Hydrophobic polypeptides

The main toxic pore forming component of P. marmoratus secretion, named pardaxin, was isolated by liquid column chromatography (5). Originally two toxic (5) polypeptides, Pardaxin I and II, were isolated. However, their primary sequences have been found to be identical (6) therefore, the two components most probably represent different aggregates of one polypeptide. This finding is in contrast to the secretion of P. pavonicuSj which contains three toxic polypeptides (8). Pardaxin is a single chain, acidic, amphipathic, hydrophobic polypeptide, composed of 33 amino acids and with a mass around 3500 daltons (5,6). The primary sequence is (6) NHj-Gly-Phe-Phe-Ala-Leu-Ile-Pro-Lys-Ile-Ile-Ser-Ser-Pro-Ile-Phe-Lys-Thr-Leu-Leu-Ser-Ala-Val-Gly-Ser-Ala-Leu-Ser-Ser-Ser-Gly-Gly-Gln-Glu-COOH. 

Fluoroalkyl side chains are hydrophobic but due to the polarization of adjacent hydrogen atoms are also polar and thus disturb hydrophobic-polypeptide interactions (Section 3.3). 

Fig. 11. A pictorial representation of the mitochondrial ATP synthesizing coupling factor interacting with the mitochondrial membrane. F, contains five polypeptide chains a, (3, 7, 8, and e and is readily solubilized. The stalk is probably made up of three polypeptide chains, 8, OSCP, and Ffi, which interact with a small group of hydrophobic polypeptides, CF0, embedded in the membrane.

The question of whether an enzyme is membrane bound or membrane associated is to some extent a matter of semantics. However, it is certainly true that some proteins are readily dissociated from membranes whereas others require quite drastic conditions before they can be dissociated from the membrane. As limiting cases, the former can be designated as membrane associated and the latter as membrane bound. Enzymes that are generally considered membrane bound are firmly embedded in the membrane structure. For example, the mitochondrial coupling factor is strongly coupled to the bilayer structure by hydrophobic polypeptides. The Na+-K+ ATPases that have been purified have a small patch of associated phospholipids when the enzyme is delipidated, enzymatic activity is lost. In fact, membrane-bound enzymes appear to be... 

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. ... 

Proteins SP-B and SP-C are small extremely hydrophobic polypeptides consisting of 79 and 35 amino acid residues, respectively." 0 Aliphatic branched amino acids constitute 23 of the 35 residues of the C-terminal part of protein C, which is also palmitoylated on two cysteine residues. SP-B is formed from a large 381-residue precursor. The mature protein contains seven cysteines and disulfide bridges. Both proteins have major effects on the properties of the surfactant mixture. They promote rapid reorganization of lipid layers, an important consideration for the functioning of the surfactant. Infants lacking SP-B suffer severe respiratory failure with high mortality.6... 

The limited solubility of membrane proteins and related polypeptides in aqueous mobile phases can also cause problems. These could be solved, e.g., by adding guanidine hydrochloride (6 M) or urea (8 M) to the portion of initial eluent used for sample preparation 69). The urea was always eluted in the breakthrough volume of the column. Thus, the retained hydrophobic polypeptides might have been temporarily precipitated upon the column. Collagen chains, dissolved in 0.5 M acetic acid, were successfully separated by RP-HPLC through gradients of 0.1 M TFA/acetonitrile 70> or (0.05 M ammonium bicarbonate + TFA)/ tetrahydrofuran 57>. 

A) Synthesis of Copolymers with a Polyvinyl Block and a Hydrophobic Polypeptide Block... 

Block copolymers with a hydrophobic polyvinyl block and a hydrophobic polypeptide block (BG, SG, SC, SL, BCK and SCK copolymers) exhibit well organized meso-phases in dioxane, 1,2-dichloroethane, 2,3-dichloro 1-propene, etc., solutions. These mesophases are observed for solvent concentrations smaller than about 60% and for dry samples obtained by evaporation of the solvent at a slow rate. 

Fig. 38. Example of variation of the geometrical parameters of the lamellar structure of copolymers with a polyvinyl block and a hydrophobic polypeptide block. Copolymer BG.530 in dioxane solution22 ...

The lamellar structure of saccharide-peptide block copolymers presents many analogies with the structure of copolymers with a hydrophobic polyvinyl block and a hydrophobic polypeptide block and exhibits a similar behaviour as a function of the solvent concentration. When the solvent concentration increases the total thickness of a sheet and the thickness of the carbohydrate layer both increase while the thickness of the polypeptide layer remains nearly constant284. ... 

A coherent interpretation for many experimental results was provided by the concept of a PS I reaction centre. This centre has now been isolated, albeit perhaps not in a definitely pure state. It is made up of a few hydrophobic polypeptides, the primary donor (P-700), several electron acceptors (Fig. 2), and about 50 molecules of pigment (chlorophyll a and /3-carotene). This composition is analogous to that of other types of reaction centres. 

The polypeptide components are small (5-6 kDa) and in most cases span the photosynthetic membrane once (hydrophobic polypeptides, insoluble in water. 

Most double emulsions are in the micron range and use a mix of surfactants in their formulations. However, recent work has shown that it is possible to form stable double emulsions with droplets under 100 nm (Hanson et al, 2008). These authors proposed the use of racemic, disordered, hydrophobic polypeptide segments which interact via hydrogen bonding to stabilize nanoscale double emulsions. 

The Fo complex making up the proton pore is composed of three subunits, a, b, and c, in the proportion ab2Cio-i2- Subunit c is a small (M 8,000), very hydrophobic polypeptide, consisting almost entirely of two transmembrane helices, with a small loop extending from the matrix side of the membrane. The crystal structure of the yeast FqFi, solved in 1999, shows the arrangement of the c subunits. The yeast complex has ten c subunits, each with two transmembrane helices roughly perpendicular to the plane of the membrane and arranged in two concentric circles (Fig. 19-23d, e). The... 

M.J. Coon (1979). Role of a hydrophobic polypeptide in the N-terminal region of NADPH-cytochrome P-450 reductase in complex formation with P-450LM. Biochem. Biophys. Res. Commun. 91, 1528-1535. 

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