Lipid mobilization functions

Bussell, S. J., Koch, D. L., and Hammer, D. A., The resistivity and mobility functions for a model system of two equal-sized proteins in a lipid bilayer. /. Fluid Mech. 243, 679 (1992). 

Glycogenolysis Lipid mobilization Smooth muscle contraction Cardiac function, vasodilation Vasoconstriction, arteriole contraction... 

Fig. 7. Relative parameters of affinity of various catecholamine and oxedrine derivatives during influencing of lipid mobilization and tracheal relaxation in vttro. N = substitution of substances H = hydrogen Me = methyl Et = ethyl nPr = n-propyl nBu = n-butyl iPr z= isopropyl tBu = tert-butyl FtBu z= phenyl-tert-butyL Relative affinities of the substances in relation to that of noradrenaline are given in mean values SE. Left scale relative (logarithmic) pD values. The leveling out of the lipid-mobilizing effects of various catecholamines is to be noted. In the case of the oxedrines the effects are equalized over both functions.

Functionally and strucmrally, the components of the respiratory chain are present in the inner mitochondrial membrane as four protein-lipid respiratory chain complexes that span the membrane. Cytochrome c is the only soluble cytochrome and, together with Q, seems to be a more mobile component of the respiratory chain connecting the fixed complexes (Figures 12-7 and 12-8). 

Capillary electrophoresis (CE) (see Section 3.5) has been used to determine partition coefficients [320-322]. Lipid vesicles or micelles are added to the buffer whose pH is adjusted to different values. Since drug molecules partition to a different extent as a function of pH, the analysis of mobility vs pH data yields log P values. 

Membrane conformational changes are observed on exposure to anesthetics, further supporting the importance of physical interactions that lead to perturbation of membrane macromolecules. For example, exposure of membranes to clinically relevant concentrations of anesthetics causes membranes to expand beyond a critical volume (critical volume hypothesis) associated with normal cellular function. Additionally, membrane structure becomes disorganized, so that the insertion of anesthetic molecules into the lipid membrane causes an increase in the mobility of the fatty acid chains in the phospholipid bilayer (membrane fluidization theory) or prevent the interconversion of membrane lipids from a gel to a liquid form, a process that is assumed necessary for normal neuronal function (lateral phase separation hypothesis). 

In addition to NAD and flavoproteins, three other types of electron-carrying molecules function in the respiratory chain a hydrophobic quinone (ubiquinone) and two different types of iron-containing proteins (cytochromes and iron-sulfur proteins). Ubiquinone (also called coenzyme Q, or simply Q) is a lipid-soluble ben-zoquinone with a long isoprenoid side chain (Fig. 19-2). The closely related compounds plastoquinone (of plant chloroplasts) and menaquinone (of bacteria) play roles analogous to that of ubiquinone, carrying electrons in membrane-associated electron-transfer chains. Ubiquinone can accept one electron to become the semi-quinone radical ( QH) or two electrons to form ubiquinol (QH2) (Fig. 19-2) and, like flavoprotein carriers, it can act at the junction between a two-electron donor and a one-electron acceptor. Because ubiquinone is both small and hydrophobic, it is freely diffusible within the lipid bilayer of the inner mitochondrial membrane and can shuttle reducing equivalents between other, less mobile electron carriers in the membrane. And because it carries both electrons and protons, it plays a central role in coupling electron flow to proton movement. 

Like Complex III of mitochondria, cytochrome b6f conveys electrons from a reduced quinone—a mobile, lipid-soluble carrier of two electrons (Q in mitochondria, PQb in chloroplasts)—to a water-soluble protein that carries one electron (cytochrome c in mitochondria, plastocyanin in chloroplasts). As in mitochondria, the function of this complex involves a Q cycle (Fig. 19-12) in which electrons pass, one at a time, from PQBH2 to cytochrome bs. This cycle results in the pumping of protons across the membrane in chloroplasts, the direction of proton movement is from the stromal compartment to the thylakoid lumen, up to four protons moving for each pair of electrons. The result is production of a proton gradient across the thylakoid membrane as electrons pass from PSII to PSI. Because the volume of the flattened thylakoid lumen is small, the influx of a small number of protons has a relatively large effect on lumenal pH. The measured difference in pH between the stroma (pH 8) and the thylakoid lumen (pH 5) represents a 1,000-fold difference in proton concentration—a powerful driving force for ATP synthesis. 

In adipose tissue, TAG is stored in the cytosol of the cells in a nearly anhydrous form. It serves as "depot fat," ready for mobilization when the body requires it for fuel. Little TAG is stored in the liver. Instead, most is exported, packaged with cholesteryl esters, cholesterol, phospholipid, and protein (apolipoprotein B-100, see p. 229) to form lipoprotein particles called very low density lipoproteins (VLDL). Nascent VLDL are secreted into the blood where they mature and function to deliver the endogenously-derived lipids to the peripheral tissues. [Note Recall that chylomicrons deliver primarily dietary (exogenously-derived) lipids.] Plasma lipoproteins are discussed in Chapter 18, p. 225. 

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