Bonding in lipids

The large number of carbon-carbon and carbon-hydrogen organic solvents and insoluble in water. Monosaccharides (from which carbohydrates are formed) and amino acids (fiom which proteins are formed), on the other hand, are very polar, so they tend to be water soluble. Because lipids share many properties with hydrocarbons, several features of lipid structure and properties have already been discussed. Table 29.1 summarizes sections of the text where aspects of lipid chemistiy were covered previously. 

TABLE 1. Energies of Light at Various Waveiengths vs. Typicai Energies of Bonds in Lipids. 

Oxidative cleavage of the O-alkyl linkage in glycerolipids is catalyzed by a microsomal tetrahydropteridine (Pte-H4)-dependent alkyl monooxygenase (Fig. 12) (T.-C. Lee, 1981). The required cofactor, Pte H4, is regenerated from Pte-Hj by an NADPH-linked pteridine reductase, a cytosolic enzyme. Oxidative attack on the ether bond in lipids is similar to the enzymatic mechanism described for the hydroxylation of phenylalanine. Fatty aldehydes produced via the cleavage reaction can be either oxidized to the corresponding acid or reduced to the alcohol by appropriate enzymes. 

H. Eibl and P. Woolley, Electrostatic Interactions at Charged Lipid Membranes. Hydrogen Bonds in Lipid Membrane Surfaces, Biophys. Chem. 10, 261-271 (1979). 

Ma, X. and Xia, Y. (2014) Pinpointing double bonds in lipids by Paterno-Biichi reactions and mass spectrometry. Angew. Chem. Int. Ed. 53, 2592-2596. 

A process is described for conducting hydrogenation reactions in a homogeneous SCF phase including hydrogenation of C=C bonds in lipids, of COOR to produce fatty alcohols, and of oxygen to hydrogen peroxide. 

Hypochlorous acid reacts with unsaturated bonds in lipids, but not saturated bonds, and the OCr ion does not participate in this reaction. This reaction occurs by hydrolysis with addition of chlorine to one of the carbons and a hydroxyl to the other. The resulting compound is a chlorhydrin. The polar chlorine disrupts lipid bilayers and could increase permeability. When chlorhydrin formation occurs in lipid bilayers of red blood cells, increased permeability occurs. Disruption could occur if enough chlorhydrin is formed. The addition of preformed chlorhydrins to red blood cells can affect permeability as well. Cholesterol chlorhydrins have also been observed, but do not greatly affect permeability, and it is believed that CI2 is responsible for this reaction. 

Hydrolases Hydrolysis reactions Proteases hydrolyze peptide bonds in proteins. Lipases hydrolyze ester bonds in lipids. Carbohydrases hydrolyze glycosidic bonds in carbohydrates. Phosphatases hydrolyze phosphoester bonds. Nucleases hydrolyze nucleic acids. 

Mitochondria Mitochondria are organelles surrounded by two membranes that differ markedly in their protein and lipid composition. The inner membrane and its interior volume, the matrix, contain many important enzymes of energy metabolism. Mitochondria are about the size of bacteria, 1 fim. Cells contain hundreds of mitochondria, which collectively occupy about one-fifth of the cell volume. Mitochondria are the power plants of eukaryotic cells where carbohydrates, fats, and amino acids are oxidized to CO9 and H9O. The energy released is trapped as high-energy phosphate bonds in ATR... 

FIGURE 10.41 (a) Gramicidin forms a double helix in organic solvents a helical dimer is the preferred strnctnre in lipid bilayers. The strnctnre is a head-to-head, left-handed helix, with the carboxy-termini of the two monomers at the ends of the strnctnre. (b) The hydrogen-bonding pattern resembles that of a parallel /3-sheet. 

Murphy, R. C. Mass Spectrometry of Lipids. New York Plenum Press, 1993. An extensive review of techniques for locating double bonds in unsaturated fatty... 

Seiler [250] proposed a way of estimating the extent of hydrogen bonding in solute partitioning between water and a lipid phase by measuring the so-called A log P parameter. The latter parameter is usually defined as the difference between the partition coefficient of a solute measured in the octanol-water system and that measured in an inert alkane-water suspension AlogP = log Kp oet — log Kp aik. 

Biological membranes consist of lipids, proteins and also sugars, sometimes mutually bonded in the form of lipoproteins, glycolipids and glycoproteins. They are highly hydrated—water forms up to 25 per cent of the dry weight of the membrane. The content of the various protein and lipid components varies with the type of biological membrane. Thus, in... 

A number of polypeptide biomarkers have also been identified in the mass range below 4000,28-31 which are cyclic secondary metabolites bonded to lipids or sugars. These peptide sequences are not directly translated from DNA,32... 

Unsaturations of lipids play a key role in lipid homeostasis, where organisms adapt to temperature variations of the environment. Plants and animals maintain physiological functions by reversibly altering the composition and conformation of lipid molecules of the cell membrane. To achieve this, they extensively and elegantly use the unsaturations (double bonds) present in their side chains. This is the process by which cell membranes adjust their flexibility (fluidity) of the bilayer and adapt themselves to perturbations in temperature, pressure, and other variations in the natural environment [11-14]. They remain indispensable for the poikilothermism exhibited by fishes, invertebrates, and amphibians [15, 16]. Commercially,... 

Another example comes from the work of Johnson, et a/.18 These workers studied spin labels dissolved in lipid bilayer dispersions of dipalmitoylphos-phatidylcholine and cholesterol (9 1 by weight) in the hope that anisotropic rotational diffusion of the spin label would mimic the motion of the bilayer components. In addition to 5-DS, which is sensitive to rotational motion about the NO bond, they used the steroidal nitroxide 8, which tends to rotate about an axis perpendicular to the N-O bond. ESR measurements were carried out at both 9 and 35 GHz and at temperatures ranging from 30 to 30 °C. Rather different results were obtained with the two spin labels, largely as a result of the different axes of rotation. Because the rotation rates were very slow, ESR spectra appeared as powder patterns rather than isotropic spectra and special methods were needed to extract the motional data. 

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