. All-P proteins invariably consist of a sandwich of two layers, each a sheet held together by a hydrophobic core formed by one side from each sheet. Often the sheet is rounded to form a P-barrel. The P interactions in these proteins are usually antiparallel because there are no helices available to form the necessary crossover connections required for parallel P-sheets. The novel family of proteins with an all-parallel P-helix (Jumak et al, 1994 Yoder et al, 1993) is a remarkable exception. Proteins with distinct regions of all-a or all-P structures are called a + P proteins. The most complex class is the o/p proteins. In these, segments of a-helix and P-sheet follow one another, and consequently each of the two classes of secondary structure is intimately involved in stabilization of the other. For this reason, the P-structures are often parallel, since the presence of helices provides the necessary crossover connections. The -class of proteins is an ill-defined catchall (Chou, 1995). It includes a number of large peptides with no globular structure. 

Lipids, lipoproteins, and proteins makeup cytoplasmic membranes. This membrane is a diffusion barrier for ions, nutrients, transport systems, and most importantly, water. It is composed of a lipid grouping with globular proteins that penetrate the lipid bilayer. Most antibacterial agents that inhibit cytoplasmic membranes do so by influencing the balance of cations, anions, or neutral compounds, thus disrupting membrane operation. Of interest is that fungal... 

Biological membranes typically contain about 50% each of lipids and proteins. Perhaps the most familiar model of the membrane make-up has been that of Singer and Nicolson (shown in Fig. 5.19). In it the lipids form a bilayer similar to those found in lamellar liquid crystals. The bilayer structure is spanned by large globular protein complexes which serve as selective barriers to the transportation of metabolites in and out of the cell. In addition to this they also confer a degree of stability on the structure, preventing it from breaking apart. The exact nature of this stabilization is obviously rather complex and extremely difficult to mimic in the synthesis of artificial membranes. Therefore, polymerization of the... 

Even though dynein, kinesin, and myosin serve similar ATPase-dependent chemomechanical functions and have structural similarities, they do not appear to be related to each other in molecular terms. Their similarity lies in the overall shape of the molecule, which is composed of a pair of globular heads that bind microtubules and a fan-shaped tail piece (not present in myosin) that is suspected to carry the attachment site for membranous vesicles and other cytoplasmic components transported by MT. The cytoplasmic and axonemal dyneins are similar in structure (Hirokawa et al., 1989 Holzbaur and Vallee, 1994). Current studies on mutant phenotypes are likely to lead to a better understanding of the cellular roles of molecular motor proteins and their mechanisms of action (Endow and Titus, 1992). 

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